Slide 1

GEOLOGICAL SOCIETY OF AMERICA 2003 ANNUAL MEETING
(ABSTRACT NO. 65672)
COMPARISON OF CLIMATIC TRENDS AND WELL RECORDS IN SOUTHERN
CALIFORNIA: IMPLICATIONS FOR GEOTECHNICAL AND HYDROGEOLOGICAL
PRACTICE
McMILLAN, Kent and VINCENT, Mark, Geo-Logic Associates, 1360 Valley Vista Dr., Diamond Bar, CA
91765, [email protected]
Geo-Logic Associates

ABSTRACT

Comparison of climate (rainfall) records from Los Angeles with water well records beginning in 1930 from the
nearby San Gabriel Valley, shows a correspondence in cycles of relative wet and dry on a decadal scale, as well
as similar longer term trends. The climate record of Los Angeles spans the termination of the Little Ice Age (+
1890 A.D.), and plots of cumulative departure from the long-term mean of the record suggest drying conditions
over the last century. With respect to the present day, these plots also suggest the last significant wet period from
the point of view of increasing groundwater recharge was the 1940’s. The well records noted above establish the
historic high groundwater levels in the area similarly occurred in the decade. Since then, however, rainfall and
groundwater level trends appear to be in decline. This raises the question of whether historic high groundwater
is an appropriate criterion in liquefaction analysis in all cases, since its use presumes the probable return of
historic conditions over the design life of a facility. Alternatively, the criterion might be based on trend analysis
of groundwater levels where there is also corresponding climatic data. Recharge trends are also analyzed by the
water-table fluctuation method.

This presentation is an outgrowth of recent work performed for the City of Irwindale, California, relating to the geotechnical
characterization of alluvial fan deposits derived from the San Gabriel River Canyon (Figure 1). We are indebted to Mr. Kwok Tam,
Director of Public Works, City of Irwindale, for his invaluable assistance, and suppor of this project. Sand and gravel mining of these
deposits to produce mineral aggregates is a major commercial activity in the City. Mining operations have excavated large, deep, steep
sided open pits, some of which have reached their permitted depths and are inactive, while others are nearing completion depths.
Reclamation of these pits is therefore of significant interest to the City, which is proceeding to assess the geotechnical and
hydrogeologic implications of post-mining reclamation.
The alluvial fan deposits constitute a significant unconfined and highly transmissive aquifer beneath the San Gabriel Valley, which
drains a major portion of the San Gabriel Mountains watershed (Figure 1). Groundwater lakes are a feature of most mining sites.
Groundwater levels fluctuate seasonally and with wet and dry climate cycles, and may vary locally depending on pumping from, or
recharge to the unconfined aquifer. Currently, groundwater levels observed in the pits are at or near historic low elevations.
All mining sites are included in seismic hazard zones defined by the California Geological Survey, because of concern for liquefaction
and seismic slope stability. As a result of this, an investigation of historical groundwater trends was conducted, seeking to identify the
historical high for the area. In the course of that analysis more general relationships between groundwater and climate trends became
apparent that are the subject of the following discussion.
Figure 1 is a Landsat, computer generated, color enhanced image of the San Gabriel Mountains watershed viewed from the west (note
the 2X vertical exaggeration). The image illustrates the spatial proximity of the mountain front, the adjacent San Gabriel Valley, and the
Los Angeles area, from which the climate records that form the basis of Figures 2, -3 and -4 originate. Also shown are the locations of
Wells 3030F, 4198A and 4198G near the mouth of San Gabriel Canyon.
Los Angeles is the nearest National Weather Service Forecast Office to the San Gabriel Mountains. It provides instrumental
measurements of precipitation, which began in 1877. Figure 2 presents the National Weather Service record plotted as annual
precipitation based on water-year. The mean of this record is 15.01 inches of precipitation annually. Periodic variation has been
partially removed by plotting the annual data as a 5-year running average, which highlights the cyclical, decadal pattern of Southern
California rainfall. A linear secular trend was also plotted, which suggests a long-term decline in annual precipitation of about 0.6-inch
over the period of record.
Figure 3 presents the instrumental record of Figure 2 expressed as cumulative departure from the mean in percent, based on the mean
for the period of 15.01 inches. Lynch (1931, 1948) and Thomas (1962) presented similar plots based on means computed for shorter
periods of record. The secular trend has a pronounced negative slope for this record reflecting precipitation deficits accumulated since
the 1940’s.

Lynch (1931) was able to extend the cumulative departure record back to the late 1770’s using “crop indices,” derived from records of
the Spanish Missions, as proxy for precipitation data. In reference to such records, Lynch (1931, p. 7) states: “For the purpose of
showing the fluctuations in rainfall, they are not much less valuable than the more recent rainfall measurements and show with great
fidelity the weather conditions at the missions.” In the absence of irrigation, crop yields are effectively a function of rainfall amount.
Crop indices were calculated using the relation: I = R/N, where, I = Crop Index for a given year, R = Harvest/Planting, and N =
Normal (i.e., mean) harvest return for the period 1800 to 1832.
To construct the cumulative departure plot of Figure 4 for the period 1770 to 2003 we combined Lynch’s (1931, Appendix D) crop
index data set and the later instrumental precipitation record. The mean utilized in Figure 4 (15.2 inches) was obtained by
extrapolating the linear secular trend from Figure 2 back to 1770, and taking its mean over the period 1770 to 2003. Cumulative
departure from this mean was then recalculated to form the curve shown in Figure 4.
If the longer cumulative departure record formed in this way is correct, it includes features not revealed in the shorter record use to
create Figure 3. For example, the cumulative departure from the mean has been positive in only about 75 years of the 233 years of
the record (approximately 32% of the time), and has been mainly in deficit since about 1845 despite significant positive fluctuations
in the 1890’s, 1910’s, and 1940’s.

Historical water level records for wells shown on Figure 5 were obtained from the County of Los Angeles Public Works Department
and the California Department of Water Resources. Three wells, 3030F, 4198A and 4198G, have historical records extending back to
the early 1930’s. Well 3030F, also known as the Key Well, possesses the most complete record (Figure 6). Wells 4198A and 4198G
are located approximately on surface contour with the Key Well. Records of these wells span the same length of time as the Key
Well, but contain data gaps, which are evident upon close inspection of the hydrographs. Nevertheless, the three well records are
highly similar and measure virtually the same water table contour. Records of many other wells in the area are more fragmentary but
when plotted against the Key Well are correlative with it over the fragmentary intervals.
It is evident from the well records that the historical high groundwater level in the vicinity, approximately elevation 325 feet, was
reached in the 1940’s, and corresponds in time to the region’s maximum historic cumulative departure from the mean precipitation,
which is also shown on Figure 5. The correlation of groundwater and precipitation fluctuations apparent in Figure 5 and the
similarity of negative secular trends (e.g., Figures 4 and 6) suggest that groundwater trends are most strongly reflecting a climatic
influence.

Average annual groundwater elevation for the Key Well is shown in Figure 7 and was derived from the periodic record of the well
shown in Figure 6. The annualized curve of Figure 7 was then analyzed by the water table fluctuation method described in Healy
(2002) to estimate net recharge. The change in storage was calculated as specific yield times the net change in head over each year,
assuming net surface-water and groundwater flow were zero, and evapotranspiration to be very small compared to specific yield.
Specific yield was estimate to be approximately 0.30 based on measurements of porosity in thin section and calculations from field
density tests; field moisture of the unsaturated zone was estimated to be less than 4% based on laboratory testing. The approximate
cumulative change in storage is also shown on Figure 7 and constitutes a net loss of 12 feet of storage over the period of record.
Comparison of the long-term rainfall (climate) record from Los Angeles with the well records illustrates a correspondence in cycles of
relative wet and dry and similarity of secular trends for the respective data sets. Since at least the 1940’s overall rainfall and
groundwater level trends appear to be in decline.
Analysis of liquefaction and seismic slope stability under assumed historically high groundwater conditions is clearly the most
conservative approach with respect to selecting design parameters. A more realistic design basis for the near-term is suggested by the
latest trends observed in Figure 6. Groundwater levels have not been higher that elevation 270 feet in about the last 20 years, and
have not exceeded 300 feet in about the last 35 years. In addition, the lowest groundwater levels were recorded in these intervals, and
at present are at the historically low level (<200 feet). It should also be considered that the higher groundwater levels are transient
and represent restricted time windows in which damaging ground motions might occur.
Relative to formulating parameters for liquefaction analysis and slope stability based on the above discussion, the following options
are under consideration by the City of Irwindale, various consultants, and interested professionals. The trend-line through the entire
data set in Figure 6 shows a steep decline in average groundwater level with time, but the trend is heavily influenced by high
groundwater levels between about 1935 and 1950. This relatively steep slope provides a poor fit for the time period since 1950, and
the fit also suggests an average groundwater elevation that appears artificially low for the present time (2003). Accordingly, a second
fit from 1950 to the present was performed, which appears to provide an unbiased fit for that time interval.
For static analysis, an appropriate groundwater level could be the sum of the 1950-2003 trend and the local deviations from this trend
associated with high rainfall years. At the present time (2003), this corresponds to a groundwater elevation of approximately 270 feet,
as shown in Figure 6; it is possible that this groundwater elevation will decrease with time in accordance with the long-term trend.
However, for design purposes a groundwater table of 270 feet could be used for static slope stability analyses. This is appropriate
because of the probability that another heavy rainfall year could occur in the very near future, before any additional negative trend in
average ground water elevation could occur.

For seismic analysis, an appropriate option could be the current average groundwater level as represented by the 1950-2003 trend line
at year 2003. It is believed that the probability of a major earthquake occurring coincident with a major rainfall excursion is
sufficiently small that the slope performance modeled by such a sequence would have a return period of occurrence that would be
significantly greater than that of the ground motion. The intent of using the average ground water level so defined is to avoid the
introduction of this bias in the seismic slope performance calculations. At the present time (2003), the average groundwater elevation
is approximately 225 feet. As noted above, it is possible that this groundwater elevation could decrease in the future, following the
1950-2003 trend. However, this option assumes no further decrease in the average, which appears to be conservative.

REFERENCES
Healy, R.W. and P.G. Cook (2002), Using groundwater levels to estimate recharge, Hydrogeology Journal, Vol. 10, No. 1, pp. 91-109.
Lynch, H.B. (1931), Rainfall and stream runoff in Southern California since 1769, consulting report for Metropolitan Water District of
Southern California, Los Angeles, 31p.

Lynch, H.B. (1948), Pacific Coast rainfall – Wide fluctuations in hundred years, Western Construction News, July 1948, pp. 76-80.
Thomas, H.E. (1962), The Meteorological Phenomenon of Drought in the Southwest, U.S. Geological Survey Professional Paper 372-A. 43p.

Figure 2 - Annual Precipitation in Los Angeles with 5 Year Running Average and Secular Trend
45

40

35

Precipitation (inches)

30

25

20

15

10

5
Slope of Trend Line = -0.0084
0
1870

1880

1890

1900

1910

1920

1930

1940

1950

1960

1970

1980

1990

2000

Year

Annual Precipitation

5 Year Running Average of Annual Precipitation

Secular Trend of Annual Precipitation

2010

Figure 3 - Precipitation in Los Angeles Expressed as Cumulative Departure from the Mean in Percent
with Secular Trend for the Instrumental Record
400

Rainfall Departure from the Mean in Percent

300

200

100

0

-100

-200
Mean = 15.01 inches
Slope of Trend Line = -1.9036
-300
1870

1880

1890

1900

1910

1920

1930

1940
Year

1950

1960

1970

1980

1990

2000

2010

Figure 4 - Precipitation in Los Angeles Expressed as Cumulative Departure fromthe Mean in Percent with Secular
Trend Derived fromCrop Yields and Precipitation Data
300

Cumulative Departure fromthe Mean in Percent

200

100

0

-100

-200
Mean = 15.2 inches
Slope of Trend Line = -1.0643

-300

-400
1760 1770 1780 1790 1800 1810 1820 1830 1840 1850 1860 1870 1880 1890 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 2010
Year

Based on Lynch's (1931, AppendixD) crop indexdata set

Based on Los Angeles precipitation data set

-

Secular Trend of Cumulative Departure fromthe Mean

350

300

300

200

250

100

200

0

150

-100

100

-200

50

-300

0
1930

1940

1950

1960

1970

1980

1990

2000

Date
Well 3030F

Well 4198A

Well 4198G

Precipitation Departure from the Mean

-400
2010

Cumulative Departure from the Mean in Percent

Groundwater Elevation (feet amsl)

Figure 5 - Combined Well Hydrographs for Wells 3030F, 4198A, and 4198G with Precipitation
Expressed as Cumulative Departure from the Mean in Percent from Figure 4

Figure 6 - Periodic Groundwater Elevation with Secular Trend Measured in Well 3030F (Key Well)

350

Groundwater Elevation (feet above mean sea level)

330
310

Groundwater Elevation for Static Slope Stability Analyses

290
270
250
Slope of Trend Line for Entire Data Set = -1.2233

230
210
Slope of Trend Line 1950 to Present = -0.5873

190

Groundwater Elevation for Dynamic Slope Stability Analyses
170
150
1930

1940

1950

1960

1970

1980

1990

2000

Year
Well 3030F Groundwater Elevation

-

Secular Trend of Groundwater Elevation

Secular Trend Since 1950

2010

330.0

120

310.0

100

290.0

Slope of Trend Line = -0.9084

80

270.0

60

250.0

40

230.0

20

210.0

0

190.0

-20
Slope of Trend Line = -0.2685

170.0
150.0
1930

-40

1940

1950

1960

1970

1980

1990

2000

Year
Annual Average Groundwater Elevation

Approximate Cumulative Change in Water Storage

Secular Trend of Annual Average Groundwater Elevation

Secular Trend of Cumulative Change in Water Storage

-60
2010

Approximate Cumulative Change in Water Storage
(feet)

Groundwater Elevation (feet above mean sea level)

Figure 7 - Annual Average Groundwater Elevation and Approximate Cumulative Change in Storage with
Secular Trends as Measured in Well 3030F (Key Well)


Slide 2

GEOLOGICAL SOCIETY OF AMERICA 2003 ANNUAL MEETING
(ABSTRACT NO. 65672)
COMPARISON OF CLIMATIC TRENDS AND WELL RECORDS IN SOUTHERN
CALIFORNIA: IMPLICATIONS FOR GEOTECHNICAL AND HYDROGEOLOGICAL
PRACTICE
McMILLAN, Kent and VINCENT, Mark, Geo-Logic Associates, 1360 Valley Vista Dr., Diamond Bar, CA
91765, [email protected]
Geo-Logic Associates

ABSTRACT

Comparison of climate (rainfall) records from Los Angeles with water well records beginning in 1930 from the
nearby San Gabriel Valley, shows a correspondence in cycles of relative wet and dry on a decadal scale, as well
as similar longer term trends. The climate record of Los Angeles spans the termination of the Little Ice Age (+
1890 A.D.), and plots of cumulative departure from the long-term mean of the record suggest drying conditions
over the last century. With respect to the present day, these plots also suggest the last significant wet period from
the point of view of increasing groundwater recharge was the 1940’s. The well records noted above establish the
historic high groundwater levels in the area similarly occurred in the decade. Since then, however, rainfall and
groundwater level trends appear to be in decline. This raises the question of whether historic high groundwater
is an appropriate criterion in liquefaction analysis in all cases, since its use presumes the probable return of
historic conditions over the design life of a facility. Alternatively, the criterion might be based on trend analysis
of groundwater levels where there is also corresponding climatic data. Recharge trends are also analyzed by the
water-table fluctuation method.

This presentation is an outgrowth of recent work performed for the City of Irwindale, California, relating to the geotechnical
characterization of alluvial fan deposits derived from the San Gabriel River Canyon (Figure 1). We are indebted to Mr. Kwok Tam,
Director of Public Works, City of Irwindale, for his invaluable assistance, and suppor of this project. Sand and gravel mining of these
deposits to produce mineral aggregates is a major commercial activity in the City. Mining operations have excavated large, deep, steep
sided open pits, some of which have reached their permitted depths and are inactive, while others are nearing completion depths.
Reclamation of these pits is therefore of significant interest to the City, which is proceeding to assess the geotechnical and
hydrogeologic implications of post-mining reclamation.
The alluvial fan deposits constitute a significant unconfined and highly transmissive aquifer beneath the San Gabriel Valley, which
drains a major portion of the San Gabriel Mountains watershed (Figure 1). Groundwater lakes are a feature of most mining sites.
Groundwater levels fluctuate seasonally and with wet and dry climate cycles, and may vary locally depending on pumping from, or
recharge to the unconfined aquifer. Currently, groundwater levels observed in the pits are at or near historic low elevations.
All mining sites are included in seismic hazard zones defined by the California Geological Survey, because of concern for liquefaction
and seismic slope stability. As a result of this, an investigation of historical groundwater trends was conducted, seeking to identify the
historical high for the area. In the course of that analysis more general relationships between groundwater and climate trends became
apparent that are the subject of the following discussion.
Figure 1 is a Landsat, computer generated, color enhanced image of the San Gabriel Mountains watershed viewed from the west (note
the 2X vertical exaggeration). The image illustrates the spatial proximity of the mountain front, the adjacent San Gabriel Valley, and the
Los Angeles area, from which the climate records that form the basis of Figures 2, -3 and -4 originate. Also shown are the locations of
Wells 3030F, 4198A and 4198G near the mouth of San Gabriel Canyon.
Los Angeles is the nearest National Weather Service Forecast Office to the San Gabriel Mountains. It provides instrumental
measurements of precipitation, which began in 1877. Figure 2 presents the National Weather Service record plotted as annual
precipitation based on water-year. The mean of this record is 15.01 inches of precipitation annually. Periodic variation has been
partially removed by plotting the annual data as a 5-year running average, which highlights the cyclical, decadal pattern of Southern
California rainfall. A linear secular trend was also plotted, which suggests a long-term decline in annual precipitation of about 0.6-inch
over the period of record.
Figure 3 presents the instrumental record of Figure 2 expressed as cumulative departure from the mean in percent, based on the mean
for the period of 15.01 inches. Lynch (1931, 1948) and Thomas (1962) presented similar plots based on means computed for shorter
periods of record. The secular trend has a pronounced negative slope for this record reflecting precipitation deficits accumulated since
the 1940’s.

Lynch (1931) was able to extend the cumulative departure record back to the late 1770’s using “crop indices,” derived from records of
the Spanish Missions, as proxy for precipitation data. In reference to such records, Lynch (1931, p. 7) states: “For the purpose of
showing the fluctuations in rainfall, they are not much less valuable than the more recent rainfall measurements and show with great
fidelity the weather conditions at the missions.” In the absence of irrigation, crop yields are effectively a function of rainfall amount.
Crop indices were calculated using the relation: I = R/N, where, I = Crop Index for a given year, R = Harvest/Planting, and N =
Normal (i.e., mean) harvest return for the period 1800 to 1832.
To construct the cumulative departure plot of Figure 4 for the period 1770 to 2003 we combined Lynch’s (1931, Appendix D) crop
index data set and the later instrumental precipitation record. The mean utilized in Figure 4 (15.2 inches) was obtained by
extrapolating the linear secular trend from Figure 2 back to 1770, and taking its mean over the period 1770 to 2003. Cumulative
departure from this mean was then recalculated to form the curve shown in Figure 4.
If the longer cumulative departure record formed in this way is correct, it includes features not revealed in the shorter record use to
create Figure 3. For example, the cumulative departure from the mean has been positive in only about 75 years of the 233 years of
the record (approximately 32% of the time), and has been mainly in deficit since about 1845 despite significant positive fluctuations
in the 1890’s, 1910’s, and 1940’s.

Historical water level records for wells shown on Figure 5 were obtained from the County of Los Angeles Public Works Department
and the California Department of Water Resources. Three wells, 3030F, 4198A and 4198G, have historical records extending back to
the early 1930’s. Well 3030F, also known as the Key Well, possesses the most complete record (Figure 6). Wells 4198A and 4198G
are located approximately on surface contour with the Key Well. Records of these wells span the same length of time as the Key
Well, but contain data gaps, which are evident upon close inspection of the hydrographs. Nevertheless, the three well records are
highly similar and measure virtually the same water table contour. Records of many other wells in the area are more fragmentary but
when plotted against the Key Well are correlative with it over the fragmentary intervals.
It is evident from the well records that the historical high groundwater level in the vicinity, approximately elevation 325 feet, was
reached in the 1940’s, and corresponds in time to the region’s maximum historic cumulative departure from the mean precipitation,
which is also shown on Figure 5. The correlation of groundwater and precipitation fluctuations apparent in Figure 5 and the
similarity of negative secular trends (e.g., Figures 4 and 6) suggest that groundwater trends are most strongly reflecting a climatic
influence.

