Transcript United States drought in the 20th century: Application of

http://nm.water.usgs.gov/drought/photos.htm
Severity-area-duration analysis
of 20th century drought in the
conterminous United States
Climate Impacts Group Weekly Seminar
Oct. 18, 2004
Elizabeth Clark, Konstantinos Andreadis, Dennis
Lettenmaier
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Background
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Outline
Motivation
VIC Model
Drought Definitions
Technique of drought classification
Preliminary results for continental U.S.
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1930s, 1950s, current drought
Comparison of most severe agricultural and hydrologic
droughts
Implications for water managers
Future research
Motivation
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Recent availability of precipitation and
temperature data make model simulation of
hydrologic conditions for long periods
possible.
Such simulations provide a spatially and
temporally continuous data set.
They also allow us to investigate historical
droughts in new ways.
VIC Model
Soil Parameterization
• 3 soil layers
• Variable infiltration curve in upper
layer partitions subsurface and quick
storm response
• Gravity-driven
vertical soil drainage
• Non-linear baseflow
drainage from lowest
layer
Maurer et al., 2002
6 Sample Hydrographs
Good agreement of
•Seasonal cycle
•Low Flows
Model •Peak Flows
Obs.
Maurer et al., 2002
Comparison with Illinois Soil Moisture
19 observing stations are compared to the 17 1/8º modeled grid
cells that contain the observation points.
Moisture Level
Moisture Flux
Variability
Obs.
Model
Persistence
Drought Definitions
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Meteorological Drought
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Agricultural Drought
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Soil Moisture
Hydrologic Drought
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Precipitation and
Temperature
Streamflow/Runoff
Socioeconomic Drought
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Disparity between supply
and demand
http://nm.water.usgs.gov/drought/photos.htm
Palmer Drought Severity Index
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PDSI : measures meteorological drought using a
method that accounts for precipitation,
evaporation, and soil moisture conditions.
Dai & Trenberth (2004) find correlation between
annual PDSI and streamflow and correlation
between PDSI and soil moisture during warm
season. Snow interferes with soil moisture
calculations.
Despite standardization, dependence on
termination criteria results in questionable
distribution of severe droughts.
PDSI-based studies
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Cook et al. (1999) used tree-ring
chronologies to reconstruct US droughts
from 1700-1978
EOFs used for regionalization purposes
Examined PDSI signal over those regions
“Dust Bowl” dominated the entire period
Other notable droughts: 1950, 1965, 1977.
PDSI-based studies
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Dai et al. (1998) used a monthly PDSI
dataset from 1860-1995
2.5o x 2.5o grid over the globe
Major droughts identified: 1930s, 1950s and
1988
Correlation between PDSI and ENSO signals
Increase in percentage areas of severe
drought during the last 2-3 decades, over
many ENSO-sensitive regions
Drought spatial analysis from
other studies
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Most other studies have used station data
Pre-defined climate regions
Statistical methods such as
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Correlation analysis, (Oladipo 1986)
Empirical Orthogonal Functions (EOF), (Cook
et al. 1999, Hisdal and Tallaksen 2003)
Simulation provides continuous spatial and
temporal mapping of hydrologic variables
Hydrologic Simulations
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Based on physical processes
Not dependent on scattered or temporally
disjoint station data
Allow for direct analysis of parameters of
interest, i.e. runoff and soil moisture
Use of percentile values standardizes over
heterogeneous regions and is independent
of initialization and termination criteria
How can we use information from
long term hydrologic model
simulations to synthesize the
following drought characteristics:
severity, intensity, extent, and
duration?
Severity-Area-Duration Analysis
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Based on the Depth-Area-Duration technique
from probable maximum precipitation analysis
Replace depth with measure of drought
severity
S=(1-ΣP/t)
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S=severity, ΣP = total percentile (soil moisture or
runoff), t = duration
How do we define drought extent?
Severity
Droughts change over time!
SAD Construction
Modified from WMO (1960) computational
method of DAD analysis
1. Rank cells by severity & identify potential
drought centers
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2. Search 3x3 neighborhood of drought
center
3. Average severities & add areas
4. Output severity and area at specified
area intervals
5. Compare the severity at ~25,000 km2 for
each potential drought center and select
center with maximum severity
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Methodology
VIC model
output
Threshold
Weibull
Total column soil
percentiles
moisture
Runoff
Temporal
contiguity
20th percentile and
lower soil moisture
30th percentile and
lower runoff
Spatial
contiguity
Final drought
and
subdrought
classification
Severity-AreaDuration
Highest
severities
Envelope
curve for
each duration
Initial
drought
classification
SAD curves
for each
event
1930s Drought Soil Moisture
Soil moisture-defined drought
80
Percent severity
100
1930s Drought Runoff
Runoff-defined drought
80
Percent severity
100
1950s Drought Soil Moisture
Soil moisture-defined drought
80
Percent severity
100
1950s Drought Runoff
Runoff-defined drought
80
Percent severity
100
Current Drought Soil Moisture
Soil moisture-defined drought
80
Percent severity
100
Current Drought Runoff
Runoff-defined drought
80
Percent severity
100
Soil Moisture
July 2002July 2002
Apr 1934June 1934
3 month
6 month
1 year
2 years
Feb 1955Feb 1956
4 years
8 years
Feb 1977Feb 1977
Jun 2002Jun 2003
Runoff
3 month
6 month
1 year
2 years
Nov 1953Nov 1956
4 years
8 years
Dec 1932Dec 1939
Rising Temperatures and
Declining Streamflow in West US
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Higher temperatures are resulting in
earlier snow melts, up to 20-30 days
earlier (Pagano et al., 2004).
Upper Colorado River basin reported to
be experiencing worst streamflow
deficit in 80 years & 7th worse in past
500 years (Piechota et al., 2004).
Implications for Water Management
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Similar to Depth-Area-Duration analysis,
Severity-Area-Duration analysis provides a
basis for a sort of “design drought”
estimation.
This estimates an upper bound for
anticipated drought severities.
Future Research
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Real-time applications!
Figure from Andy Wood.
Conclusions
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The most severe historical US droughts
occurred during the 1930s and 1950s.
The current drought ranks among the
most severe droughts, especially when
averaged over smaller areas.
Future research promises to provide water
managers with new tools for real-time
drought forecasting.
Acknowledgements
Thanks to Alan Hamlet, Andy Wood, and
HyoSeok Park.
http://www.usask.ca/geology/classes/geol243/243notes/sedlab3.html