Average annual groundwater elevation for the Key Well is shown in Figure 7 and was derived from the periodic record of the well
shown in Figure 6. The annualized curve of Figure 7 was then analyzed by the water table fluctuation method described in Healy
(2002) to estimate net recharge. The change in storage was calculated as specific yield times the net change in head over each year,
assuming net surface-water and groundwater flow were zero, and evapotranspiration to be very small compared to specific yield.
Specific yield was estimate to be approximately 0.30 based on measurements of porosity in thin section and calculations from field
density tests; field moisture of the unsaturated zone was estimated to be less than 4% based on laboratory testing. The approximate
cumulative change in storage is also shown on Figure 7 and constitutes a net loss of 12 feet of storage over the period of record.
Comparison of the long-term rainfall (climate) record from Los Angeles with the well records illustrates a correspondence in cycles of
relative wet and dry and similarity of secular trends for the respective data sets. Since at least the 1940’s overall rainfall and
groundwater level trends appear to be in decline.
Analysis of liquefaction and seismic slope stability under assumed historically high groundwater conditions is clearly the most
conservative approach with respect to selecting design parameters. A more realistic design basis for the near-term is suggested by the
latest trends observed in Figure 6. Groundwater levels have not been higher that elevation 270 feet in about the last 20 years, and
have not exceeded 300 feet in about the last 35 years. In addition, the lowest groundwater levels were recorded in these intervals, and
at present are at the historically low level (<200 feet). It should also be considered that the higher groundwater levels are transient
and represent restricted time windows in which damaging ground motions might occur.
Relative to formulating parameters for liquefaction analysis and slope stability based on the above discussion, the following options
are under consideration by the City of Irwindale, various consultants, and interested professionals. The trend-line through the entire
data set in Figure 6 shows a steep decline in average groundwater level with time, but the trend is heavily influenced by high
groundwater levels between about 1935 and 1950. This relatively steep slope provides a poor fit for the time period since 1950, and
the fit also suggests an average groundwater elevation that appears artificially low for the present time (2003). Accordingly, a second
fit from 1950 to the present was performed, which appears to provide an unbiased fit for that time interval.
For static analysis, an appropriate groundwater level could be the sum of the 1950-2003 trend and the local deviations from this trend
associated with high rainfall years. At the present time (2003), this corresponds to a groundwater elevation of approximately 270 feet,
as shown in Figure 6; it is possible that this groundwater elevation will decrease with time in accordance with the long-term trend.
However, for design purposes a groundwater table of 270 feet could be used for static slope stability analyses. This is appropriate
because of the probability that another heavy rainfall year could occur in the very near future, before any additional negative trend in
average ground water elevation could occur.

For seismic analysis, an appropriate option could be the current average groundwater level as represented by the 1950-2003 trend line
at year 2003. It is believed that the probability of a major earthquake occurring coincident with a major rainfall excursion is
sufficiently small that the slope performance modeled by such a sequence would have a return period of occurrence that would be
significantly greater than that of the ground motion. The intent of using the average ground water level so defined is to avoid the
introduction of this bias in the seismic slope performance calculations. At the present time (2003), the average groundwater elevation
is approximately 225 feet. As noted above, it is possible that this groundwater elevation could decrease in the future, following the
1950-2003 trend. However, this option assumes no further decrease in the average, which appears to be conservative.

REFERENCES
Healy, R.W. and P.G. Cook (2002), Using groundwater levels to estimate recharge, Hydrogeology Journal, Vol. 10, No. 1, pp. 91-109.
Lynch, H.B. (1931), Rainfall and stream runoff in Southern California since 1769, consulting report for Metropolitan Water District of
Southern California, Los Angeles, 31p.

Lynch, H.B. (1948), Pacific Coast rainfall – Wide fluctuations in hundred years, Western Construction News, July 1948, pp. 76-80.
Thomas, H.E. (1962), The Meteorological Phenomenon of Drought in the Southwest, U.S. Geological Survey Professional Paper 372-A. 43p.

Figure 2 - Annual Precipitation in Los Angeles with 5 Year Running Average and Secular Trend
45

40

35

Precipitation (inches)

30

25

20

15

10

5
Slope of Trend Line = -0.0084
0
1870

1880

1890

1900

1910

1920

1930

1940

1950

1960

1970

1980

1990

2000

Year

Annual Precipitation

5 Year Running Average of Annual Precipitation

Secular Trend of Annual Precipitation

2010

Figure 3 - Precipitation in Los Angeles Expressed as Cumulative Departure from the Mean in Percent
with Secular Trend for the Instrumental Record
400

Rainfall Departure from the Mean in Percent

300

200

100

0

-100

-200
Mean = 15.01 inches
Slope of Trend Line = -1.9036
-300
1870

1880

1890

1900

1910

1920

1930

1940
Year

1950

1960

1970

1980

1990

2000

2010

Figure 4 - Precipitation in Los Angeles Expressed as Cumulative Departure fromthe Mean in Percent with Secular
Trend Derived fromCrop Yields and Precipitation Data
300

Cumulative Departure fromthe Mean in Percent

200

100

0

-100

-200
Mean = 15.2 inches
Slope of Trend Line = -1.0643

-300

-400
1760 1770 1780 1790 1800 1810 1820 1830 1840 1850 1860 1870 1880 1890 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 2010
Year

Based on Lynch's (1931, AppendixD) crop indexdata set

Based on Los Angeles precipitation data set

-

Secular Trend of Cumulative Departure fromthe Mean

350

300

300

200

250

100

200

0

150

-100

100

-200

50

-300

0
1930

1940

1950

1960

1970

1980

1990

2000

Date
Well 3030F

Well 4198A

Well 4198G

Precipitation Departure from the Mean

-400
2010

Cumulative Departure from the Mean in Percent

Groundwater Elevation (feet amsl)

Figure 5 - Combined Well Hydrographs for Wells 3030F, 4198A, and 4198G with Precipitation
Expressed as Cumulative Departure from the Mean in Percent from Figure 4

Figure 6 - Periodic Groundwater Elevation with Secular Trend Measured in Well 3030F (Key Well)

350

Groundwater Elevation (feet above mean sea level)

330
310

Groundwater Elevation for Static Slope Stability Analyses

290
270
250
Slope of Trend Line for Entire Data Set = -1.2233

230
210
Slope of Trend Line 1950 to Present = -0.5873

190

Groundwater Elevation for Dynamic Slope Stability Analyses
170
150
1930

1940

1950

1960

1970

1980

1990

2000

Year
Well 3030F Groundwater Elevation

-

Secular Trend of Groundwater Elevation

Secular Trend Since 1950

2010

330.0

120

310.0

100

290.0

Slope of Trend Line = -0.9084

80

270.0

60

250.0

40

230.0

20

210.0

0

190.0

-20
Slope of Trend Line = -0.2685

170.0
150.0
1930

-40

1940

1950

1960

1970

1980

1990

2000

Year
Annual Average Groundwater Elevation

Approximate Cumulative Change in Water Storage

Secular Trend of Annual Average Groundwater Elevation

Secular Trend of Cumulative Change in Water Storage

-60
2010

Approximate Cumulative Change in Water Storage
(feet)

Groundwater Elevation (feet above mean sea level)

Figure 7 - Annual Average Groundwater Elevation and Approximate Cumulative Change in Storage with
Secular Trends as Measured in Well 3030F (Key Well)


Slide 3

GEOLOGICAL SOCIETY OF AMERICA 2003 ANNUAL MEETING
(ABSTRACT NO. 65672)
COMPARISON OF CLIMATIC TRENDS AND WELL RECORDS IN SOUTHERN
CALIFORNIA: IMPLICATIONS FOR GEOTECHNICAL AND HYDROGEOLOGICAL
PRACTICE
McMILLAN, Kent and VINCENT, Mark, Geo-Logic Associates, 1360 Valley Vista Dr., Diamond Bar, CA
91765, [email protected]
Geo-Logic Associates

ABSTRACT

Comparison of climate (rainfall) records from Los Angeles with water well records beginning in 1930 from the
nearby San Gabriel Valley, shows a correspondence in cycles of relative wet and dry on a decadal scale, as well
as similar longer term trends. The climate record of Los Angeles spans the termination of the Little Ice Age (+
1890 A.D.), and plots of cumulative departure from the long-term mean of the record suggest drying conditions
over the last century. With respect to the present day, these plots also suggest the last significant wet period from
the point of view of increasing groundwater recharge was the 1940’s. The well records noted above establish the
historic high groundwater levels in the area similarly occurred in the decade. Since then, however, rainfall and
groundwater level trends appear to be in decline. This raises the question of whether historic high groundwater
is an appropriate criterion in liquefaction analysis in all cases, since its use presumes the probable return of
historic conditions over the design life of a facility. Alternatively, the criterion might be based on trend analysis
of groundwater levels where there is also corresponding climatic data. Recharge trends are also analyzed by the
water-table fluctuation method.

This presentation is an outgrowth of recent work performed for the City of Irwindale, California, relating to the geotechnical
characterization of alluvial fan deposits derived from the San Gabriel River Canyon (Figure 1). We are indebted to Mr. Kwok Tam,
Director of Public Works, City of Irwindale, for his invaluable assistance, and suppor of this project. Sand and gravel mining of these
deposits to produce mineral aggregates is a major commercial activity in the City. Mining operations have excavated large, deep, steep
sided open pits, some of which have reached their permitted depths and are inactive, while others are nearing completion depths.
Reclamation of these pits is therefore of significant interest to the City, which is proceeding to assess the geotechnical and
hydrogeologic implications of post-mining reclamation.
The alluvial fan deposits constitute a significant unconfined and highly transmissive aquifer beneath the San Gabriel Valley, which
drains a major portion of the San Gabriel Mountains watershed (Figure 1). Groundwater lakes are a feature of most mining sites.
Groundwater levels fluctuate seasonally and with wet and dry climate cycles, and may vary locally depending on pumping from, or
recharge to the unconfined aquifer. Currently, groundwater levels observed in the pits are at or near historic low elevations.
All mining sites are included in seismic hazard zones defined by the California Geological Survey, because of concern for liquefaction
and seismic slope stability. As a result of this, an investigation of historical groundwater trends was conducted, seeking to identify the
historical high for the area. In the course of that analysis more general relationships between groundwater and climate trends became
apparent that are the subject of the following discussion.
Figure 1 is a Landsat, computer generated, color enhanced image of the San Gabriel Mountains watershed viewed from the west (note
the 2X vertical exaggeration). The image illustrates the spatial proximity of the mountain front, the adjacent San Gabriel Valley, and the
Los Angeles area, from which the climate records that form the basis of Figures 2, -3 and -4 originate. Also shown are the locations of
Wells 3030F, 4198A and 4198G near the mouth of San Gabriel Canyon.
Los Angeles is the nearest National Weather Service Forecast Office to the San Gabriel Mountains. It provides instrumental
measurements of precipitation, which began in 1877. Figure 2 presents the National Weather Service record plotted as annual
precipitation based on water-year. The mean of this record is 15.01 inches of precipitation annually. Periodic variation has been
partially removed by plotting the annual data as a 5-year running average, which highlights the cyclical, decadal pattern of Southern
California rainfall. A linear secular trend was also plotted, which suggests a long-term decline in annual precipitation of about 0.6-inch
over the period of record.
Figure 3 presents the instrumental record of Figure 2 expressed as cumulative departure from the mean in percent, based on the mean
for the period of 15.01 inches. Lynch (1931, 1948) and Thomas (1962) presented similar plots based on means computed for shorter
periods of record. The secular trend has a pronounced negative slope for this record reflecting precipitation deficits accumulated since
the 1940’s.

Lynch (1931) was able to extend the cumulative departure record back to the late 1770’s using “crop indices,” derived from records of
the Spanish Missions, as proxy for precipitation data. In reference to such records, Lynch (1931, p. 7) states: “For the purpose of
showing the fluctuations in rainfall, they are not much less valuable than the more recent rainfall measurements and show with great
fidelity the weather conditions at the missions.” In the absence of irrigation, crop yields are effectively a function of rainfall amount.
Crop indices were calculated using the relation: I = R/N, where, I = Crop Index for a given year, R = Harvest/Planting, and N =
Normal (i.e., mean) harvest return for the period 1800 to 1832.
To construct the cumulative departure plot of Figure 4 for the period 1770 to 2003 we combined Lynch’s (1931, Appendix D) crop
index data set and the later instrumental precipitation record. The mean utilized in Figure 4 (15.2 inches) was obtained by
extrapolating the linear secular trend from Figure 2 back to 1770, and taking its mean over the period 1770 to 2003. Cumulative
departure from this mean was then recalculated to form the curve shown in Figure 4.
If the longer cumulative departure record formed in this way is correct, it includes features not revealed in the shorter record use to
create Figure 3. For example, the cumulative departure from the mean has been positive in only about 75 years of the 233 years of
the record (approximately 32% of the time), and has been mainly in deficit since about 1845 despite significant positive fluctuations
in the 1890’s, 1910’s, and 1940’s.

Historical water level records for wells shown on Figure 5 were obtained from the County of Los Angeles Public Works Department
and the California Department of Water Resources. Three wells, 3030F, 4198A and 4198G, have historical records extending back to
the early 1930’s. Well 3030F, also known as the Key Well, possesses the most complete record (Figure 6). Wells 4198A and 4198G
are located approximately on surface contour with the Key Well. Records of these wells span the same length of time as the Key
Well, but contain data gaps, which are evident upon close inspection of the hydrographs. Nevertheless, the three well records are
highly similar and measure virtually the same water table contour. Records of many other wells in the area are more fragmentary but
when plotted against the Key Well are correlative with it over the fragmentary intervals.
It is evident from the well records that the historical high groundwater level in the vicinity, approximately elevation 325 feet, was
reached in the 1940’s, and corresponds in time to the region’s maximum historic cumulative departure from the mean precipitation,
which is also shown on Figure 5. The correlation of groundwater and precipitation fluctuations apparent in Figure 5 and the
similarity of negative secular trends (e.g., Figures 4 and 6) suggest that groundwater trends are most strongly reflecting a climatic
influence.

Average annual groundwater elevation for the Key Well is shown in Figure 7 and was derived from the periodic record of the well
shown in Figure 6. The annualized curve of Figure 7 was then analyzed by the water table fluctuation method described in Healy
(2002) to estimate net recharge. The change in storage was calculated as specific yield times the net change in head over each year,
assuming net surface-water and groundwater flow were zero, and evapotranspiration to be very small compared to specific yield.
Specific yield was estimate to be approximately 0.30 based on measurements of porosity in thin section and calculations from field
density tests; field moisture of the unsaturated zone was estimated to be less than 4% based on laboratory testing. The approximate
cumulative change in storage is also shown on Figure 7 and constitutes a net loss of 12 feet of storage over the period of record.
Comparison of the long-term rainfall (climate) record from Los Angeles with the well records illustrates a correspondence in cycles of
relative wet and dry and similarity of secular trends for the respective data sets. Since at least the 1940’s overall rainfall and
groundwater level trends appear to be in decline.
Analysis of liquefaction and seismic slope stability under assumed historically high groundwater conditions is clearly the most
conservative approach with respect to selecting design parameters. A more realistic design basis for the near-term is suggested by the
latest trends observed in Figure 6. Groundwater levels have not been higher that elevation 270 feet in about the last 20 years, and
have not exceeded 300 feet in about the last 35 years. In addition, the lowest groundwater levels were recorded in these intervals, and
at present are at the historically low level (<200 feet). It should also be considered that the higher groundwater levels are transient
and represent restricted time windows in which damaging ground motions might occur.
Relative to formulating parameters for liquefaction analysis and slope stability based on the above discussion, the following options
are under consideration by the City of Irwindale, various consultants, and interested professionals. The trend-line through the entire
data set in Figure 6 shows a steep decline in average groundwater level with time, but the trend is heavily influenced by high
groundwater levels between about 1935 and 1950. This relatively steep slope provides a poor fit for the time period since 1950, and
the fit also suggests an average groundwater elevation that appears artificially low for the present time (2003). Accordingly, a second
fit from 1950 to the present was performed, which appears to provide an unbiased fit for that time interval.
For static analysis, an appropriate groundwater level could be the sum of the 1950-2003 trend and the local deviations from this trend
associated with high rainfall years. At the present time (2003), this corresponds to a groundwater elevation of approximately 270 feet,
as shown in Figure 6; it is possible that this groundwater elevation will decrease with time in accordance with the long-term trend.
However, for design purposes a groundwater table of 270 feet could be used for static slope stability analyses. This is appropriate
because of the probability that another heavy rainfall year could occur in the very near future, before any additional negative trend in
average ground water elevation could occur.

For seismic analysis, an appropriate option could be the current average groundwater level as represented by the 1950-2003 trend line
at year 2003. It is believed that the probability of a major earthquake occurring coincident with a major rainfall excursion is
sufficiently small that the slope performance modeled by such a sequence would have a return period of occurrence that would be
significantly greater than that of the ground motion. The intent of using the average ground water level so defined is to avoid the
introduction of this bias in the seismic slope performance calculations. At the present time (2003), the average groundwater elevation
is approximately 225 feet. As noted above, it is possible that this groundwater elevation could decrease in the future, following the
1950-2003 trend. However, this option assumes no further decrease in the average, which appears to be conservative.

REFERENCES
Healy, R.W. and P.G. Cook (2002), Using groundwater levels to estimate recharge, Hydrogeology Journal, Vol. 10, No. 1, pp. 91-109.
Lynch, H.B. (1931), Rainfall and stream runoff in Southern California since 1769, consulting report for Metropolitan Water District of
Southern California, Los Angeles, 31p.

Lynch, H.B. (1948), Pacific Coast rainfall – Wide fluctuations in hundred years, Western Construction News, July 1948, pp. 76-80.
Thomas, H.E. (1962), The Meteorological Phenomenon of Drought in the Southwest, U.S. Geological Survey Professional Paper 372-A. 43p.

Figure 2 - Annual Precipitation in Los Angeles with 5 Year Running Average and Secular Trend
45

40

35

Precipitation (inches)

30

25

20

15

10

5
Slope of Trend Line = -0.0084
0
1870

1880

1890

1900

1910

1920

1930

1940

1950

1960

1970

1980

1990

2000

Year

Annual Precipitation

5 Year Running Average of Annual Precipitation

Secular Trend of Annual Precipitation

2010

Figure 3 - Precipitation in Los Angeles Expressed as Cumulative Departure from the Mean in Percent
with Secular Trend for the Instrumental Record
400

Rainfall Departure from the Mean in Percent

300

200

100

0

-100

-200
Mean = 15.01 inches
Slope of Trend Line = -1.9036
-300
1870

1880

1890

1900

1910

1920

1930

1940
Year

1950

1960

1970

1980

1990

2000

2010

Figure 4 - Precipitation in Los Angeles Expressed as Cumulative Departure fromthe Mean in Percent with Secular
Trend Derived fromCrop Yields and Precipitation Data
300

Cumulative Departure fromthe Mean in Percent

200

100

0

-100

-200
Mean = 15.2 inches
Slope of Trend Line = -1.0643

-300

-400
1760 1770 1780 1790 1800 1810 1820 1830 1840 1850 1860 1870 1880 1890 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 2010
Year

Based on Lynch's (1931, AppendixD) crop indexdata set

Based on Los Angeles precipitation data set

-

Secular Trend of Cumulative Departure fromthe Mean

350

300

300

200

250

100

200

0

150

-100

100

-200

50

-300

0
1930

1940

1950

1960

1970

1980

1990

2000

Date
Well 3030F

Well 4198A

Well 4198G

Precipitation Departure from the Mean

-400
2010

Cumulative Departure from the Mean in Percent

Groundwater Elevation (feet amsl)

Figure 5 - Combined Well Hydrographs for Wells 3030F, 4198A, and 4198G with Precipitation
Expressed as Cumulative Departure from the Mean in Percent from Figure 4

Figure 6 - Periodic Groundwater Elevation with Secular Trend Measured in Well 3030F (Key Well)

350

Groundwater Elevation (feet above mean sea level)

330
310

Groundwater Elevation for Static Slope Stability Analyses

290
270
250
Slope of Trend Line for Entire Data Set = -1.2233

230
210
Slope of Trend Line 1950 to Present = -0.5873

190

Groundwater Elevation for Dynamic Slope Stability Analyses
170
150
1930

1940

1950

1960

1970

1980

1990

2000

Year
Well 3030F Groundwater Elevation

-

Secular Trend of Groundwater Elevation

Secular Trend Since 1950

2010

330.0

120

310.0

100

290.0

Slope of Trend Line = -0.9084

80

270.0

60

250.0

40

230.0

20

210.0

0

190.0

-20
Slope of Trend Line = -0.2685

170.0
150.0
1930

-40

1940

1950

1960

1970

1980

1990

2000

Year
Annual Average Groundwater Elevation

Approximate Cumulative Change in Water Storage

Secular Trend of Annual Average Groundwater Elevation

Secular Trend of Cumulative Change in Water Storage

-60
2010

Approximate Cumulative Change in Water Storage
(feet)

Groundwater Elevation (feet above mean sea level)

Figure 7 - Annual Average Groundwater Elevation and Approximate Cumulative Change in Storage with
Secular Trends as Measured in Well 3030F (Key Well)


Slide 4

GEOLOGICAL SOCIETY OF AMERICA 2003 ANNUAL MEETING
(ABSTRACT NO. 65672)
COMPARISON OF CLIMATIC TRENDS AND WELL RECORDS IN SOUTHERN
CALIFORNIA: IMPLICATIONS FOR GEOTECHNICAL AND HYDROGEOLOGICAL
PRACTICE
McMILLAN, Kent and VINCENT, Mark, Geo-Logic Associates, 1360 Valley Vista Dr., Diamond Bar, CA
91765, [email protected]
Geo-Logic Associates

ABSTRACT

Comparison of climate (rainfall) records from Los Angeles with water well records beginning in 1930 from the
nearby San Gabriel Valley, shows a correspondence in cycles of relative wet and dry on a decadal scale, as well
as similar longer term trends. The climate record of Los Angeles spans the termination of the Little Ice Age (+
1890 A.D.), and plots of cumulative departure from the long-term mean of the record suggest drying conditions
over the last century. With respect to the present day, these plots also suggest the last significant wet period from
the point of view of increasing groundwater recharge was the 1940’s. The well records noted above establish the
historic high groundwater levels in the area similarly occurred in the decade. Since then, however, rainfall and
groundwater level trends appear to be in decline. This raises the question of whether historic high groundwater
is an appropriate criterion in liquefaction analysis in all cases, since its use presumes the probable return of
historic conditions over the design life of a facility. Alternatively, the criterion might be based on trend analysis
of groundwater levels where there is also corresponding climatic data. Recharge trends are also analyzed by the
water-table fluctuation method.

This presentation is an outgrowth of recent work performed for the City of Irwindale, California, relating to the geotechnical
characterization of alluvial fan deposits derived from the San Gabriel River Canyon (Figure 1). We are indebted to Mr. Kwok Tam,
Director of Public Works, City of Irwindale, for his invaluable assistance, and suppor of this project. Sand and gravel mining of these
deposits to produce mineral aggregates is a major commercial activity in the City. Mining operations have excavated large, deep, steep
sided open pits, some of which have reached their permitted depths and are inactive, while others are nearing completion depths.
Reclamation of these pits is therefore of significant interest to the City, which is proceeding to assess the geotechnical and
hydrogeologic implications of post-mining reclamation.
The alluvial fan deposits constitute a significant unconfined and highly transmissive aquifer beneath the San Gabriel Valley, which
drains a major portion of the San Gabriel Mountains watershed (Figure 1). Groundwater lakes are a feature of most mining sites.
Groundwater levels fluctuate seasonally and with wet and dry climate cycles, and may vary locally depending on pumping from, or
recharge to the unconfined aquifer. Currently, groundwater levels observed in the pits are at or near historic low elevations.
All mining sites are included in seismic hazard zones defined by the California Geological Survey, because of concern for liquefaction
and seismic slope stability. As a result of this, an investigation of historical groundwater trends was conducted, seeking to identify the
historical high for the area. In the course of that analysis more general relationships between groundwater and climate trends became
apparent that are the subject of the following discussion.
Figure 1 is a Landsat, computer generated, color enhanced image of the San Gabriel Mountains watershed viewed from the west (note
the 2X vertical exaggeration). The image illustrates the spatial proximity of the mountain front, the adjacent San Gabriel Valley, and the
Los Angeles area, from which the climate records that form the basis of Figures 2, -3 and -4 originate. Also shown are the locations of
Wells 3030F, 4198A and 4198G near the mouth of San Gabriel Canyon.
Los Angeles is the nearest National Weather Service Forecast Office to the San Gabriel Mountains. It provides instrumental
measurements of precipitation, which began in 1877. Figure 2 presents the National Weather Service record plotted as annual
precipitation based on water-year. The mean of this record is 15.01 inches of precipitation annually. Periodic variation has been
partially removed by plotting the annual data as a 5-year running average, which highlights the cyclical, decadal pattern of Southern
California rainfall. A linear secular trend was also plotted, which suggests a long-term decline in annual precipitation of about 0.6-inch
over the period of record.
Figure 3 presents the instrumental record of Figure 2 expressed as cumulative departure from the mean in percent, based on the mean
for the period of 15.01 inches. Lynch (1931, 1948) and Thomas (1962) presented similar plots based on means computed for shorter
periods of record. The secular trend has a pronounced negative slope for this record reflecting precipitation deficits accumulated since
the 1940’s.

Lynch (1931) was able to extend the cumulative departure record back to the late 1770’s using “crop indices,” derived from records of
the Spanish Missions, as proxy for precipitation data. In reference to such records, Lynch (1931, p. 7) states: “For the purpose of
showing the fluctuations in rainfall, they are not much less valuable than the more recent rainfall measurements and show with great
fidelity the weather conditions at the missions.” In the absence of irrigation, crop yields are effectively a function of rainfall amount.
Crop indices were calculated using the relation: I = R/N, where, I = Crop Index for a given year, R = Harvest/Planting, and N =
Normal (i.e., mean) harvest return for the period 1800 to 1832.
To construct the cumulative departure plot of Figure 4 for the period 1770 to 2003 we combined Lynch’s (1931, Appendix D) crop
index data set and the later instrumental precipitation record. The mean utilized in Figure 4 (15.2 inches) was obtained by
extrapolating the linear secular trend from Figure 2 back to 1770, and taking its mean over the period 1770 to 2003. Cumulative
departure from this mean was then recalculated to form the curve shown in Figure 4.
If the longer cumulative departure record formed in this way is correct, it includes features not revealed in the shorter record use to
create Figure 3. For example, the cumulative departure from the mean has been positive in only about 75 years of the 233 years of
the record (approximately 32% of the time), and has been mainly in deficit since about 1845 despite significant positive fluctuations
in the 1890’s, 1910’s, and 1940’s.

Historical water level records for wells shown on Figure 5 were obtained from the County of Los Angeles Public Works Department
and the California Department of Water Resources. Three wells, 3030F, 4198A and 4198G, have historical records extending back to
the early 1930’s. Well 3030F, also known as the Key Well, possesses the most complete record (Figure 6). Wells 4198A and 4198G
are located approximately on surface contour with the Key Well. Records of these wells span the same length of time as the Key
Well, but contain data gaps, which are evident upon close inspection of the hydrographs. Nevertheless, the three well records are
highly similar and measure virtually the same water table contour. Records of many other wells in the area are more fragmentary but
when plotted against the Key Well are correlative with it over the fragmentary intervals.
It is evident from the well records that the historical high groundwater level in the vicinity, approximately elevation 325 feet, was
reached in the 1940’s, and corresponds in time to the region’s maximum historic cumulative departure from the mean precipitation,
which is also shown on Figure 5. The correlation of groundwater and precipitation fluctuations apparent in Figure 5 and the
similarity of negative secular trends (e.g., Figures 4 and 6) suggest that groundwater trends are most strongly reflecting a climatic
influence.

Average annual groundwater elevation for the Key Well is shown in Figure 7 and was derived from the periodic record of the well
shown in Figure 6. The annualized curve of Figure 7 was then analyzed by the water table fluctuation method described in Healy
(2002) to estimate net recharge. The change in storage was calculated as specific yield times the net change in head over each year,
assuming net surface-water and groundwater flow were zero, and evapotranspiration to be very small compared to specific yield.
Specific yield was estimate to be approximately 0.30 based on measurements of porosity in thin section and calculations from field
density tests; field moisture of the unsaturated zone was estimated to be less than 4% based on laboratory testing. The approximate
cumulative change in storage is also shown on Figure 7 and constitutes a net loss of 12 feet of storage over the period of record.
Comparison of the long-term rainfall (climate) record from Los Angeles with the well records illustrates a correspondence in cycles of
relative wet and dry and similarity of secular trends for the respective data sets. Since at least the 1940’s overall rainfall and
groundwater level trends appear to be in decline.
Analysis of liquefaction and seismic slope stability under assumed historically high groundwater conditions is clearly the most
conservative approach with respect to selecting design parameters. A more realistic design basis for the near-term is suggested by the
latest trends observed in Figure 6. Groundwater levels have not been higher that elevation 270 feet in about the last 20 years, and
have not exceeded 300 feet in about the last 35 years. In addition, the lowest groundwater levels were recorded in these intervals, and
at present are at the historically low level (<200 feet). It should also be considered that the higher groundwater levels are transient
and represent restricted time windows in which damaging ground motions might occur.
Relative to formulating parameters for liquefaction analysis and slope stability based on the above discussion, the following options
are under consideration by the City of Irwindale, various consultants, and interested professionals. The trend-line through the entire
data set in Figure 6 shows a steep decline in average groundwater level with time, but the trend is heavily influenced by high
groundwater levels between about 1935 and 1950. This relatively steep slope provides a poor fit for the time period since 1950, and
the fit also suggests an average groundwater elevation that appears artificially low for the present time (2003). Accordingly, a second
fit from 1950 to the present was performed, which appears to provide an unbiased fit for that time interval.
For static analysis, an appropriate groundwater level could be the sum of the 1950-2003 trend and the local deviations from this trend
associated with high rainfall years. At the present time (2003), this corresponds to a groundwater elevation of approximately 270 feet,
as shown in Figure 6; it is possible that this groundwater elevation will decrease with time in accordance with the long-term trend.
However, for design purposes a groundwater table of 270 feet could be used for static slope stability analyses. This is appropriate
because of the probability that another heavy rainfall year could occur in the very near future, before any additional negative trend in
average ground water elevation could occur.

For seismic analysis, an appropriate option could be the current average groundwater level as represented by the 1950-2003 trend line
at year 2003. It is believed that the probability of a major earthquake occurring coincident with a major rainfall excursion is
sufficiently small that the slope performance modeled by such a sequence would have a return period of occurrence that would be
significantly greater than that of the ground motion. The intent of using the average ground water level so defined is to avoid the
introduction of this bias in the seismic slope performance calculations. At the present time (2003), the average groundwater elevation
is approximately 225 feet. As noted above, it is possible that this groundwater elevation could decrease in the future, following the
1950-2003 trend. However, this option assumes no further decrease in the average, which appears to be conservative.

REFERENCES
Healy, R.W. and P.G. Cook (2002), Using groundwater levels to estimate recharge, Hydrogeology Journal, Vol. 10, No. 1, pp. 91-109.
Lynch, H.B. (1931), Rainfall and stream runoff in Southern California since 1769, consulting report for Metropolitan Water District of
Southern California, Los Angeles, 31p.

Lynch, H.B. (1948), Pacific Coast rainfall – Wide fluctuations in hundred years, Western Construction News, July 1948, pp. 76-80.
Thomas, H.E. (1962), The Meteorological Phenomenon of Drought in the Southwest, U.S. Geological Survey Professional Paper 372-A. 43p.

Figure 2 - Annual Precipitation in Los Angeles with 5 Year Running Average and Secular Trend
45

40

35

Precipitation (inches)

30

25

20

15

10

5
Slope of Trend Line = -0.0084
0
1870

1880

1890

1900

1910

1920

1930

1940

1950

1960

1970

1980

1990

2000

Year

Annual Precipitation

5 Year Running Average of Annual Precipitation

Secular Trend of Annual Precipitation

2010

Figure 3 - Precipitation in Los Angeles Expressed as Cumulative Departure from the Mean in Percent
with Secular Trend for the Instrumental Record
400

Rainfall Departure from the Mean in Percent

300

200

100

0

-100

-200
Mean = 15.01 inches
Slope of Trend Line = -1.9036
-300
1870

1880

1890

1900

1910

1920

1930

1940
Year

1950

1960

1970

1980

1990

2000

2010

Figure 4 - Precipitation in Los Angeles Expressed as Cumulative Departure fromthe Mean in Percent with Secular
Trend Derived fromCrop Yields and Precipitation Data
300

Cumulative Departure fromthe Mean in Percent

200

100

0

-100

-200
Mean = 15.2 inches
Slope of Trend Line = -1.0643

-300

-400
1760 1770 1780 1790 1800 1810 1820 1830 1840 1850 1860 1870 1880 1890 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 2010
Year

Based on Lynch's (1931, AppendixD) crop indexdata set

Based on Los Angeles precipitation data set

-

Secular Trend of Cumulative Departure fromthe Mean

350

300

300

200

250

100

200

0

150

-100

100

-200

50

-300

0
1930

1940

1950

1960

1970

1980

1990

2000

Date
Well 3030F

Well 4198A

Well 4198G

Precipitation Departure from the Mean

-400
2010

Cumulative Departure from the Mean in Percent

Groundwater Elevation (feet amsl)

Figure 5 - Combined Well Hydrographs for Wells 3030F, 4198A, and 4198G with Precipitation
Expressed as Cumulative Departure from the Mean in Percent from Figure 4

Figure 6 - Periodic Groundwater Elevation with Secular Trend Measured in Well 3030F (Key Well)

350

Groundwater Elevation (feet above mean sea level)

330
310

Groundwater Elevation for Static Slope Stability Analyses

290
270
250
Slope of Trend Line for Entire Data Set = -1.2233

230
210
Slope of Trend Line 1950 to Present = -0.5873

190

Groundwater Elevation for Dynamic Slope Stability Analyses
170
150
1930

1940

1950

1960

1970

1980

1990

2000

Year
Well 3030F Groundwater Elevation

-

Secular Trend of Groundwater Elevation

Secular Trend Since 1950

2010

330.0

120

310.0

100

290.0

Slope of Trend Line = -0.9084

80

270.0

60

250.0

40

230.0

20

210.0

0

190.0

-20
Slope of Trend Line = -0.2685

170.0
150.0
1930

-40

1940

1950

1960

1970

1980

1990

2000

Year
Annual Average Groundwater Elevation

Approximate Cumulative Change in Water Storage

Secular Trend of Annual Average Groundwater Elevation

Secular Trend of Cumulative Change in Water Storage

-60
2010

Approximate Cumulative Change in Water Storage
(feet)

Groundwater Elevation (feet above mean sea level)

Figure 7 - Annual Average Groundwater Elevation and Approximate Cumulative Change in Storage with
Secular Trends as Measured in Well 3030F (Key Well)


Slide 5

GEOLOGICAL SOCIETY OF AMERICA 2003 ANNUAL MEETING
(ABSTRACT NO. 65672)
COMPARISON OF CLIMATIC TRENDS AND WELL RECORDS IN SOUTHERN
CALIFORNIA: IMPLICATIONS FOR GEOTECHNICAL AND HYDROGEOLOGICAL
PRACTICE
McMILLAN, Kent and VINCENT, Mark, Geo-Logic Associates, 1360 Valley Vista Dr., Diamond Bar, CA
91765, [email protected]
Geo-Logic Associates

ABSTRACT

Comparison of climate (rainfall) records from Los Angeles with water well records beginning in 1930 from the
nearby San Gabriel Valley, shows a correspondence in cycles of relative wet and dry on a decadal scale, as well
as similar longer term trends. The climate record of Los Angeles spans the termination of the Little Ice Age (+
1890 A.D.), and plots of cumulative departure from the long-term mean of the record suggest drying conditions
over the last century. With respect to the present day, these plots also suggest the last significant wet period from
the point of view of increasing groundwater recharge was the 1940’s. The well records noted above establish the
historic high groundwater levels in the area similarly occurred in the decade. Since then, however, rainfall and
groundwater level trends appear to be in decline. This raises the question of whether historic high groundwater
is an appropriate criterion in liquefaction analysis in all cases, since its use presumes the probable return of
historic conditions over the design life of a facility. Alternatively, the criterion might be based on trend analysis
of groundwater levels where there is also corresponding climatic data. Recharge trends are also analyzed by the
water-table fluctuation method.

This presentation is an outgrowth of recent work performed for the City of Irwindale, California, relating to the geotechnical
characterization of alluvial fan deposits derived from the San Gabriel River Canyon (Figure 1). We are indebted to Mr. Kwok Tam,
Director of Public Works, City of Irwindale, for his invaluable assistance, and suppor of this project. Sand and gravel mining of these
deposits to produce mineral aggregates is a major commercial activity in the City. Mining operations have excavated large, deep, steep
sided open pits, some of which have reached their permitted depths and are inactive, while others are nearing completion depths.
Reclamation of these pits is therefore of significant interest to the City, which is proceeding to assess the geotechnical and
hydrogeologic implications of post-mining reclamation.
The alluvial fan deposits constitute a significant unconfined and highly transmissive aquifer beneath the San Gabriel Valley, which
drains a major portion of the San Gabriel Mountains watershed (Figure 1). Groundwater lakes are a feature of most mining sites.
Groundwater levels fluctuate seasonally and with wet and dry climate cycles, and may vary locally depending on pumping from, or
recharge to the unconfined aquifer. Currently, groundwater levels observed in the pits are at or near historic low elevations.
All mining sites are included in seismic hazard zones defined by the California Geological Survey, because of concern for liquefaction
and seismic slope stability. As a result of this, an investigation of historical groundwater trends was conducted, seeking to identify the
historical high for the area. In the course of that analysis more general relationships between groundwater and climate trends became
apparent that are the subject of the following discussion.
Figure 1 is a Landsat, computer generated, color enhanced image of the San Gabriel Mountains watershed viewed from the west (note
the 2X vertical exaggeration). The image illustrates the spatial proximity of the mountain front, the adjacent San Gabriel Valley, and the
Los Angeles area, from which the climate records that form the basis of Figures 2, -3 and -4 originate. Also shown are the locations of
Wells 3030F, 4198A and 4198G near the mouth of San Gabriel Canyon.
Los Angeles is the nearest National Weather Service Forecast Office to the San Gabriel Mountains. It provides instrumental
measurements of precipitation, which began in 1877. Figure 2 presents the National Weather Service record plotted as annual
precipitation based on water-year. The mean of this record is 15.01 inches of precipitation annually. Periodic variation has been
partially removed by plotting the annual data as a 5-year running average, which highlights the cyclical, decadal pattern of Southern
California rainfall. A linear secular trend was also plotted, which suggests a long-term decline in annual precipitation of about 0.6-inch
over the period of record.
Figure 3 presents the instrumental record of Figure 2 expressed as cumulative departure from the mean in percent, based on the mean
for the period of 15.01 inches. Lynch (1931, 1948) and Thomas (1962) presented similar plots based on means computed for shorter
periods of record. The secular trend has a pronounced negative slope for this record reflecting precipitation deficits accumulated since
the 1940’s.

Lynch (1931) was able to extend the cumulative departure record back to the late 1770’s using “crop indices,” derived from records of
the Spanish Missions, as proxy for precipitation data. In reference to such records, Lynch (1931, p. 7) states: “For the purpose of
showing the fluctuations in rainfall, they are not much less valuable than the more recent rainfall measurements and show with great
fidelity the weather conditions at the missions.” In the absence of irrigation, crop yields are effectively a function of rainfall amount.
Crop indices were calculated using the relation: I = R/N, where, I = Crop Index for a given year, R = Harvest/Planting, and N =
Normal (i.e., mean) harvest return for the period 1800 to 1832.
To construct the cumulative departure plot of Figure 4 for the period 1770 to 2003 we combined Lynch’s (1931, Appendix D) crop
index data set and the later instrumental precipitation record. The mean utilized in Figure 4 (15.2 inches) was obtained by
extrapolating the linear secular trend from Figure 2 back to 1770, and taking its mean over the period 1770 to 2003. Cumulative
departure from this mean was then recalculated to form the curve shown in Figure 4.
If the longer cumulative departure record formed in this way is correct, it includes features not revealed in the shorter record use to
create Figure 3. For example, the cumulative departure from the mean has been positive in only about 75 years of the 233 years of
the record (approximately 32% of the time), and has been mainly in deficit since about 1845 despite significant positive fluctuations
in the 1890’s, 1910’s, and 1940’s.

Historical water level records for wells shown on Figure 5 were obtained from the County of Los Angeles Public Works Department
and the California Department of Water Resources. Three wells, 3030F, 4198A and 4198G, have historical records extending back to
the early 1930’s. Well 3030F, also known as the Key Well, possesses the most complete record (Figure 6). Wells 4198A and 4198G
are located approximately on surface contour with the Key Well. Records of these wells span the same length of time as the Key
Well, but contain data gaps, which are evident upon close inspection of the hydrographs. Nevertheless, the three well records are
highly similar and measure virtually the same water table contour. Records of many other wells in the area are more fragmentary but
when plotted against the Key Well are correlative with it over the fragmentary intervals.
It is evident from the well records that the historical high groundwater level in the vicinity, approximately elevation 325 feet, was
reached in the 1940’s, and corresponds in time to the region’s maximum historic cumulative departure from the mean precipitation,
which is also shown on Figure 5. The correlation of groundwater and precipitation fluctuations apparent in Figure 5 and the
similarity of negative secular trends (e.g., Figures 4 and 6) suggest that groundwater trends are most strongly reflecting a climatic
influence.

Average annual groundwater elevation for the Key Well is shown in Figure 7 and was derived from the periodic record of the well
shown in Figure 6. The annualized curve of Figure 7 was then analyzed by the water table fluctuation method described in Healy
(2002) to estimate net recharge. The change in storage was calculated as specific yield times the net change in head over each year,
assuming net surface-water and groundwater flow were zero, and evapotranspiration to be very small compared to specific yield.
Specific yield was estimate to be approximately 0.30 based on measurements of porosity in thin section and calculations from field
density tests; field moisture of the unsaturated zone was estimated to be less than 4% based on laboratory testing. The approximate
cumulative change in storage is also shown on Figure 7 and constitutes a net loss of 12 feet of storage over the period of record.
Comparison of the long-term rainfall (climate) record from Los Angeles with the well records illustrates a correspondence in cycles of
relative wet and dry and similarity of secular trends for the respective data sets. Since at least the 1940’s overall rainfall and
groundwater level trends appear to be in decline.
Analysis of liquefaction and seismic slope stability under assumed historically high groundwater conditions is clearly the most
conservative approach with respect to selecting design parameters. A more realistic design basis for the near-term is suggested by the
latest trends observed in Figure 6. Groundwater levels have not been higher that elevation 270 feet in about the last 20 years, and
have not exceeded 300 feet in about the last 35 years. In addition, the lowest groundwater levels were recorded in these intervals, and
at present are at the historically low level (<200 feet). It should also be considered that the higher groundwater levels are transient
and represent restricted time windows in which damaging ground motions might occur.
Relative to formulating parameters for liquefaction analysis and slope stability based on the above discussion, the following options
are under consideration by the City of Irwindale, various consultants, and interested professionals. The trend-line through the entire
data set in Figure 6 shows a steep decline in average groundwater level with time, but the trend is heavily influenced by high
groundwater levels between about 1935 and 1950. This relatively steep slope provides a poor fit for the time period since 1950, and
the fit also suggests an average groundwater elevation that appears artificially low for the present time (2003). Accordingly, a second
fit from 1950 to the present was performed, which appears to provide an unbiased fit for that time interval.
For static analysis, an appropriate groundwater level could be the sum of the 1950-2003 trend and the local deviations from this trend
associated with high rainfall years. At the present time (2003), this corresponds to a groundwater elevation of approximately 270 feet,
as shown in Figure 6; it is possible that this groundwater elevation will decrease with time in accordance with the long-term trend.
However, for design purposes a groundwater table of 270 feet could be used for static slope stability analyses. This is appropriate
because of the probability that another heavy rainfall year could occur in the very near future, before any additional negative trend in
average ground water elevation could occur.

For seismic analysis, an appropriate option could be the current average groundwater level as represented by the 1950-2003 trend line
at year 2003. It is believed that the probability of a major earthquake occurring coincident with a major rainfall excursion is
sufficiently small that the slope performance modeled by such a sequence would have a return period of occurrence that would be
significantly greater than that of the ground motion. The intent of using the average ground water level so defined is to avoid the
introduction of this bias in the seismic slope performance calculations. At the present time (2003), the average groundwater elevation
is approximately 225 feet. As noted above, it is possible that this groundwater elevation could decrease in the future, following the
1950-2003 trend. However, this option assumes no further decrease in the average, which appears to be conservative.

REFERENCES
Healy, R.W. and P.G. Cook (2002), Using groundwater levels to estimate recharge, Hydrogeology Journal, Vol. 10, No. 1, pp. 91-109.
Lynch, H.B. (1931), Rainfall and stream runoff in Southern California since 1769, consulting report for Metropolitan Water District of
Southern California, Los Angeles, 31p.

Lynch, H.B. (1948), Pacific Coast rainfall – Wide fluctuations in hundred years, Western Construction News, July 1948, pp. 76-80.
Thomas, H.E. (1962), The Meteorological Phenomenon of Drought in the Southwest, U.S. Geological Survey Professional Paper 372-A. 43p.

Figure 2 - Annual Precipitation in Los Angeles with 5 Year Running Average and Secular Trend
45

40

35

Precipitation (inches)

30

25

20

15

10

5
Slope of Trend Line = -0.0084
0
1870

1880

1890

1900

1910

1920

1930

1940

1950

1960

1970

1980

1990

2000

Year

Annual Precipitation

5 Year Running Average of Annual Precipitation

Secular Trend of Annual Precipitation

2010

Figure 3 - Precipitation in Los Angeles Expressed as Cumulative Departure from the Mean in Percent
with Secular Trend for the Instrumental Record
400

Rainfall Departure from the Mean in Percent

300

200

100

0

-100

-200
Mean = 15.01 inches
Slope of Trend Line = -1.9036
-300
1870

1880

1890

1900

1910

1920

1930

1940
Year

1950

1960

1970

1980

1990

2000

2010

Figure 4 - Precipitation in Los Angeles Expressed as Cumulative Departure fromthe Mean in Percent with Secular
Trend Derived fromCrop Yields and Precipitation Data
300

Cumulative Departure fromthe Mean in Percent

200

100

0

-100

-200
Mean = 15.2 inches
Slope of Trend Line = -1.0643

-300

-400
1760 1770 1780 1790 1800 1810 1820 1830 1840 1850 1860 1870 1880 1890 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 2010
Year

Based on Lynch's (1931, AppendixD) crop indexdata set

Based on Los Angeles precipitation data set

-

Secular Trend of Cumulative Departure fromthe Mean

350

300

300

200

250

100

200

0

150

-100

100

-200

50

-300

0
1930

1940

1950

1960

1970

1980

1990

2000

Date
Well 3030F

Well 4198A

Well 4198G

Precipitation Departure from the Mean

-400
2010

Cumulative Departure from the Mean in Percent

Groundwater Elevation (feet amsl)

Figure 5 - Combined Well Hydrographs for Wells 3030F, 4198A, and 4198G with Precipitation
Expressed as Cumulative Departure from the Mean in Percent from Figure 4

Figure 6 - Periodic Groundwater Elevation with Secular Trend Measured in Well 3030F (Key Well)

350

Groundwater Elevation (feet above mean sea level)

330
310

Groundwater Elevation for Static Slope Stability Analyses

290
270
250
Slope of Trend Line for Entire Data Set = -1.2233

230
210
Slope of Trend Line 1950 to Present = -0.5873

190

Groundwater Elevation for Dynamic Slope Stability Analyses
170
150
1930

1940

1950

1960

1970

1980

1990

2000

Year
Well 3030F Groundwater Elevation

-

Secular Trend of Groundwater Elevation

Secular Trend Since 1950

2010

330.0

120

310.0

100

290.0

Slope of Trend Line = -0.9084

80

270.0

60

250.0

40

230.0

20

210.0

0

190.0

-20
Slope of Trend Line = -0.2685

170.0
150.0
1930

-40

1940

1950

1960

1970

1980

1990

2000

Year
Annual Average Groundwater Elevation

Approximate Cumulative Change in Water Storage

Secular Trend of Annual Average Groundwater Elevation

Secular Trend of Cumulative Change in Water Storage

-60
2010

Approximate Cumulative Change in Water Storage
(feet)

Groundwater Elevation (feet above mean sea level)

Figure 7 - Annual Average Groundwater Elevation and Approximate Cumulative Change in Storage with
Secular Trends as Measured in Well 3030F (Key Well)


Slide 6

GEOLOGICAL SOCIETY OF AMERICA 2003 ANNUAL MEETING
(ABSTRACT NO. 65672)
COMPARISON OF CLIMATIC TRENDS AND WELL RECORDS IN SOUTHERN
CALIFORNIA: IMPLICATIONS FOR GEOTECHNICAL AND HYDROGEOLOGICAL
PRACTICE
McMILLAN, Kent and VINCENT, Mark, Geo-Logic Associates, 1360 Valley Vista Dr., Diamond Bar, CA
91765, [email protected]
Geo-Logic Associates

ABSTRACT

Comparison of climate (rainfall) records from Los Angeles with water well records beginning in 1930 from the
nearby San Gabriel Valley, shows a correspondence in cycles of relative wet and dry on a decadal scale, as well
as similar longer term trends. The climate record of Los Angeles spans the termination of the Little Ice Age (+
1890 A.D.), and plots of cumulative departure from the long-term mean of the record suggest drying conditions
over the last century. With respect to the present day, these plots also suggest the last significant wet period from
the point of view of increasing groundwater recharge was the 1940’s. The well records noted above establish the
historic high groundwater levels in the area similarly occurred in the decade. Since then, however, rainfall and
groundwater level trends appear to be in decline. This raises the question of whether historic high groundwater
is an appropriate criterion in liquefaction analysis in all cases, since its use presumes the probable return of
historic conditions over the design life of a facility. Alternatively, the criterion might be based on trend analysis
of groundwater levels where there is also corresponding climatic data. Recharge trends are also analyzed by the
water-table fluctuation method.

This presentation is an outgrowth of recent work performed for the City of Irwindale, California, relating to the geotechnical
characterization of alluvial fan deposits derived from the San Gabriel River Canyon (Figure 1). We are indebted to Mr. Kwok Tam,
Director of Public Works, City of Irwindale, for his invaluable assistance, and suppor of this project. Sand and gravel mining of these
deposits to produce mineral aggregates is a major commercial activity in the City. Mining operations have excavated large, deep, steep
sided open pits, some of which have reached their permitted depths and are inactive, while others are nearing completion depths.
Reclamation of these pits is therefore of significant interest to the City, which is proceeding to assess the geotechnical and
hydrogeologic implications of post-mining reclamation.
The alluvial fan deposits constitute a significant unconfined and highly transmissive aquifer beneath the San Gabriel Valley, which
drains a major portion of the San Gabriel Mountains watershed (Figure 1). Groundwater lakes are a feature of most mining sites.
Groundwater levels fluctuate seasonally and with wet and dry climate cycles, and may vary locally depending on pumping from, or
recharge to the unconfined aquifer. Currently, groundwater levels observed in the pits are at or near historic low elevations.
All mining sites are included in seismic hazard zones defined by the California Geological Survey, because of concern for liquefaction
and seismic slope stability. As a result of this, an investigation of historical groundwater trends was conducted, seeking to identify the
historical high for the area. In the course of that analysis more general relationships between groundwater and climate trends became
apparent that are the subject of the following discussion.
Figure 1 is a Landsat, computer generated, color enhanced image of the San Gabriel Mountains watershed viewed from the west (note
the 2X vertical exaggeration). The image illustrates the spatial proximity of the mountain front, the adjacent San Gabriel Valley, and the
Los Angeles area, from which the climate records that form the basis of Figures 2, -3 and -4 originate. Also shown are the locations of
Wells 3030F, 4198A and 4198G near the mouth of San Gabriel Canyon.
Los Angeles is the nearest National Weather Service Forecast Office to the San Gabriel Mountains. It provides instrumental
measurements of precipitation, which began in 1877. Figure 2 presents the National Weather Service record plotted as annual
precipitation based on water-year. The mean of this record is 15.01 inches of precipitation annually. Periodic variation has been
partially removed by plotting the annual data as a 5-year running average, which highlights the cyclical, decadal pattern of Southern
California rainfall. A linear secular trend was also plotted, which suggests a long-term decline in annual precipitation of about 0.6-inch
over the period of record.
Figure 3 presents the instrumental record of Figure 2 expressed as cumulative departure from the mean in percent, based on the mean
for the period of 15.01 inches. Lynch (1931, 1948) and Thomas (1962) presented similar plots based on means computed for shorter
periods of record. The secular trend has a pronounced negative slope for this record reflecting precipitation deficits accumulated since
the 1940’s.

Lynch (1931) was able to extend the cumulative departure record back to the late 1770’s using “crop indices,” derived from records of
the Spanish Missions, as proxy for precipitation data. In reference to such records, Lynch (1931, p. 7) states: “For the purpose of
showing the fluctuations in rainfall, they are not much less valuable than the more recent rainfall measurements and show with great
fidelity the weather conditions at the missions.” In the absence of irrigation, crop yields are effectively a function of rainfall amount.
Crop indices were calculated using the relation: I = R/N, where, I = Crop Index for a given year, R = Harvest/Planting, and N =
Normal (i.e., mean) harvest return for the period 1800 to 1832.
To construct the cumulative departure plot of Figure 4 for the period 1770 to 2003 we combined Lynch’s (1931, Appendix D) crop
index data set and the later instrumental precipitation record. The mean utilized in Figure 4 (15.2 inches) was obtained by
extrapolating the linear secular trend from Figure 2 back to 1770, and taking its mean over the period 1770 to 2003. Cumulative
departure from this mean was then recalculated to form the curve shown in Figure 4.
If the longer cumulative departure record formed in this way is correct, it includes features not revealed in the shorter record use to
create Figure 3. For example, the cumulative departure from the mean has been positive in only about 75 years of the 233 years of
the record (approximately 32% of the time), and has been mainly in deficit since about 1845 despite significant positive fluctuations
in the 1890’s, 1910’s, and 1940’s.

Historical water level records for wells shown on Figure 5 were obtained from the County of Los Angeles Public Works Department
and the California Department of Water Resources. Three wells, 3030F, 4198A and 4198G, have historical records extending back to
the early 1930’s. Well 3030F, also known as the Key Well, possesses the most complete record (Figure 6). Wells 4198A and 4198G
are located approximately on surface contour with the Key Well. Records of these wells span the same length of time as the Key
Well, but contain data gaps, which are evident upon close inspection of the hydrographs. Nevertheless, the three well records are
highly similar and measure virtually the same water table contour. Records of many other wells in the area are more fragmentary but
when plotted against the Key Well are correlative with it over the fragmentary intervals.
It is evident from the well records that the historical high groundwater level in the vicinity, approximately elevation 325 feet, was
reached in the 1940’s, and corresponds in time to the region’s maximum historic cumulative departure from the mean precipitation,
which is also shown on Figure 5. The correlation of groundwater and precipitation fluctuations apparent in Figure 5 and the
similarity of negative secular trends (e.g., Figures 4 and 6) suggest that groundwater trends are most strongly reflecting a climatic
influence.

Average annual groundwater elevation for the Key Well is shown in Figure 7 and was derived from the periodic record of the well
shown in Figure 6. The annualized curve of Figure 7 was then analyzed by the water table fluctuation method described in Healy
(2002) to estimate net recharge. The change in storage was calculated as specific yield times the net change in head over each year,
assuming net surface-water and groundwater flow were zero, and evapotranspiration to be very small compared to specific yield.
Specific yield was estimate to be approximately 0.30 based on measurements of porosity in thin section and calculations from field
density tests; field moisture of the unsaturated zone was estimated to be less than 4% based on laboratory testing. The approximate
cumulative change in storage is also shown on Figure 7 and constitutes a net loss of 12 feet of storage over the period of record.
Comparison of the long-term rainfall (climate) record from Los Angeles with the well records illustrates a correspondence in cycles of
relative wet and dry and similarity of secular trends for the respective data sets. Since at least the 1940’s overall rainfall and
groundwater level trends appear to be in decline.
Analysis of liquefaction and seismic slope stability under assumed historically high groundwater conditions is clearly the most
conservative approach with respect to selecting design parameters. A more realistic design basis for the near-term is suggested by the
latest trends observed in Figure 6. Groundwater levels have not been higher that elevation 270 feet in about the last 20 years, and
have not exceeded 300 feet in about the last 35 years. In addition, the lowest groundwater levels were recorded in these intervals, and
at present are at the historically low level (<200 feet). It should also be considered that the higher groundwater levels are transient
and represent restricted time windows in which damaging ground motions might occur.
Relative to formulating parameters for liquefaction analysis and slope stability based on the above discussion, the following options
are under consideration by the City of Irwindale, various consultants, and interested professionals. The trend-line through the entire
data set in Figure 6 shows a steep decline in average groundwater level with time, but the trend is heavily influenced by high
groundwater levels between about 1935 and 1950. This relatively steep slope provides a poor fit for the time period since 1950, and
the fit also suggests an average groundwater elevation that appears artificially low for the present time (2003). Accordingly, a second
fit from 1950 to the present was performed, which appears to provide an unbiased fit for that time interval.
For static analysis, an appropriate groundwater level could be the sum of the 1950-2003 trend and the local deviations from this trend
associated with high rainfall years. At the present time (2003), this corresponds to a groundwater elevation of approximately 270 feet,
as shown in Figure 6; it is possible that this groundwater elevation will decrease with time in accordance with the long-term trend.
However, for design purposes a groundwater table of 270 feet could be used for static slope stability analyses. This is appropriate
because of the probability that another heavy rainfall year could occur in the very near future, before any additional negative trend in
average ground water elevation could occur.

For seismic analysis, an appropriate option could be the current average groundwater level as represented by the 1950-2003 trend line
at year 2003. It is believed that the probability of a major earthquake occurring coincident with a major rainfall excursion is
sufficiently small that the slope performance modeled by such a sequence would have a return period of occurrence that would be
significantly greater than that of the ground motion. The intent of using the average ground water level so defined is to avoid the
introduction of this bias in the seismic slope performance calculations. At the present time (2003), the average groundwater elevation
is approximately 225 feet. As noted above, it is possible that this groundwater elevation could decrease in the future, following the
1950-2003 trend. However, this option assumes no further decrease in the average, which appears to be conservative.

REFERENCES
Healy, R.W. and P.G. Cook (2002), Using groundwater levels to estimate recharge, Hydrogeology Journal, Vol. 10, No. 1, pp. 91-109.
Lynch, H.B. (1931), Rainfall and stream runoff in Southern California since 1769, consulting report for Metropolitan Water District of
Southern California, Los Angeles, 31p.

Lynch, H.B. (1948), Pacific Coast rainfall – Wide fluctuations in hundred years, Western Construction News, July 1948, pp. 76-80.
Thomas, H.E. (1962), The Meteorological Phenomenon of Drought in the Southwest, U.S. Geological Survey Professional Paper 372-A. 43p.

Figure 2 - Annual Precipitation in Los Angeles with 5 Year Running Average and Secular Trend
45

40

35

Precipitation (inches)

30

25

20

15

10

5
Slope of Trend Line = -0.0084
0
1870

1880

1890

1900

1910

1920

1930

1940

1950

1960

1970

1980

1990

2000

Year

Annual Precipitation

5 Year Running Average of Annual Precipitation

Secular Trend of Annual Precipitation

2010

Figure 3 - Precipitation in Los Angeles Expressed as Cumulative Departure from the Mean in Percent
with Secular Trend for the Instrumental Record
400

Rainfall Departure from the Mean in Percent

300

200

100

0

-100

-200
Mean = 15.01 inches
Slope of Trend Line = -1.9036
-300
1870

1880

1890

1900

1910

1920

1930

1940
Year

1950

1960

1970

1980

1990

2000

2010

Figure 4 - Precipitation in Los Angeles Expressed as Cumulative Departure fromthe Mean in Percent with Secular
Trend Derived fromCrop Yields and Precipitation Data
300

Cumulative Departure fromthe Mean in Percent

200

100

0

-100

-200
Mean = 15.2 inches
Slope of Trend Line = -1.0643

-300

-400
1760 1770 1780 1790 1800 1810 1820 1830 1840 1850 1860 1870 1880 1890 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 2010
Year

Based on Lynch's (1931, AppendixD) crop indexdata set

Based on Los Angeles precipitation data set

-

Secular Trend of Cumulative Departure fromthe Mean

350

300

300

200

250

100

200

0

150

-100

100

-200

50

-300

0
1930

1940

1950

1960

1970

1980

1990

2000

Date
Well 3030F

Well 4198A

Well 4198G

Precipitation Departure from the Mean

-400
2010

Cumulative Departure from the Mean in Percent

Groundwater Elevation (feet amsl)

Figure 5 - Combined Well Hydrographs for Wells 3030F, 4198A, and 4198G with Precipitation
Expressed as Cumulative Departure from the Mean in Percent from Figure 4

Figure 6 - Periodic Groundwater Elevation with Secular Trend Measured in Well 3030F (Key Well)

350

Groundwater Elevation (feet above mean sea level)

330
310

Groundwater Elevation for Static Slope Stability Analyses

290
270
250
Slope of Trend Line for Entire Data Set = -1.2233

230
210
Slope of Trend Line 1950 to Present = -0.5873

190

Groundwater Elevation for Dynamic Slope Stability Analyses
170
150
1930

1940

1950

1960

1970

1980

1990

2000

Year
Well 3030F Groundwater Elevation

-

Secular Trend of Groundwater Elevation

Secular Trend Since 1950

2010

330.0

120

310.0

100

290.0

Slope of Trend Line = -0.9084

80

270.0

60

250.0

40

230.0

20

210.0

0

190.0

-20
Slope of Trend Line = -0.2685

170.0
150.0
1930

-40

1940

1950

1960

1970

1980

1990

2000

Year
Annual Average Groundwater Elevation

Approximate Cumulative Change in Water Storage

Secular Trend of Annual Average Groundwater Elevation

Secular Trend of Cumulative Change in Water Storage

-60
2010

Approximate Cumulative Change in Water Storage
(feet)

Groundwater Elevation (feet above mean sea level)

Figure 7 - Annual Average Groundwater Elevation and Approximate Cumulative Change in Storage with
Secular Trends as Measured in Well 3030F (Key Well)


Slide 7

GEOLOGICAL SOCIETY OF AMERICA 2003 ANNUAL MEETING
(ABSTRACT NO. 65672)
COMPARISON OF CLIMATIC TRENDS AND WELL RECORDS IN SOUTHERN
CALIFORNIA: IMPLICATIONS FOR GEOTECHNICAL AND HYDROGEOLOGICAL
PRACTICE
McMILLAN, Kent and VINCENT, Mark, Geo-Logic Associates, 1360 Valley Vista Dr., Diamond Bar, CA
91765, [email protected]
Geo-Logic Associates

ABSTRACT

Comparison of climate (rainfall) records from Los Angeles with water well records beginning in 1930 from the
nearby San Gabriel Valley, shows a correspondence in cycles of relative wet and dry on a decadal scale, as well
as similar longer term trends. The climate record of Los Angeles spans the termination of the Little Ice Age (+
1890 A.D.), and plots of cumulative departure from the long-term mean of the record suggest drying conditions
over the last century. With respect to the present day, these plots also suggest the last significant wet period from
the point of view of increasing groundwater recharge was the 1940’s. The well records noted above establish the
historic high groundwater levels in the area similarly occurred in the decade. Since then, however, rainfall and
groundwater level trends appear to be in decline. This raises the question of whether historic high groundwater
is an appropriate criterion in liquefaction analysis in all cases, since its use presumes the probable return of
historic conditions over the design life of a facility. Alternatively, the criterion might be based on trend analysis
of groundwater levels where there is also corresponding climatic data. Recharge trends are also analyzed by the
water-table fluctuation method.

This presentation is an outgrowth of recent work performed for the City of Irwindale, California, relating to the geotechnical
characterization of alluvial fan deposits derived from the San Gabriel River Canyon (Figure 1). We are indebted to Mr. Kwok Tam,
Director of Public Works, City of Irwindale, for his invaluable assistance, and suppor of this project. Sand and gravel mining of these
deposits to produce mineral aggregates is a major commercial activity in the City. Mining operations have excavated large, deep, steep
sided open pits, some of which have reached their permitted depths and are inactive, while others are nearing completion depths.
Reclamation of these pits is therefore of significant interest to the City, which is proceeding to assess the geotechnical and
hydrogeologic implications of post-mining reclamation.
The alluvial fan deposits constitute a significant unconfined and highly transmissive aquifer beneath the San Gabriel Valley, which
drains a major portion of the San Gabriel Mountains watershed (Figure 1). Groundwater lakes are a feature of most mining sites.
Groundwater levels fluctuate seasonally and with wet and dry climate cycles, and may vary locally depending on pumping from, or
recharge to the unconfined aquifer. Currently, groundwater levels observed in the pits are at or near historic low elevations.
All mining sites are included in seismic hazard zones defined by the California Geological Survey, because of concern for liquefaction
and seismic slope stability. As a result of this, an investigation of historical groundwater trends was conducted, seeking to identify the
historical high for the area. In the course of that analysis more general relationships between groundwater and climate trends became
apparent that are the subject of the following discussion.
Figure 1 is a Landsat, computer generated, color enhanced image of the San Gabriel Mountains watershed viewed from the west (note
the 2X vertical exaggeration). The image illustrates the spatial proximity of the mountain front, the adjacent San Gabriel Valley, and the
Los Angeles area, from which the climate records that form the basis of Figures 2, -3 and -4 originate. Also shown are the locations of
Wells 3030F, 4198A and 4198G near the mouth of San Gabriel Canyon.
Los Angeles is the nearest National Weather Service Forecast Office to the San Gabriel Mountains. It provides instrumental
measurements of precipitation, which began in 1877. Figure 2 presents the National Weather Service record plotted as annual
precipitation based on water-year. The mean of this record is 15.01 inches of precipitation annually. Periodic variation has been
partially removed by plotting the annual data as a 5-year running average, which highlights the cyclical, decadal pattern of Southern
California rainfall. A linear secular trend was also plotted, which suggests a long-term decline in annual precipitation of about 0.6-inch
over the period of record.
Figure 3 presents the instrumental record of Figure 2 expressed as cumulative departure from the mean in percent, based on the mean
for the period of 15.01 inches. Lynch (1931, 1948) and Thomas (1962) presented similar plots based on means computed for shorter
periods of record. The secular trend has a pronounced negative slope for this record reflecting precipitation deficits accumulated since
the 1940’s.

Lynch (1931) was able to extend the cumulative departure record back to the late 1770’s using “crop indices,” derived from records of
the Spanish Missions, as proxy for precipitation data. In reference to such records, Lynch (1931, p. 7) states: “For the purpose of
showing the fluctuations in rainfall, they are not much less valuable than the more recent rainfall measurements and show with great
fidelity the weather conditions at the missions.” In the absence of irrigation, crop yields are effectively a function of rainfall amount.
Crop indices were calculated using the relation: I = R/N, where, I = Crop Index for a given year, R = Harvest/Planting, and N =
Normal (i.e., mean) harvest return for the period 1800 to 1832.
To construct the cumulative departure plot of Figure 4 for the period 1770 to 2003 we combined Lynch’s (1931, Appendix D) crop
index data set and the later instrumental precipitation record. The mean utilized in Figure 4 (15.2 inches) was obtained by
extrapolating the linear secular trend from Figure 2 back to 1770, and taking its mean over the period 1770 to 2003. Cumulative
departure from this mean was then recalculated to form the curve shown in Figure 4.
If the longer cumulative departure record formed in this way is correct, it includes features not revealed in the shorter record use to
create Figure 3. For example, the cumulative departure from the mean has been positive in only about 75 years of the 233 years of
the record (approximately 32% of the time), and has been mainly in deficit since about 1845 despite significant positive fluctuations
in the 1890’s, 1910’s, and 1940’s.

Historical water level records for wells shown on Figure 5 were obtained from the County of Los Angeles Public Works Department
and the California Department of Water Resources. Three wells, 3030F, 4198A and 4198G, have historical records extending back to
the early 1930’s. Well 3030F, also known as the Key Well, possesses the most complete record (Figure 6). Wells 4198A and 4198G
are located approximately on surface contour with the Key Well. Records of these wells span the same length of time as the Key
Well, but contain data gaps, which are evident upon close inspection of the hydrographs. Nevertheless, the three well records are
highly similar and measure virtually the same water table contour. Records of many other wells in the area are more fragmentary but
when plotted against the Key Well are correlative with it over the fragmentary intervals.
It is evident from the well records that the historical high groundwater level in the vicinity, approximately elevation 325 feet, was
reached in the 1940’s, and corresponds in time to the region’s maximum historic cumulative departure from the mean precipitation,
which is also shown on Figure 5. The correlation of groundwater and precipitation fluctuations apparent in Figure 5 and the
similarity of negative secular trends (e.g., Figures 4 and 6) suggest that groundwater trends are most strongly reflecting a climatic
influence.

Average annual groundwater elevation for the Key Well is shown in Figure 7 and was derived from the periodic record of the well
shown in Figure 6. The annualized curve of Figure 7 was then analyzed by the water table fluctuation method described in Healy
(2002) to estimate net recharge. The change in storage was calculated as specific yield times the net change in head over each year,
assuming net surface-water and groundwater flow were zero, and evapotranspiration to be very small compared to specific yield.
Specific yield was estimate to be approximately 0.30 based on measurements of porosity in thin section and calculations from field
density tests; field moisture of the unsaturated zone was estimated to be less than 4% based on laboratory testing. The approximate
cumulative change in storage is also shown on Figure 7 and constitutes a net loss of 12 feet of storage over the period of record.
Comparison of the long-term rainfall (climate) record from Los Angeles with the well records illustrates a correspondence in cycles of
relative wet and dry and similarity of secular trends for the respective data sets. Since at least the 1940’s overall rainfall and
groundwater level trends appear to be in decline.
Analysis of liquefaction and seismic slope stability under assumed historically high groundwater conditions is clearly the most
conservative approach with respect to selecting design parameters. A more realistic design basis for the near-term is suggested by the
latest trends observed in Figure 6. Groundwater levels have not been higher that elevation 270 feet in about the last 20 years, and
have not exceeded 300 feet in about the last 35 years. In addition, the lowest groundwater levels were recorded in these intervals, and
at present are at the historically low level (<200 feet). It should also be considered that the higher groundwater levels are transient
and represent restricted time windows in which damaging ground motions might occur.
Relative to formulating parameters for liquefaction analysis and slope stability based on the above discussion, the following options
are under consideration by the City of Irwindale, various consultants, and interested professionals. The trend-line through the entire
data set in Figure 6 shows a steep decline in average groundwater level with time, but the trend is heavily influenced by high
groundwater levels between about 1935 and 1950. This relatively steep slope provides a poor fit for the time period since 1950, and
the fit also suggests an average groundwater elevation that appears artificially low for the present time (2003). Accordingly, a second
fit from 1950 to the present was performed, which appears to provide an unbiased fit for that time interval.
For static analysis, an appropriate groundwater level could be the sum of the 1950-2003 trend and the local deviations from this trend
associated with high rainfall years. At the present time (2003), this corresponds to a groundwater elevation of approximately 270 feet,
as shown in Figure 6; it is possible that this groundwater elevation will decrease with time in accordance with the long-term trend.
However, for design purposes a groundwater table of 270 feet could be used for static slope stability analyses. This is appropriate
because of the probability that another heavy rainfall year could occur in the very near future, before any additional negative trend in
average ground water elevation could occur.

For seismic analysis, an appropriate option could be the current average groundwater level as represented by the 1950-2003 trend line
at year 2003. It is believed that the probability of a major earthquake occurring coincident with a major rainfall excursion is
sufficiently small that the slope performance modeled by such a sequence would have a return period of occurrence that would be
significantly greater than that of the ground motion. The intent of using the average ground water level so defined is to avoid the
introduction of this bias in the seismic slope performance calculations. At the present time (2003), the average groundwater elevation
is approximately 225 feet. As noted above, it is possible that this groundwater elevation could decrease in the future, following the
1950-2003 trend. However, this option assumes no further decrease in the average, which appears to be conservative.

REFERENCES
Healy, R.W. and P.G. Cook (2002), Using groundwater levels to estimate recharge, Hydrogeology Journal, Vol. 10, No. 1, pp. 91-109.
Lynch, H.B. (1931), Rainfall and stream runoff in Southern California since 1769, consulting report for Metropolitan Water District of
Southern California, Los Angeles, 31p.

Lynch, H.B. (1948), Pacific Coast rainfall – Wide fluctuations in hundred years, Western Construction News, July 1948, pp. 76-80.
Thomas, H.E. (1962), The Meteorological Phenomenon of Drought in the Southwest, U.S. Geological Survey Professional Paper 372-A. 43p.

Figure 2 - Annual Precipitation in Los Angeles with 5 Year Running Average and Secular Trend
45

40

35

Precipitation (inches)

30

25

20

15

10

5
Slope of Trend Line = -0.0084
0
1870

1880

1890

1900

1910

1920

1930

1940

1950

1960

1970

1980

1990

2000

Year

Annual Precipitation

5 Year Running Average of Annual Precipitation

Secular Trend of Annual Precipitation

2010

Figure 3 - Precipitation in Los Angeles Expressed as Cumulative Departure from the Mean in Percent
with Secular Trend for the Instrumental Record
400

Rainfall Departure from the Mean in Percent

300

200

100

0

-100

-200
Mean = 15.01 inches
Slope of Trend Line = -1.9036
-300
1870

1880

1890

1900

1910

1920

1930

1940
Year

1950

1960

1970

1980

1990

2000

2010

Figure 4 - Precipitation in Los Angeles Expressed as Cumulative Departure fromthe Mean in Percent with Secular
Trend Derived fromCrop Yields and Precipitation Data
300

Cumulative Departure fromthe Mean in Percent

200

100

0

-100

-200
Mean = 15.2 inches
Slope of Trend Line = -1.0643

-300

-400
1760 1770 1780 1790 1800 1810 1820 1830 1840 1850 1860 1870 1880 1890 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 2010
Year

Based on Lynch's (1931, AppendixD) crop indexdata set

Based on Los Angeles precipitation data set

-

Secular Trend of Cumulative Departure fromthe Mean

350

300

300

200

250

100

200

0

150

-100

100

-200

50

-300

0
1930

1940

1950

1960

1970

1980

1990

2000

Date
Well 3030F

Well 4198A

Well 4198G

Precipitation Departure from the Mean

-400
2010

Cumulative Departure from the Mean in Percent

Groundwater Elevation (feet amsl)

Figure 5 - Combined Well Hydrographs for Wells 3030F, 4198A, and 4198G with Precipitation
Expressed as Cumulative Departure from the Mean in Percent from Figure 4

Figure 6 - Periodic Groundwater Elevation with Secular Trend Measured in Well 3030F (Key Well)

350

Groundwater Elevation (feet above mean sea level)

330
310

Groundwater Elevation for Static Slope Stability Analyses

290
270
250
Slope of Trend Line for Entire Data Set = -1.2233

230
210
Slope of Trend Line 1950 to Present = -0.5873

190

Groundwater Elevation for Dynamic Slope Stability Analyses
170
150
1930

1940

1950

1960

1970

1980

1990

2000

Year
Well 3030F Groundwater Elevation

-

Secular Trend of Groundwater Elevation

Secular Trend Since 1950

2010

330.0

120

310.0

100

290.0

Slope of Trend Line = -0.9084

80

270.0

60

250.0

40

230.0

20

210.0

0

190.0

-20
Slope of Trend Line = -0.2685

170.0
150.0
1930

-40

1940

1950

1960

1970

1980

1990

2000

Year
Annual Average Groundwater Elevation

Approximate Cumulative Change in Water Storage

Secular Trend of Annual Average Groundwater Elevation

Secular Trend of Cumulative Change in Water Storage

-60
2010

Approximate Cumulative Change in Water Storage
(feet)

Groundwater Elevation (feet above mean sea level)

Figure 7 - Annual Average Groundwater Elevation and Approximate Cumulative Change in Storage with
Secular Trends as Measured in Well 3030F (Key Well)


Slide 8

GEOLOGICAL SOCIETY OF AMERICA 2003 ANNUAL MEETING
(ABSTRACT NO. 65672)
COMPARISON OF CLIMATIC TRENDS AND WELL RECORDS IN SOUTHERN
CALIFORNIA: IMPLICATIONS FOR GEOTECHNICAL AND HYDROGEOLOGICAL
PRACTICE
McMILLAN, Kent and VINCENT, Mark, Geo-Logic Associates, 1360 Valley Vista Dr., Diamond Bar, CA
91765, [email protected]
Geo-Logic Associates

ABSTRACT

Comparison of climate (rainfall) records from Los Angeles with water well records beginning in 1930 from the
nearby San Gabriel Valley, shows a correspondence in cycles of relative wet and dry on a decadal scale, as well
as similar longer term trends. The climate record of Los Angeles spans the termination of the Little Ice Age (+
1890 A.D.), and plots of cumulative departure from the long-term mean of the record suggest drying conditions
over the last century. With respect to the present day, these plots also suggest the last significant wet period from
the point of view of increasing groundwater recharge was the 1940’s. The well records noted above establish the
historic high groundwater levels in the area similarly occurred in the decade. Since then, however, rainfall and
groundwater level trends appear to be in decline. This raises the question of whether historic high groundwater
is an appropriate criterion in liquefaction analysis in all cases, since its use presumes the probable return of
historic conditions over the design life of a facility. Alternatively, the criterion might be based on trend analysis
of groundwater levels where there is also corresponding climatic data. Recharge trends are also analyzed by the
water-table fluctuation method.

This presentation is an outgrowth of recent work performed for the City of Irwindale, California, relating to the geotechnical
characterization of alluvial fan deposits derived from the San Gabriel River Canyon (Figure 1). We are indebted to Mr. Kwok Tam,
Director of Public Works, City of Irwindale, for his invaluable assistance, and suppor of this project. Sand and gravel mining of these
deposits to produce mineral aggregates is a major commercial activity in the City. Mining operations have excavated large, deep, steep
sided open pits, some of which have reached their permitted depths and are inactive, while others are nearing completion depths.
Reclamation of these pits is therefore of significant interest to the City, which is proceeding to assess the geotechnical and
hydrogeologic implications of post-mining reclamation.
The alluvial fan deposits constitute a significant unconfined and highly transmissive aquifer beneath the San Gabriel Valley, which
drains a major portion of the San Gabriel Mountains watershed (Figure 1). Groundwater lakes are a feature of most mining sites.
Groundwater levels fluctuate seasonally and with wet and dry climate cycles, and may vary locally depending on pumping from, or
recharge to the unconfined aquifer. Currently, groundwater levels observed in the pits are at or near historic low elevations.
All mining sites are included in seismic hazard zones defined by the California Geological Survey, because of concern for liquefaction
and seismic slope stability. As a result of this, an investigation of historical groundwater trends was conducted, seeking to identify the
historical high for the area. In the course of that analysis more general relationships between groundwater and climate trends became
apparent that are the subject of the following discussion.
Figure 1 is a Landsat, computer generated, color enhanced image of the San Gabriel Mountains watershed viewed from the west (note
the 2X vertical exaggeration). The image illustrates the spatial proximity of the mountain front, the adjacent San Gabriel Valley, and the
Los Angeles area, from which the climate records that form the basis of Figures 2, -3 and -4 originate. Also shown are the locations of
Wells 3030F, 4198A and 4198G near the mouth of San Gabriel Canyon.
Los Angeles is the nearest National Weather Service Forecast Office to the San Gabriel Mountains. It provides instrumental
measurements of precipitation, which began in 1877. Figure 2 presents the National Weather Service record plotted as annual
precipitation based on water-year. The mean of this record is 15.01 inches of precipitation annually. Periodic variation has been
partially removed by plotting the annual data as a 5-year running average, which highlights the cyclical, decadal pattern of Southern
California rainfall. A linear secular trend was also plotted, which suggests a long-term decline in annual precipitation of about 0.6-inch
over the period of record.
Figure 3 presents the instrumental record of Figure 2 expressed as cumulative departure from the mean in percent, based on the mean
for the period of 15.01 inches. Lynch (1931, 1948) and Thomas (1962) presented similar plots based on means computed for shorter
periods of record. The secular trend has a pronounced negative slope for this record reflecting precipitation deficits accumulated since
the 1940’s.

Lynch (1931) was able to extend the cumulative departure record back to the late 1770’s using “crop indices,” derived from records of
the Spanish Missions, as proxy for precipitation data. In reference to such records, Lynch (1931, p. 7) states: “For the purpose of
showing the fluctuations in rainfall, they are not much less valuable than the more recent rainfall measurements and show with great
fidelity the weather conditions at the missions.” In the absence of irrigation, crop yields are effectively a function of rainfall amount.
Crop indices were calculated using the relation: I = R/N, where, I = Crop Index for a given year, R = Harvest/Planting, and N =
Normal (i.e., mean) harvest return for the period 1800 to 1832.
To construct the cumulative departure plot of Figure 4 for the period 1770 to 2003 we combined Lynch’s (1931, Appendix D) crop
index data set and the later instrumental precipitation record. The mean utilized in Figure 4 (15.2 inches) was obtained by
extrapolating the linear secular trend from Figure 2 back to 1770, and taking its mean over the period 1770 to 2003. Cumulative
departure from this mean was then recalculated to form the curve shown in Figure 4.
If the longer cumulative departure record formed in this way is correct, it includes features not revealed in the shorter record use to
create Figure 3. For example, the cumulative departure from the mean has been positive in only about 75 years of the 233 years of
the record (approximately 32% of the time), and has been mainly in deficit since about 1845 despite significant positive fluctuations
in the 1890’s, 1910’s, and 1940’s.

Historical water level records for wells shown on Figure 5 were obtained from the County of Los Angeles Public Works Department
and the California Department of Water Resources. Three wells, 3030F, 4198A and 4198G, have historical records extending back to
the early 1930’s. Well 3030F, also known as the Key Well, possesses the most complete record (Figure 6). Wells 4198A and 4198G
are located approximately on surface contour with the Key Well. Records of these wells span the same length of time as the Key
Well, but contain data gaps, which are evident upon close inspection of the hydrographs. Nevertheless, the three well records are
highly similar and measure virtually the same water table contour. Records of many other wells in the area are more fragmentary but
when plotted against the Key Well are correlative with it over the fragmentary intervals.
It is evident from the well records that the historical high groundwater level in the vicinity, approximately elevation 325 feet, was
reached in the 1940’s, and corresponds in time to the region’s maximum historic cumulative departure from the mean precipitation,
which is also shown on Figure 5. The correlation of groundwater and precipitation fluctuations apparent in Figure 5 and the
similarity of negative secular trends (e.g., Figures 4 and 6) suggest that groundwater trends are most strongly reflecting a climatic
influence.

Average annual groundwater elevation for the Key Well is shown in Figure 7 and was derived from the periodic record of the well
shown in Figure 6. The annualized curve of Figure 7 was then analyzed by the water table fluctuation method described in Healy
(2002) to estimate net recharge. The change in storage was calculated as specific yield times the net change in head over each year,
assuming net surface-water and groundwater flow were zero, and evapotranspiration to be very small compared to specific yield.
Specific yield was estimate to be approximately 0.30 based on measurements of porosity in thin section and calculations from field
density tests; field moisture of the unsaturated zone was estimated to be less than 4% based on laboratory testing. The approximate
cumulative change in storage is also shown on Figure 7 and constitutes a net loss of 12 feet of storage over the period of record.
Comparison of the long-term rainfall (climate) record from Los Angeles with the well records illustrates a correspondence in cycles of
relative wet and dry and similarity of secular trends for the respective data sets. Since at least the 1940’s overall rainfall and
groundwater level trends appear to be in decline.
Analysis of liquefaction and seismic slope stability under assumed historically high groundwater conditions is clearly the most
conservative approach with respect to selecting design parameters. A more realistic design basis for the near-term is suggested by the
latest trends observed in Figure 6. Groundwater levels have not been higher that elevation 270 feet in about the last 20 years, and
have not exceeded 300 feet in about the last 35 years. In addition, the lowest groundwater levels were recorded in these intervals, and
at present are at the historically low level (<200 feet). It should also be considered that the higher groundwater levels are transient
and represent restricted time windows in which damaging ground motions might occur.
Relative to formulating parameters for liquefaction analysis and slope stability based on the above discussion, the following options
are under consideration by the City of Irwindale, various consultants, and interested professionals. The trend-line through the entire
data set in Figure 6 shows a steep decline in average groundwater level with time, but the trend is heavily influenced by high
groundwater levels between about 1935 and 1950. This relatively steep slope provides a poor fit for the time period since 1950, and
the fit also suggests an average groundwater elevation that appears artificially low for the present time (2003). Accordingly, a second
fit from 1950 to the present was performed, which appears to provide an unbiased fit for that time interval.
For static analysis, an appropriate groundwater level could be the sum of the 1950-2003 trend and the local deviations from this trend
associated with high rainfall years. At the present time (2003), this corresponds to a groundwater elevation of approximately 270 feet,
as shown in Figure 6; it is possible that this groundwater elevation will decrease with time in accordance with the long-term trend.
However, for design purposes a groundwater table of 270 feet could be used for static slope stability analyses. This is appropriate
because of the probability that another heavy rainfall year could occur in the very near future, before any additional negative trend in
average ground water elevation could occur.

For seismic analysis, an appropriate option could be the current average groundwater level as represented by the 1950-2003 trend line
at year 2003. It is believed that the probability of a major earthquake occurring coincident with a major rainfall excursion is
sufficiently small that the slope performance modeled by such a sequence would have a return period of occurrence that would be
significantly greater than that of the ground motion. The intent of using the average ground water level so defined is to avoid the
introduction of this bias in the seismic slope performance calculations. At the present time (2003), the average groundwater elevation
is approximately 225 feet. As noted above, it is possible that this groundwater elevation could decrease in the future, following the
1950-2003 trend. However, this option assumes no further decrease in the average, which appears to be conservative.

REFERENCES
Healy, R.W. and P.G. Cook (2002), Using groundwater levels to estimate recharge, Hydrogeology Journal, Vol. 10, No. 1, pp. 91-109.
Lynch, H.B. (1931), Rainfall and stream runoff in Southern California since 1769, consulting report for Metropolitan Water District of
Southern California, Los Angeles, 31p.

Lynch, H.B. (1948), Pacific Coast rainfall – Wide fluctuations in hundred years, Western Construction News, July 1948, pp. 76-80.
Thomas, H.E. (1962), The Meteorological Phenomenon of Drought in the Southwest, U.S. Geological Survey Professional Paper 372-A. 43p.

Figure 2 - Annual Precipitation in Los Angeles with 5 Year Running Average and Secular Trend
45

40

35

Precipitation (inches)

30

25

20

15

10

5
Slope of Trend Line = -0.0084
0
1870

1880

1890

1900

1910

1920

1930

1940

1950

1960

1970

1980

1990

2000

Year

Annual Precipitation

5 Year Running Average of Annual Precipitation

Secular Trend of Annual Precipitation

2010

Figure 3 - Precipitation in Los Angeles Expressed as Cumulative Departure from the Mean in Percent
with Secular Trend for the Instrumental Record
400

Rainfall Departure from the Mean in Percent

300

200

100

0

-100

-200
Mean = 15.01 inches
Slope of Trend Line = -1.9036
-300
1870

1880

1890

1900

1910

1920

1930

1940
Year

1950

1960

1970

1980

1990

2000

2010

Figure 4 - Precipitation in Los Angeles Expressed as Cumulative Departure fromthe Mean in Percent with Secular
Trend Derived fromCrop Yields and Precipitation Data
300

Cumulative Departure fromthe Mean in Percent

200

100

0

-100

-200
Mean = 15.2 inches
Slope of Trend Line = -1.0643

-300

-400
1760 1770 1780 1790 1800 1810 1820 1830 1840 1850 1860 1870 1880 1890 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 2010
Year

Based on Lynch's (1931, AppendixD) crop indexdata set

Based on Los Angeles precipitation data set

-

Secular Trend of Cumulative Departure fromthe Mean

350

300

300

200

250

100

200

0

150

-100

100

-200

50

-300

0
1930

1940

1950

1960

1970

1980

1990

2000

Date
Well 3030F

Well 4198A

Well 4198G

Precipitation Departure from the Mean

-400
2010

Cumulative Departure from the Mean in Percent

Groundwater Elevation (feet amsl)

Figure 5 - Combined Well Hydrographs for Wells 3030F, 4198A, and 4198G with Precipitation
Expressed as Cumulative Departure from the Mean in Percent from Figure 4

Figure 6 - Periodic Groundwater Elevation with Secular Trend Measured in Well 3030F (Key Well)

350

Groundwater Elevation (feet above mean sea level)

330
310

Groundwater Elevation for Static Slope Stability Analyses

290
270
250
Slope of Trend Line for Entire Data Set = -1.2233

230
210
Slope of Trend Line 1950 to Present = -0.5873

190

Groundwater Elevation for Dynamic Slope Stability Analyses
170
150
1930

1940

1950

1960

1970

1980

1990

2000

Year
Well 3030F Groundwater Elevation

-

Secular Trend of Groundwater Elevation

Secular Trend Since 1950

2010

330.0

120

310.0

100

290.0

Slope of Trend Line = -0.9084

80

270.0

60

250.0

40

230.0

20

210.0

0

190.0

-20
Slope of Trend Line = -0.2685

170.0
150.0
1930

-40

1940

1950

1960

1970

1980

1990

2000

Year
Annual Average Groundwater Elevation

Approximate Cumulative Change in Water Storage

Secular Trend of Annual Average Groundwater Elevation

Secular Trend of Cumulative Change in Water Storage

-60
2010

Approximate Cumulative Change in Water Storage
(feet)

Groundwater Elevation (feet above mean sea level)

Figure 7 - Annual Average Groundwater Elevation and Approximate Cumulative Change in Storage with
Secular Trends as Measured in Well 3030F (Key Well)


Slide 9

GEOLOGICAL SOCIETY OF AMERICA 2003 ANNUAL MEETING
(ABSTRACT NO. 65672)
COMPARISON OF CLIMATIC TRENDS AND WELL RECORDS IN SOUTHERN
CALIFORNIA: IMPLICATIONS FOR GEOTECHNICAL AND HYDROGEOLOGICAL
PRACTICE
McMILLAN, Kent and VINCENT, Mark, Geo-Logic Associates, 1360 Valley Vista Dr., Diamond Bar, CA
91765, [email protected]
Geo-Logic Associates

ABSTRACT

Comparison of climate (rainfall) records from Los Angeles with water well records beginning in 1930 from the
nearby San Gabriel Valley, shows a correspondence in cycles of relative wet and dry on a decadal scale, as well
as similar longer term trends. The climate record of Los Angeles spans the termination of the Little Ice Age (+
1890 A.D.), and plots of cumulative departure from the long-term mean of the record suggest drying conditions
over the last century. With respect to the present day, these plots also suggest the last significant wet period from
the point of view of increasing groundwater recharge was the 1940’s. The well records noted above establish the
historic high groundwater levels in the area similarly occurred in the decade. Since then, however, rainfall and
groundwater level trends appear to be in decline. This raises the question of whether historic high groundwater
is an appropriate criterion in liquefaction analysis in all cases, since its use presumes the probable return of
historic conditions over the design life of a facility. Alternatively, the criterion might be based on trend analysis
of groundwater levels where there is also corresponding climatic data. Recharge trends are also analyzed by the
water-table fluctuation method.

This presentation is an outgrowth of recent work performed for the City of Irwindale, California, relating to the geotechnical
characterization of alluvial fan deposits derived from the San Gabriel River Canyon (Figure 1). We are indebted to Mr. Kwok Tam,
Director of Public Works, City of Irwindale, for his invaluable assistance, and suppor of this project. Sand and gravel mining of these
deposits to produce mineral aggregates is a major commercial activity in the City. Mining operations have excavated large, deep, steep
sided open pits, some of which have reached their permitted depths and are inactive, while others are nearing completion depths.
Reclamation of these pits is therefore of significant interest to the City, which is proceeding to assess the geotechnical and
hydrogeologic implications of post-mining reclamation.
The alluvial fan deposits constitute a significant unconfined and highly transmissive aquifer beneath the San Gabriel Valley, which
drains a major portion of the San Gabriel Mountains watershed (Figure 1). Groundwater lakes are a feature of most mining sites.
Groundwater levels fluctuate seasonally and with wet and dry climate cycles, and may vary locally depending on pumping from, or
recharge to the unconfined aquifer. Currently, groundwater levels observed in the pits are at or near historic low elevations.
All mining sites are included in seismic hazard zones defined by the California Geological Survey, because of concern for liquefaction
and seismic slope stability. As a result of this, an investigation of historical groundwater trends was conducted, seeking to identify the
historical high for the area. In the course of that analysis more general relationships between groundwater and climate trends became
apparent that are the subject of the following discussion.
Figure 1 is a Landsat, computer generated, color enhanced image of the San Gabriel Mountains watershed viewed from the west (note
the 2X vertical exaggeration). The image illustrates the spatial proximity of the mountain front, the adjacent San Gabriel Valley, and the
Los Angeles area, from which the climate records that form the basis of Figures 2, -3 and -4 originate. Also shown are the locations of
Wells 3030F, 4198A and 4198G near the mouth of San Gabriel Canyon.
Los Angeles is the nearest National Weather Service Forecast Office to the San Gabriel Mountains. It provides instrumental
measurements of precipitation, which began in 1877. Figure 2 presents the National Weather Service record plotted as annual
precipitation based on water-year. The mean of this record is 15.01 inches of precipitation annually. Periodic variation has been
partially removed by plotting the annual data as a 5-year running average, which highlights the cyclical, decadal pattern of Southern
California rainfall. A linear secular trend was also plotted, which suggests a long-term decline in annual precipitation of about 0.6-inch
over the period of record.
Figure 3 presents the instrumental record of Figure 2 expressed as cumulative departure from the mean in percent, based on the mean
for the period of 15.01 inches. Lynch (1931, 1948) and Thomas (1962) presented similar plots based on means computed for shorter
periods of record. The secular trend has a pronounced negative slope for this record reflecting precipitation deficits accumulated since
the 1940’s.

Lynch (1931) was able to extend the cumulative departure record back to the late 1770’s using “crop indices,” derived from records of
the Spanish Missions, as proxy for precipitation data. In reference to such records, Lynch (1931, p. 7) states: “For the purpose of
showing the fluctuations in rainfall, they are not much less valuable than the more recent rainfall measurements and show with great
fidelity the weather conditions at the missions.” In the absence of irrigation, crop yields are effectively a function of rainfall amount.
Crop indices were calculated using the relation: I = R/N, where, I = Crop Index for a given year, R = Harvest/Planting, and N =
Normal (i.e., mean) harvest return for the period 1800 to 1832.
To construct the cumulative departure plot of Figure 4 for the period 1770 to 2003 we combined Lynch’s (1931, Appendix D) crop
index data set and the later instrumental precipitation record. The mean utilized in Figure 4 (15.2 inches) was obtained by
extrapolating the linear secular trend from Figure 2 back to 1770, and taking its mean over the period 1770 to 2003. Cumulative
departure from this mean was then recalculated to form the curve shown in Figure 4.
If the longer cumulative departure record formed in this way is correct, it includes features not revealed in the shorter record use to
create Figure 3. For example, the cumulative departure from the mean has been positive in only about 75 years of the 233 years of
the record (approximately 32% of the time), and has been mainly in deficit since about 1845 despite significant positive fluctuations
in the 1890’s, 1910’s, and 1940’s.

Historical water level records for wells shown on Figure 5 were obtained from the County of Los Angeles Public Works Department
and the California Department of Water Resources. Three wells, 3030F, 4198A and 4198G, have historical records extending back to
the early 1930’s. Well 3030F, also known as the Key Well, possesses the most complete record (Figure 6). Wells 4198A and 4198G
are located approximately on surface contour with the Key Well. Records of these wells span the same length of time as the Key
Well, but contain data gaps, which are evident upon close inspection of the hydrographs. Nevertheless, the three well records are
highly similar and measure virtually the same water table contour. Records of many other wells in the area are more fragmentary but
when plotted against the Key Well are correlative with it over the fragmentary intervals.
It is evident from the well records that the historical high groundwater level in the vicinity, approximately elevation 325 feet, was
reached in the 1940’s, and corresponds in time to the region’s maximum historic cumulative departure from the mean precipitation,
which is also shown on Figure 5. The correlation of groundwater and precipitation fluctuations apparent in Figure 5 and the
similarity of negative secular trends (e.g., Figures 4 and 6) suggest that groundwater trends are most strongly reflecting a climatic
influence.

Average annual groundwater elevation for the Key Well is shown in Figure 7 and was derived from the periodic record of the well
shown in Figure 6. The annualized curve of Figure 7 was then analyzed by the water table fluctuation method described in Healy
(2002) to estimate net recharge. The change in storage was calculated as specific yield times the net change in head over each year,
assuming net surface-water and groundwater flow were zero, and evapotranspiration to be very small compared to specific yield.
Specific yield was estimate to be approximately 0.30 based on measurements of porosity in thin section and calculations from field
density tests; field moisture of the unsaturated zone was estimated to be less than 4% based on laboratory testing. The approximate
cumulative change in storage is also shown on Figure 7 and constitutes a net loss of 12 feet of storage over the period of record.
Comparison of the long-term rainfall (climate) record from Los Angeles with the well records illustrates a correspondence in cycles of
relative wet and dry and similarity of secular trends for the respective data sets. Since at least the 1940’s overall rainfall and
groundwater level trends appear to be in decline.
Analysis of liquefaction and seismic slope stability under assumed historically high groundwater conditions is clearly the most
conservative approach with respect to selecting design parameters. A more realistic design basis for the near-term is suggested by the
latest trends observed in Figure 6. Groundwater levels have not been higher that elevation 270 feet in about the last 20 years, and
have not exceeded 300 feet in about the last 35 years. In addition, the lowest groundwater levels were recorded in these intervals, and
at present are at the historically low level (<200 feet). It should also be considered that the higher groundwater levels are transient
and represent restricted time windows in which damaging ground motions might occur.
Relative to formulating parameters for liquefaction analysis and slope stability based on the above discussion, the following options
are under consideration by the City of Irwindale, various consultants, and interested professionals. The trend-line through the entire
data set in Figure 6 shows a steep decline in average groundwater level with time, but the trend is heavily influenced by high
groundwater levels between about 1935 and 1950. This relatively steep slope provides a poor fit for the time period since 1950, and
the fit also suggests an average groundwater elevation that appears artificially low for the present time (2003). Accordingly, a second
fit from 1950 to the present was performed, which appears to provide an unbiased fit for that time interval.
For static analysis, an appropriate groundwater level could be the sum of the 1950-2003 trend and the local deviations from this trend
associated with high rainfall years. At the present time (2003), this corresponds to a groundwater elevation of approximately 270 feet,
as shown in Figure 6; it is possible that this groundwater elevation will decrease with time in accordance with the long-term trend.
However, for design purposes a groundwater table of 270 feet could be used for static slope stability analyses. This is appropriate
because of the probability that another heavy rainfall year could occur in the very near future, before any additional negative trend in
average ground water elevation could occur.

For seismic analysis, an appropriate option could be the current average groundwater level as represented by the 1950-2003 trend line
at year 2003. It is believed that the probability of a major earthquake occurring coincident with a major rainfall excursion is
sufficiently small that the slope performance modeled by such a sequence would have a return period of occurrence that would be
significantly greater than that of the ground motion. The intent of using the average ground water level so defined is to avoid the
introduction of this bias in the seismic slope performance calculations. At the present time (2003), the average groundwater elevation
is approximately 225 feet. As noted above, it is possible that this groundwater elevation could decrease in the future, following the
1950-2003 trend. However, this option assumes no further decrease in the average, which appears to be conservative.

REFERENCES
Healy, R.W. and P.G. Cook (2002), Using groundwater levels to estimate recharge, Hydrogeology Journal, Vol. 10, No. 1, pp. 91-109.
Lynch, H.B. (1931), Rainfall and stream runoff in Southern California since 1769, consulting report for Metropolitan Water District of
Southern California, Los Angeles, 31p.

Lynch, H.B. (1948), Pacific Coast rainfall – Wide fluctuations in hundred years, Western Construction News, July 1948, pp. 76-80.
Thomas, H.E. (1962), The Meteorological Phenomenon of Drought in the Southwest, U.S. Geological Survey Professional Paper 372-A. 43p.

Figure 2 - Annual Precipitation in Los Angeles with 5 Year Running Average and Secular Trend
45

40

35

Precipitation (inches)

30

25

20

15

10

5
Slope of Trend Line = -0.0084
0
1870

1880

1890

1900

1910

1920

1930

1940

1950

1960

1970

1980

1990

2000

Year

Annual Precipitation

5 Year Running Average of Annual Precipitation

Secular Trend of Annual Precipitation

2010

Figure 3 - Precipitation in Los Angeles Expressed as Cumulative Departure from the Mean in Percent
with Secular Trend for the Instrumental Record
400

Rainfall Departure from the Mean in Percent

300

200

100

0

-100

-200
Mean = 15.01 inches
Slope of Trend Line = -1.9036
-300
1870

1880

1890

1900

1910

1920

1930

1940
Year

1950

1960

1970

1980

1990

2000

2010

Figure 4 - Precipitation in Los Angeles Expressed as Cumulative Departure fromthe Mean in Percent with Secular
Trend Derived fromCrop Yields and Precipitation Data
300

Cumulative Departure fromthe Mean in Percent

200

100

0

-100

-200
Mean = 15.2 inches
Slope of Trend Line = -1.0643

-300

-400
1760 1770 1780 1790 1800 1810 1820 1830 1840 1850 1860 1870 1880 1890 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 2010
Year

Based on Lynch's (1931, AppendixD) crop indexdata set

Based on Los Angeles precipitation data set

-

Secular Trend of Cumulative Departure fromthe Mean

350

300

300

200

250

100

200

0

150

-100

100

-200

50

-300

0
1930

1940

1950

1960

1970

1980

1990

2000

Date
Well 3030F

Well 4198A

Well 4198G

Precipitation Departure from the Mean

-400
2010

Cumulative Departure from the Mean in Percent

Groundwater Elevation (feet amsl)

Figure 5 - Combined Well Hydrographs for Wells 3030F, 4198A, and 4198G with Precipitation
Expressed as Cumulative Departure from the Mean in Percent from Figure 4

Figure 6 - Periodic Groundwater Elevation with Secular Trend Measured in Well 3030F (Key Well)

350

Groundwater Elevation (feet above mean sea level)

330
310

Groundwater Elevation for Static Slope Stability Analyses

290
270
250
Slope of Trend Line for Entire Data Set = -1.2233

230
210
Slope of Trend Line 1950 to Present = -0.5873

190

Groundwater Elevation for Dynamic Slope Stability Analyses
170
150
1930

1940

1950

1960

1970

1980

1990

2000

Year
Well 3030F Groundwater Elevation

-

Secular Trend of Groundwater Elevation

Secular Trend Since 1950

2010

330.0

120

310.0

100

290.0

Slope of Trend Line = -0.9084

80

270.0

60

250.0

40

230.0

20

210.0

0

190.0

-20
Slope of Trend Line = -0.2685

170.0
150.0
1930

-40

1940

1950

1960

1970

1980

1990

2000

Year
Annual Average Groundwater Elevation

Approximate Cumulative Change in Water Storage

Secular Trend of Annual Average Groundwater Elevation

Secular Trend of Cumulative Change in Water Storage

-60
2010

Approximate Cumulative Change in Water Storage
(feet)

Groundwater Elevation (feet above mean sea level)

Figure 7 - Annual Average Groundwater Elevation and Approximate Cumulative Change in Storage with
Secular Trends as Measured in Well 3030F (Key Well)


Slide 10

GEOLOGICAL SOCIETY OF AMERICA 2003 ANNUAL MEETING
(ABSTRACT NO. 65672)
COMPARISON OF CLIMATIC TRENDS AND WELL RECORDS IN SOUTHERN
CALIFORNIA: IMPLICATIONS FOR GEOTECHNICAL AND HYDROGEOLOGICAL
PRACTICE
McMILLAN, Kent and VINCENT, Mark, Geo-Logic Associates, 1360 Valley Vista Dr., Diamond Bar, CA
91765, [email protected]
Geo-Logic Associates

ABSTRACT

Comparison of climate (rainfall) records from Los Angeles with water well records beginning in 1930 from the
nearby San Gabriel Valley, shows a correspondence in cycles of relative wet and dry on a decadal scale, as well
as similar longer term trends. The climate record of Los Angeles spans the termination of the Little Ice Age (+
1890 A.D.), and plots of cumulative departure from the long-term mean of the record suggest drying conditions
over the last century. With respect to the present day, these plots also suggest the last significant wet period from
the point of view of increasing groundwater recharge was the 1940’s. The well records noted above establish the
historic high groundwater levels in the area similarly occurred in the decade. Since then, however, rainfall and
groundwater level trends appear to be in decline. This raises the question of whether historic high groundwater
is an appropriate criterion in liquefaction analysis in all cases, since its use presumes the probable return of
historic conditions over the design life of a facility. Alternatively, the criterion might be based on trend analysis
of groundwater levels where there is also corresponding climatic data. Recharge trends are also analyzed by the
water-table fluctuation method.

This presentation is an outgrowth of recent work performed for the City of Irwindale, California, relating to the geotechnical
characterization of alluvial fan deposits derived from the San Gabriel River Canyon (Figure 1). We are indebted to Mr. Kwok Tam,
Director of Public Works, City of Irwindale, for his invaluable assistance, and suppor of this project. Sand and gravel mining of these
deposits to produce mineral aggregates is a major commercial activity in the City. Mining operations have excavated large, deep, steep
sided open pits, some of which have reached their permitted depths and are inactive, while others are nearing completion depths.
Reclamation of these pits is therefore of significant interest to the City, which is proceeding to assess the geotechnical and
hydrogeologic implications of post-mining reclamation.
The alluvial fan deposits constitute a significant unconfined and highly transmissive aquifer beneath the San Gabriel Valley, which
drains a major portion of the San Gabriel Mountains watershed (Figure 1). Groundwater lakes are a feature of most mining sites.
Groundwater levels fluctuate seasonally and with wet and dry climate cycles, and may vary locally depending on pumping from, or
recharge to the unconfined aquifer. Currently, groundwater levels observed in the pits are at or near historic low elevations.
All mining sites are included in seismic hazard zones defined by the California Geological Survey, because of concern for liquefaction
and seismic slope stability. As a result of this, an investigation of historical groundwater trends was conducted, seeking to identify the
historical high for the area. In the course of that analysis more general relationships between groundwater and climate trends became
apparent that are the subject of the following discussion.
Figure 1 is a Landsat, computer generated, color enhanced image of the San Gabriel Mountains watershed viewed from the west (note
the 2X vertical exaggeration). The image illustrates the spatial proximity of the mountain front, the adjacent San Gabriel Valley, and the
Los Angeles area, from which the climate records that form the basis of Figures 2, -3 and -4 originate. Also shown are the locations of
Wells 3030F, 4198A and 4198G near the mouth of San Gabriel Canyon.
Los Angeles is the nearest National Weather Service Forecast Office to the San Gabriel Mountains. It provides instrumental
measurements of precipitation, which began in 1877. Figure 2 presents the National Weather Service record plotted as annual
precipitation based on water-year. The mean of this record is 15.01 inches of precipitation annually. Periodic variation has been
partially removed by plotting the annual data as a 5-year running average, which highlights the cyclical, decadal pattern of Southern
California rainfall. A linear secular trend was also plotted, which suggests a long-term decline in annual precipitation of about 0.6-inch
over the period of record.
Figure 3 presents the instrumental record of Figure 2 expressed as cumulative departure from the mean in percent, based on the mean
for the period of 15.01 inches. Lynch (1931, 1948) and Thomas (1962) presented similar plots based on means computed for shorter
periods of record. The secular trend has a pronounced negative slope for this record reflecting precipitation deficits accumulated since
the 1940’s.

Lynch (1931) was able to extend the cumulative departure record back to the late 1770’s using “crop indices,” derived from records of
the Spanish Missions, as proxy for precipitation data. In reference to such records, Lynch (1931, p. 7) states: “For the purpose of
showing the fluctuations in rainfall, they are not much less valuable than the more recent rainfall measurements and show with great
fidelity the weather conditions at the missions.” In the absence of irrigation, crop yields are effectively a function of rainfall amount.
Crop indices were calculated using the relation: I = R/N, where, I = Crop Index for a given year, R = Harvest/Planting, and N =
Normal (i.e., mean) harvest return for the period 1800 to 1832.
To construct the cumulative departure plot of Figure 4 for the period 1770 to 2003 we combined Lynch’s (1931, Appendix D) crop
index data set and the later instrumental precipitation record. The mean utilized in Figure 4 (15.2 inches) was obtained by
extrapolating the linear secular trend from Figure 2 back to 1770, and taking its mean over the period 1770 to 2003. Cumulative
departure from this mean was then recalculated to form the curve shown in Figure 4.
If the longer cumulative departure record formed in this way is correct, it includes features not revealed in the shorter record use to
create Figure 3. For example, the cumulative departure from the mean has been positive in only about 75 years of the 233 years of
the record (approximately 32% of the time), and has been mainly in deficit since about 1845 despite significant positive fluctuations
in the 1890’s, 1910’s, and 1940’s.

Historical water level records for wells shown on Figure 5 were obtained from the County of Los Angeles Public Works Department
and the California Department of Water Resources. Three wells, 3030F, 4198A and 4198G, have historical records extending back to
the early 1930’s. Well 3030F, also known as the Key Well, possesses the most complete record (Figure 6). Wells 4198A and 4198G
are located approximately on surface contour with the Key Well. Records of these wells span the same length of time as the Key
Well, but contain data gaps, which are evident upon close inspection of the hydrographs. Nevertheless, the three well records are
highly similar and measure virtually the same water table contour. Records of many other wells in the area are more fragmentary but
when plotted against the Key Well are correlative with it over the fragmentary intervals.
It is evident from the well records that the historical high groundwater level in the vicinity, approximately elevation 325 feet, was
reached in the 1940’s, and corresponds in time to the region’s maximum historic cumulative departure from the mean precipitation,
which is also shown on Figure 5. The correlation of groundwater and precipitation fluctuations apparent in Figure 5 and the
similarity of negative secular trends (e.g., Figures 4 and 6) suggest that groundwater trends are most strongly reflecting a climatic
influence.

Average annual groundwater elevation for the Key Well is shown in Figure 7 and was derived from the periodic record of the well
shown in Figure 6. The annualized curve of Figure 7 was then analyzed by the water table fluctuation method described in Healy
(2002) to estimate net recharge. The change in storage was calculated as specific yield times the net change in head over each year,
assuming net surface-water and groundwater flow were zero, and evapotranspiration to be very small compared to specific yield.
Specific yield was estimate to be approximately 0.30 based on measurements of porosity in thin section and calculations from field
density tests; field moisture of the unsaturated zone was estimated to be less than 4% based on laboratory testing. The approximate
cumulative change in storage is also shown on Figure 7 and constitutes a net loss of 12 feet of storage over the period of record.
Comparison of the long-term rainfall (climate) record from Los Angeles with the well records illustrates a correspondence in cycles of
relative wet and dry and similarity of secular trends for the respective data sets. Since at least the 1940’s overall rainfall and
groundwater level trends appear to be in decline.
Analysis of liquefaction and seismic slope stability under assumed historically high groundwater conditions is clearly the most
conservative approach with respect to selecting design parameters. A more realistic design basis for the near-term is suggested by the
latest trends observed in Figure 6. Groundwater levels have not been higher that elevation 270 feet in about the last 20 years, and
have not exceeded 300 feet in about the last 35 years. In addition, the lowest groundwater levels were recorded in these intervals, and
at present are at the historically low level (<200 feet). It should also be considered that the higher groundwater levels are transient
and represent restricted time windows in which damaging ground motions might occur.
Relative to formulating parameters for liquefaction analysis and slope stability based on the above discussion, the following options
are under consideration by the City of Irwindale, various consultants, and interested professionals. The trend-line through the entire
data set in Figure 6 shows a steep decline in average groundwater level with time, but the trend is heavily influenced by high
groundwater levels between about 1935 and 1950. This relatively steep slope provides a poor fit for the time period since 1950, and
the fit also suggests an average groundwater elevation that appears artificially low for the present time (2003). Accordingly, a second
fit from 1950 to the present was performed, which appears to provide an unbiased fit for that time interval.
For static analysis, an appropriate groundwater level could be the sum of the 1950-2003 trend and the local deviations from this trend
associated with high rainfall years. At the present time (2003), this corresponds to a groundwater elevation of approximately 270 feet,
as shown in Figure 6; it is possible that this groundwater elevation will decrease with time in accordance with the long-term trend.
However, for design purposes a groundwater table of 270 feet could be used for static slope stability analyses. This is appropriate
because of the probability that another heavy rainfall year could occur in the very near future, before any additional negative trend in
average ground water elevation could occur.

For seismic analysis, an appropriate option could be the current average groundwater level as represented by the 1950-2003 trend line
at year 2003. It is believed that the probability of a major earthquake occurring coincident with a major rainfall excursion is
sufficiently small that the slope performance modeled by such a sequence would have a return period of occurrence that would be
significantly greater than that of the ground motion. The intent of using the average ground water level so defined is to avoid the
introduction of this bias in the seismic slope performance calculations. At the present time (2003), the average groundwater elevation
is approximately 225 feet. As noted above, it is possible that this groundwater elevation could decrease in the future, following the
1950-2003 trend. However, this option assumes no further decrease in the average, which appears to be conservative.

REFERENCES
Healy, R.W. and P.G. Cook (2002), Using groundwater levels to estimate recharge, Hydrogeology Journal, Vol. 10, No. 1, pp. 91-109.
Lynch, H.B. (1931), Rainfall and stream runoff in Southern California since 1769, consulting report for Metropolitan Water District of
Southern California, Los Angeles, 31p.

Lynch, H.B. (1948), Pacific Coast rainfall – Wide fluctuations in hundred years, Western Construction News, July 1948, pp. 76-80.
Thomas, H.E. (1962), The Meteorological Phenomenon of Drought in the Southwest, U.S. Geological Survey Professional Paper 372-A. 43p.

Figure 2 - Annual Precipitation in Los Angeles with 5 Year Running Average and Secular Trend
45

40

35

Precipitation (inches)

30

25

20

15

10

5
Slope of Trend Line = -0.0084
0
1870

1880

1890

1900

1910

1920

1930

1940

1950

1960

1970

1980

1990

2000

Year

Annual Precipitation

5 Year Running Average of Annual Precipitation

Secular Trend of Annual Precipitation

2010

Figure 3 - Precipitation in Los Angeles Expressed as Cumulative Departure from the Mean in Percent
with Secular Trend for the Instrumental Record
400

Rainfall Departure from the Mean in Percent

300

200

100

0

-100

-200
Mean = 15.01 inches
Slope of Trend Line = -1.9036
-300
1870

1880

1890

1900

1910

1920

1930

1940
Year

1950

1960

1970

1980

1990

2000

2010

Figure 4 - Precipitation in Los Angeles Expressed as Cumulative Departure fromthe Mean in Percent with Secular
Trend Derived fromCrop Yields and Precipitation Data
300

Cumulative Departure fromthe Mean in Percent

200

100

0

-100

-200
Mean = 15.2 inches
Slope of Trend Line = -1.0643

-300

-400
1760 1770 1780 1790 1800 1810 1820 1830 1840 1850 1860 1870 1880 1890 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 2010
Year

Based on Lynch's (1931, AppendixD) crop indexdata set

Based on Los Angeles precipitation data set

-

Secular Trend of Cumulative Departure fromthe Mean

350

300

300

200

250

100

200

0

150

-100

100

-200

50

-300

0
1930

1940

1950

1960

1970

1980

1990

2000

Date
Well 3030F

Well 4198A

Well 4198G

Precipitation Departure from the Mean

-400
2010

Cumulative Departure from the Mean in Percent

Groundwater Elevation (feet amsl)

Figure 5 - Combined Well Hydrographs for Wells 3030F, 4198A, and 4198G with Precipitation
Expressed as Cumulative Departure from the Mean in Percent from Figure 4

Figure 6 - Periodic Groundwater Elevation with Secular Trend Measured in Well 3030F (Key Well)

350

Groundwater Elevation (feet above mean sea level)

330
310

Groundwater Elevation for Static Slope Stability Analyses

290
270
250
Slope of Trend Line for Entire Data Set = -1.2233

230
210
Slope of Trend Line 1950 to Present = -0.5873

190

Groundwater Elevation for Dynamic Slope Stability Analyses
170
150
1930

1940

1950

1960

1970

1980

1990

2000

Year
Well 3030F Groundwater Elevation

-

Secular Trend of Groundwater Elevation

Secular Trend Since 1950

2010

330.0

120

310.0

100

290.0

Slope of Trend Line = -0.9084

80

270.0

60

250.0

40

230.0

20

210.0

0

190.0

-20
Slope of Trend Line = -0.2685

170.0
150.0
1930

-40

1940

1950

1960

1970

1980

1990

2000

Year
Annual Average Groundwater Elevation

Approximate Cumulative Change in Water Storage

Secular Trend of Annual Average Groundwater Elevation

Secular Trend of Cumulative Change in Water Storage

-60
2010

Approximate Cumulative Change in Water Storage
(feet)

Groundwater Elevation (feet above mean sea level)

Figure 7 - Annual Average Groundwater Elevation and Approximate Cumulative Change in Storage with
Secular Trends as Measured in Well 3030F (Key Well)


Slide 11

GEOLOGICAL SOCIETY OF AMERICA 2003 ANNUAL MEETING
(ABSTRACT NO. 65672)
COMPARISON OF CLIMATIC TRENDS AND WELL RECORDS IN SOUTHERN
CALIFORNIA: IMPLICATIONS FOR GEOTECHNICAL AND HYDROGEOLOGICAL
PRACTICE
McMILLAN, Kent and VINCENT, Mark, Geo-Logic Associates, 1360 Valley Vista Dr., Diamond Bar, CA
91765, [email protected]
Geo-Logic Associates

ABSTRACT

Comparison of climate (rainfall) records from Los Angeles with water well records beginning in 1930 from the
nearby San Gabriel Valley, shows a correspondence in cycles of relative wet and dry on a decadal scale, as well
as similar longer term trends. The climate record of Los Angeles spans the termination of the Little Ice Age (+
1890 A.D.), and plots of cumulative departure from the long-term mean of the record suggest drying conditions
over the last century. With respect to the present day, these plots also suggest the last significant wet period from
the point of view of increasing groundwater recharge was the 1940’s. The well records noted above establish the
historic high groundwater levels in the area similarly occurred in the decade. Since then, however, rainfall and
groundwater level trends appear to be in decline. This raises the question of whether historic high groundwater
is an appropriate criterion in liquefaction analysis in all cases, since its use presumes the probable return of
historic conditions over the design life of a facility. Alternatively, the criterion might be based on trend analysis
of groundwater levels where there is also corresponding climatic data. Recharge trends are also analyzed by the
water-table fluctuation method.

This presentation is an outgrowth of recent work performed for the City of Irwindale, California, relating to the geotechnical
characterization of alluvial fan deposits derived from the San Gabriel River Canyon (Figure 1). We are indebted to Mr. Kwok Tam,
Director of Public Works, City of Irwindale, for his invaluable assistance, and suppor of this project. Sand and gravel mining of these
deposits to produce mineral aggregates is a major commercial activity in the City. Mining operations have excavated large, deep, steep
sided open pits, some of which have reached their permitted depths and are inactive, while others are nearing completion depths.
Reclamation of these pits is therefore of significant interest to the City, which is proceeding to assess the geotechnical and
hydrogeologic implications of post-mining reclamation.
The alluvial fan deposits constitute a significant unconfined and highly transmissive aquifer beneath the San Gabriel Valley, which
drains a major portion of the San Gabriel Mountains watershed (Figure 1). Groundwater lakes are a feature of most mining sites.
Groundwater levels fluctuate seasonally and with wet and dry climate cycles, and may vary locally depending on pumping from, or
recharge to the unconfined aquifer. Currently, groundwater levels observed in the pits are at or near historic low elevations.
All mining sites are included in seismic hazard zones defined by the California Geological Survey, because of concern for liquefaction
and seismic slope stability. As a result of this, an investigation of historical groundwater trends was conducted, seeking to identify the
historical high for the area. In the course of that analysis more general relationships between groundwater and climate trends became
apparent that are the subject of the following discussion.
Figure 1 is a Landsat, computer generated, color enhanced image of the San Gabriel Mountains watershed viewed from the west (note
the 2X vertical exaggeration). The image illustrates the spatial proximity of the mountain front, the adjacent San Gabriel Valley, and the
Los Angeles area, from which the climate records that form the basis of Figures 2, -3 and -4 originate. Also shown are the locations of
Wells 3030F, 4198A and 4198G near the mouth of San Gabriel Canyon.
Los Angeles is the nearest National Weather Service Forecast Office to the San Gabriel Mountains. It provides instrumental
measurements of precipitation, which began in 1877. Figure 2 presents the National Weather Service record plotted as annual
precipitation based on water-year. The mean of this record is 15.01 inches of precipitation annually. Periodic variation has been
partially removed by plotting the annual data as a 5-year running average, which highlights the cyclical, decadal pattern of Southern
California rainfall. A linear secular trend was also plotted, which suggests a long-term decline in annual precipitation of about 0.6-inch
over the period of record.
Figure 3 presents the instrumental record of Figure 2 expressed as cumulative departure from the mean in percent, based on the mean
for the period of 15.01 inches. Lynch (1931, 1948) and Thomas (1962) presented similar plots based on means computed for shorter
periods of record. The secular trend has a pronounced negative slope for this record reflecting precipitation deficits accumulated since
the 1940’s.

Lynch (1931) was able to extend the cumulative departure record back to the late 1770’s using “crop indices,” derived from records of
the Spanish Missions, as proxy for precipitation data. In reference to such records, Lynch (1931, p. 7) states: “For the purpose of
showing the fluctuations in rainfall, they are not much less valuable than the more recent rainfall measurements and show with great
fidelity the weather conditions at the missions.” In the absence of irrigation, crop yields are effectively a function of rainfall amount.
Crop indices were calculated using the relation: I = R/N, where, I = Crop Index for a given year, R = Harvest/Planting, and N =
Normal (i.e., mean) harvest return for the period 1800 to 1832.
To construct the cumulative departure plot of Figure 4 for the period 1770 to 2003 we combined Lynch’s (1931, Appendix D) crop
index data set and the later instrumental precipitation record. The mean utilized in Figure 4 (15.2 inches) was obtained by
extrapolating the linear secular trend from Figure 2 back to 1770, and taking its mean over the period 1770 to 2003. Cumulative
departure from this mean was then recalculated to form the curve shown in Figure 4.
If the longer cumulative departure record formed in this way is correct, it includes features not revealed in the shorter record use to
create Figure 3. For example, the cumulative departure from the mean has been positive in only about 75 years of the 233 years of
the record (approximately 32% of the time), and has been mainly in deficit since about 1845 despite significant positive fluctuations
in the 1890’s, 1910’s, and 1940’s.

Historical water level records for wells shown on Figure 5 were obtained from the County of Los Angeles Public Works Department
and the California Department of Water Resources. Three wells, 3030F, 4198A and 4198G, have historical records extending back to
the early 1930’s. Well 3030F, also known as the Key Well, possesses the most complete record (Figure 6). Wells 4198A and 4198G
are located approximately on surface contour with the Key Well. Records of these wells span the same length of time as the Key
Well, but contain data gaps, which are evident upon close inspection of the hydrographs. Nevertheless, the three well records are
highly similar and measure virtually the same water table contour. Records of many other wells in the area are more fragmentary but
when plotted against the Key Well are correlative with it over the fragmentary intervals.
It is evident from the well records that the historical high groundwater level in the vicinity, approximately elevation 325 feet, was
reached in the 1940’s, and corresponds in time to the region’s maximum historic cumulative departure from the mean precipitation,
which is also shown on Figure 5. The correlation of groundwater and precipitation fluctuations apparent in Figure 5 and the
similarity of negative secular trends (e.g., Figures 4 and 6) suggest that groundwater trends are most strongly reflecting a climatic
influence.

Average annual groundwater elevation for the Key Well is shown in Figure 7 and was derived from the periodic record of the well
shown in Figure 6. The annualized curve of Figure 7 was then analyzed by the water table fluctuation method described in Healy
(2002) to estimate net recharge. The change in storage was calculated as specific yield times the net change in head over each year,
assuming net surface-water and groundwater flow were zero, and evapotranspiration to be very small compared to specific yield.
Specific yield was estimate to be approximately 0.30 based on measurements of porosity in thin section and calculations from field
density tests; field moisture of the unsaturated zone was estimated to be less than 4% based on laboratory testing. The approximate
cumulative change in storage is also shown on Figure 7 and constitutes a net loss of 12 feet of storage over the period of record.
Comparison of the long-term rainfall (climate) record from Los Angeles with the well records illustrates a correspondence in cycles of
relative wet and dry and similarity of secular trends for the respective data sets. Since at least the 1940’s overall rainfall and
groundwater level trends appear to be in decline.
Analysis of liquefaction and seismic slope stability under assumed historically high groundwater conditions is clearly the most
conservative approach with respect to selecting design parameters. A more realistic design basis for the near-term is suggested by the
latest trends observed in Figure 6. Groundwater levels have not been higher that elevation 270 feet in about the last 20 years, and
have not exceeded 300 feet in about the last 35 years. In addition, the lowest groundwater levels were recorded in these intervals, and
at present are at the historically low level (<200 feet). It should also be considered that the higher groundwater levels are transient
and represent restricted time windows in which damaging ground motions might occur.
Relative to formulating parameters for liquefaction analysis and slope stability based on the above discussion, the following options
are under consideration by the City of Irwindale, various consultants, and interested professionals. The trend-line through the entire
data set in Figure 6 shows a steep decline in average groundwater level with time, but the trend is heavily influenced by high
groundwater levels between about 1935 and 1950. This relatively steep slope provides a poor fit for the time period since 1950, and
the fit also suggests an average groundwater elevation that appears artificially low for the present time (2003). Accordingly, a second
fit from 1950 to the present was performed, which appears to provide an unbiased fit for that time interval.
For static analysis, an appropriate groundwater level could be the sum of the 1950-2003 trend and the local deviations from this trend
associated with high rainfall years. At the present time (2003), this corresponds to a groundwater elevation of approximately 270 feet,
as shown in Figure 6; it is possible that this groundwater elevation will decrease with time in accordance with the long-term trend.
However, for design purposes a groundwater table of 270 feet could be used for static slope stability analyses. This is appropriate
because of the probability that another heavy rainfall year could occur in the very near future, before any additional negative trend in
average ground water elevation could occur.

For seismic analysis, an appropriate option could be the current average groundwater level as represented by the 1950-2003 trend line
at year 2003. It is believed that the probability of a major earthquake occurring coincident with a major rainfall excursion is
sufficiently small that the slope performance modeled by such a sequence would have a return period of occurrence that would be
significantly greater than that of the ground motion. The intent of using the average ground water level so defined is to avoid the
introduction of this bias in the seismic slope performance calculations. At the present time (2003), the average groundwater elevation
is approximately 225 feet. As noted above, it is possible that this groundwater elevation could decrease in the future, following the
1950-2003 trend. However, this option assumes no further decrease in the average, which appears to be conservative.

REFERENCES
Healy, R.W. and P.G. Cook (2002), Using groundwater levels to estimate recharge, Hydrogeology Journal, Vol. 10, No. 1, pp. 91-109.
Lynch, H.B. (1931), Rainfall and stream runoff in Southern California since 1769, consulting report for Metropolitan Water District of
Southern California, Los Angeles, 31p.

Lynch, H.B. (1948), Pacific Coast rainfall – Wide fluctuations in hundred years, Western Construction News, July 1948, pp. 76-80.
Thomas, H.E. (1962), The Meteorological Phenomenon of Drought in the Southwest, U.S. Geological Survey Professional Paper 372-A. 43p.

Figure 2 - Annual Precipitation in Los Angeles with 5 Year Running Average and Secular Trend
45

40

35

Precipitation (inches)

30

25

20

15

10

5
Slope of Trend Line = -0.0084
0
1870

1880

1890

1900

1910

1920

1930

1940

1950

1960

1970

1980

1990

2000

Year

Annual Precipitation

5 Year Running Average of Annual Precipitation

Secular Trend of Annual Precipitation

2010

Figure 3 - Precipitation in Los Angeles Expressed as Cumulative Departure from the Mean in Percent
with Secular Trend for the Instrumental Record
400

Rainfall Departure from the Mean in Percent

300

200

100

0

-100

-200
Mean = 15.01 inches
Slope of Trend Line = -1.9036
-300
1870

1880

1890

1900

1910

1920

1930

1940
Year

1950

1960

1970

1980

1990

2000

2010

Figure 4 - Precipitation in Los Angeles Expressed as Cumulative Departure fromthe Mean in Percent with Secular
Trend Derived fromCrop Yields and Precipitation Data
300

Cumulative Departure fromthe Mean in Percent

200

100

0

-100

-200
Mean = 15.2 inches
Slope of Trend Line = -1.0643

-300

-400
1760 1770 1780 1790 1800 1810 1820 1830 1840 1850 1860 1870 1880 1890 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 2010
Year

Based on Lynch's (1931, AppendixD) crop indexdata set

Based on Los Angeles precipitation data set

-

Secular Trend of Cumulative Departure fromthe Mean

350

300

300

200

250

100

200

0

150

-100

100

-200

50

-300

0
1930

1940

1950

1960

1970

1980

1990

2000

Date
Well 3030F

Well 4198A

Well 4198G

Precipitation Departure from the Mean

-400
2010

Cumulative Departure from the Mean in Percent

Groundwater Elevation (feet amsl)

Figure 5 - Combined Well Hydrographs for Wells 3030F, 4198A, and 4198G with Precipitation
Expressed as Cumulative Departure from the Mean in Percent from Figure 4

Figure 6 - Periodic Groundwater Elevation with Secular Trend Measured in Well 3030F (Key Well)

350

Groundwater Elevation (feet above mean sea level)

330
310

Groundwater Elevation for Static Slope Stability Analyses

290
270
250
Slope of Trend Line for Entire Data Set = -1.2233

230
210
Slope of Trend Line 1950 to Present = -0.5873

190

Groundwater Elevation for Dynamic Slope Stability Analyses
170
150
1930

1940

1950

1960

1970

1980

1990

2000

Year
Well 3030F Groundwater Elevation

-

Secular Trend of Groundwater Elevation

Secular Trend Since 1950

2010

330.0

120

310.0

100

290.0

Slope of Trend Line = -0.9084

80

270.0

60

250.0

40

230.0

20

210.0

0

190.0

-20
Slope of Trend Line = -0.2685

170.0
150.0
1930

-40

1940

1950

1960

1970

1980

1990

2000

Year
Annual Average Groundwater Elevation

Approximate Cumulative Change in Water Storage

Secular Trend of Annual Average Groundwater Elevation

Secular Trend of Cumulative Change in Water Storage

-60
2010

Approximate Cumulative Change in Water Storage
(feet)

Groundwater Elevation (feet above mean sea level)

Figure 7 - Annual Average Groundwater Elevation and Approximate Cumulative Change in Storage with
Secular Trends as Measured in Well 3030F (Key Well)


Slide 12

GEOLOGICAL SOCIETY OF AMERICA 2003 ANNUAL MEETING
(ABSTRACT NO. 65672)
COMPARISON OF CLIMATIC TRENDS AND WELL RECORDS IN SOUTHERN
CALIFORNIA: IMPLICATIONS FOR GEOTECHNICAL AND HYDROGEOLOGICAL
PRACTICE
McMILLAN, Kent and VINCENT, Mark, Geo-Logic Associates, 1360 Valley Vista Dr., Diamond Bar, CA
91765, [email protected]
Geo-Logic Associates

ABSTRACT

Comparison of climate (rainfall) records from Los Angeles with water well records beginning in 1930 from the
nearby San Gabriel Valley, shows a correspondence in cycles of relative wet and dry on a decadal scale, as well
as similar longer term trends. The climate record of Los Angeles spans the termination of the Little Ice Age (+
1890 A.D.), and plots of cumulative departure from the long-term mean of the record suggest drying conditions
over the last century. With respect to the present day, these plots also suggest the last significant wet period from
the point of view of increasing groundwater recharge was the 1940’s. The well records noted above establish the
historic high groundwater levels in the area similarly occurred in the decade. Since then, however, rainfall and
groundwater level trends appear to be in decline. This raises the question of whether historic high groundwater
is an appropriate criterion in liquefaction analysis in all cases, since its use presumes the probable return of
historic conditions over the design life of a facility. Alternatively, the criterion might be based on trend analysis
of groundwater levels where there is also corresponding climatic data. Recharge trends are also analyzed by the
water-table fluctuation method.

This presentation is an outgrowth of recent work performed for the City of Irwindale, California, relating to the geotechnical
characterization of alluvial fan deposits derived from the San Gabriel River Canyon (Figure 1). We are indebted to Mr. Kwok Tam,
Director of Public Works, City of Irwindale, for his invaluable assistance, and suppor of this project. Sand and gravel mining of these
deposits to produce mineral aggregates is a major commercial activity in the City. Mining operations have excavated large, deep, steep
sided open pits, some of which have reached their permitted depths and are inactive, while others are nearing completion depths.
Reclamation of these pits is therefore of significant interest to the City, which is proceeding to assess the geotechnical and
hydrogeologic implications of post-mining reclamation.
The alluvial fan deposits constitute a significant unconfined and highly transmissive aquifer beneath the San Gabriel Valley, which
drains a major portion of the San Gabriel Mountains watershed (Figure 1). Groundwater lakes are a feature of most mining sites.
Groundwater levels fluctuate seasonally and with wet and dry climate cycles, and may vary locally depending on pumping from, or
recharge to the unconfined aquifer. Currently, groundwater levels observed in the pits are at or near historic low elevations.
All mining sites are included in seismic hazard zones defined by the California Geological Survey, because of concern for liquefaction
and seismic slope stability. As a result of this, an investigation of historical groundwater trends was conducted, seeking to identify the
historical high for the area. In the course of that analysis more general relationships between groundwater and climate trends became
apparent that are the subject of the following discussion.
Figure 1 is a Landsat, computer generated, color enhanced image of the San Gabriel Mountains watershed viewed from the west (note
the 2X vertical exaggeration). The image illustrates the spatial proximity of the mountain front, the adjacent San Gabriel Valley, and the
Los Angeles area, from which the climate records that form the basis of Figures 2, -3 and -4 originate. Also shown are the locations of
Wells 3030F, 4198A and 4198G near the mouth of San Gabriel Canyon.
Los Angeles is the nearest National Weather Service Forecast Office to the San Gabriel Mountains. It provides instrumental
measurements of precipitation, which began in 1877. Figure 2 presents the National Weather Service record plotted as annual
precipitation based on water-year. The mean of this record is 15.01 inches of precipitation annually. Periodic variation has been
partially removed by plotting the annual data as a 5-year running average, which highlights the cyclical, decadal pattern of Southern
California rainfall. A linear secular trend was also plotted, which suggests a long-term decline in annual precipitation of about 0.6-inch
over the period of record.
Figure 3 presents the instrumental record of Figure 2 expressed as cumulative departure from the mean in percent, based on the mean
for the period of 15.01 inches. Lynch (1931, 1948) and Thomas (1962) presented similar plots based on means computed for shorter
periods of record. The secular trend has a pronounced negative slope for this record reflecting precipitation deficits accumulated since
the 1940’s.

Lynch (1931) was able to extend the cumulative departure record back to the late 1770’s using “crop indices,” derived from records of
the Spanish Missions, as proxy for precipitation data. In reference to such records, Lynch (1931, p. 7) states: “For the purpose of
showing the fluctuations in rainfall, they are not much less valuable than the more recent rainfall measurements and show with great
fidelity the weather conditions at the missions.” In the absence of irrigation, crop yields are effectively a function of rainfall amount.
Crop indices were calculated using the relation: I = R/N, where, I = Crop Index for a given year, R = Harvest/Planting, and N =
Normal (i.e., mean) harvest return for the period 1800 to 1832.
To construct the cumulative departure plot of Figure 4 for the period 1770 to 2003 we combined Lynch’s (1931, Appendix D) crop
index data set and the later instrumental precipitation record. The mean utilized in Figure 4 (15.2 inches) was obtained by
extrapolating the linear secular trend from Figure 2 back to 1770, and taking its mean over the period 1770 to 2003. Cumulative
departure from this mean was then recalculated to form the curve shown in Figure 4.
If the longer cumulative departure record formed in this way is correct, it includes features not revealed in the shorter record use to
create Figure 3. For example, the cumulative departure from the mean has been positive in only about 75 years of the 233 years of
the record (approximately 32% of the time), and has been mainly in deficit since about 1845 despite significant positive fluctuations
in the 1890’s, 1910’s, and 1940’s.

Historical water level records for wells shown on Figure 5 were obtained from the County of Los Angeles Public Works Department
and the California Department of Water Resources. Three wells, 3030F, 4198A and 4198G, have historical records extending back to
the early 1930’s. Well 3030F, also known as the Key Well, possesses the most complete record (Figure 6). Wells 4198A and 4198G
are located approximately on surface contour with the Key Well. Records of these wells span the same length of time as the Key
Well, but contain data gaps, which are evident upon close inspection of the hydrographs. Nevertheless, the three well records are
highly similar and measure virtually the same water table contour. Records of many other wells in the area are more fragmentary but
when plotted against the Key Well are correlative with it over the fragmentary intervals.
It is evident from the well records that the historical high groundwater level in the vicinity, approximately elevation 325 feet, was
reached in the 1940’s, and corresponds in time to the region’s maximum historic cumulative departure from the mean precipitation,
which is also shown on Figure 5. The correlation of groundwater and precipitation fluctuations apparent in Figure 5 and the
similarity of negative secular trends (e.g., Figures 4 and 6) suggest that groundwater trends are most strongly reflecting a climatic
influence.

Average annual groundwater elevation for the Key Well is shown in Figure 7 and was derived from the periodic record of the well
shown in Figure 6. The annualized curve of Figure 7 was then analyzed by the water table fluctuation method described in Healy
(2002) to estimate net recharge. The change in storage was calculated as specific yield times the net change in head over each year,
assuming net surface-water and groundwater flow were zero, and evapotranspiration to be very small compared to specific yield.
Specific yield was estimate to be approximately 0.30 based on measurements of porosity in thin section and calculations from field
density tests; field moisture of the unsaturated zone was estimated to be less than 4% based on laboratory testing. The approximate
cumulative change in storage is also shown on Figure 7 and constitutes a net loss of 12 feet of storage over the period of record.
Comparison of the long-term rainfall (climate) record from Los Angeles with the well records illustrates a correspondence in cycles of
relative wet and dry and similarity of secular trends for the respective data sets. Since at least the 1940’s overall rainfall and
groundwater level trends appear to be in decline.
Analysis of liquefaction and seismic slope stability under assumed historically high groundwater conditions is clearly the most
conservative approach with respect to selecting design parameters. A more realistic design basis for the near-term is suggested by the
latest trends observed in Figure 6. Groundwater levels have not been higher that elevation 270 feet in about the last 20 years, and
have not exceeded 300 feet in about the last 35 years. In addition, the lowest groundwater levels were recorded in these intervals, and
at present are at the historically low level (<200 feet). It should also be considered that the higher groundwater levels are transient
and represent restricted time windows in which damaging ground motions might occur.
Relative to formulating parameters for liquefaction analysis and slope stability based on the above discussion, the following options
are under consideration by the City of Irwindale, various consultants, and interested professionals. The trend-line through the entire
data set in Figure 6 shows a steep decline in average groundwater level with time, but the trend is heavily influenced by high
groundwater levels between about 1935 and 1950. This relatively steep slope provides a poor fit for the time period since 1950, and
the fit also suggests an average groundwater elevation that appears artificially low for the present time (2003). Accordingly, a second
fit from 1950 to the present was performed, which appears to provide an unbiased fit for that time interval.
For static analysis, an appropriate groundwater level could be the sum of the 1950-2003 trend and the local deviations from this trend
associated with high rainfall years. At the present time (2003), this corresponds to a groundwater elevation of approximately 270 feet,
as shown in Figure 6; it is possible that this groundwater elevation will decrease with time in accordance with the long-term trend.
However, for design purposes a groundwater table of 270 feet could be used for static slope stability analyses. This is appropriate
because of the probability that another heavy rainfall year could occur in the very near future, before any additional negative trend in
average ground water elevation could occur.

For seismic analysis, an appropriate option could be the current average groundwater level as represented by the 1950-2003 trend line
at year 2003. It is believed that the probability of a major earthquake occurring coincident with a major rainfall excursion is
sufficiently small that the slope performance modeled by such a sequence would have a return period of occurrence that would be
significantly greater than that of the ground motion. The intent of using the average ground water level so defined is to avoid the
introduction of this bias in the seismic slope performance calculations. At the present time (2003), the average groundwater elevation
is approximately 225 feet. As noted above, it is possible that this groundwater elevation could decrease in the future, following the
1950-2003 trend. However, this option assumes no further decrease in the average, which appears to be conservative.

REFERENCES
Healy, R.W. and P.G. Cook (2002), Using groundwater levels to estimate recharge, Hydrogeology Journal, Vol. 10, No. 1, pp. 91-109.
Lynch, H.B. (1931), Rainfall and stream runoff in Southern California since 1769, consulting report for Metropolitan Water District of
Southern California, Los Angeles, 31p.

Lynch, H.B. (1948), Pacific Coast rainfall – Wide fluctuations in hundred years, Western Construction News, July 1948, pp. 76-80.
Thomas, H.E. (1962), The Meteorological Phenomenon of Drought in the Southwest, U.S. Geological Survey Professional Paper 372-A. 43p.

Figure 2 - Annual Precipitation in Los Angeles with 5 Year Running Average and Secular Trend
45

40

35

Precipitation (inches)

30

25

20

15

10

5
Slope of Trend Line = -0.0084
0
1870

1880

1890

1900

1910

1920

1930

1940

1950

1960

1970

1980

1990

2000

Year

Annual Precipitation

5 Year Running Average of Annual Precipitation

Secular Trend of Annual Precipitation

2010

Figure 3 - Precipitation in Los Angeles Expressed as Cumulative Departure from the Mean in Percent
with Secular Trend for the Instrumental Record
400

Rainfall Departure from the Mean in Percent

300

200

100

0

-100

-200
Mean = 15.01 inches
Slope of Trend Line = -1.9036
-300
1870

1880

1890

1900

1910

1920

1930

1940
Year

1950

1960

1970

1980

1990

2000

2010

Figure 4 - Precipitation in Los Angeles Expressed as Cumulative Departure fromthe Mean in Percent with Secular
Trend Derived fromCrop Yields and Precipitation Data
300

Cumulative Departure fromthe Mean in Percent

200

100

0

-100

-200
Mean = 15.2 inches
Slope of Trend Line = -1.0643

-300

-400
1760 1770 1780 1790 1800 1810 1820 1830 1840 1850 1860 1870 1880 1890 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 2010
Year

Based on Lynch's (1931, AppendixD) crop indexdata set

Based on Los Angeles precipitation data set

-

Secular Trend of Cumulative Departure fromthe Mean

350

300

300

200

250

100

200

0

150

-100

100

-200

50

-300

0
1930

1940

1950

1960

1970

1980

1990

2000

Date
Well 3030F

Well 4198A

Well 4198G

Precipitation Departure from the Mean

-400
2010

Cumulative Departure from the Mean in Percent

Groundwater Elevation (feet amsl)

Figure 5 - Combined Well Hydrographs for Wells 3030F, 4198A, and 4198G with Precipitation
Expressed as Cumulative Departure from the Mean in Percent from Figure 4

Figure 6 - Periodic Groundwater Elevation with Secular Trend Measured in Well 3030F (Key Well)

350

Groundwater Elevation (feet above mean sea level)

330
310

Groundwater Elevation for Static Slope Stability Analyses

290
270
250
Slope of Trend Line for Entire Data Set = -1.2233

230
210
Slope of Trend Line 1950 to Present = -0.5873

190

Groundwater Elevation for Dynamic Slope Stability Analyses
170
150
1930

1940

1950

1960

1970

1980

1990

2000

Year
Well 3030F Groundwater Elevation

-

Secular Trend of Groundwater Elevation

Secular Trend Since 1950

2010

330.0

120

310.0

100

290.0

Slope of Trend Line = -0.9084

80

270.0

60

250.0

40

230.0

20

210.0

0

190.0

-20
Slope of Trend Line = -0.2685

170.0
150.0
1930

-40

1940

1950

1960

1970

1980

1990

2000

Year
Annual Average Groundwater Elevation

Approximate Cumulative Change in Water Storage

Secular Trend of Annual Average Groundwater Elevation

Secular Trend of Cumulative Change in Water Storage

-60
2010

Approximate Cumulative Change in Water Storage
(feet)

Groundwater Elevation (feet above mean sea level)

Figure 7 - Annual Average Groundwater Elevation and Approximate Cumulative Change in Storage with
Secular Trends as Measured in Well 3030F (Key Well)