Transcript Rainfall-runoff relationships Methods of assessment

3. Rainfall-runoff
relationships
Methods of assessment
Factors that influence surface runoff
Physical-geographic factors (natural, non-manageable)
Climatic (meteorological):

Precipitation
 Type of precipitation (rain, snow
etc.)
 The rate (amount) and intensity
[initially less runoff, but ! melting season],
sleet,
 Duration of rainfall
 Direction of storm movement
 Distribution of rainfall over the drainage basin


Previous weather (e.g. precipitation that occurred earlier and
resulting soil moisture)
Time of year/season
– Summer - evapotranspiration rates higher, photosynthesis in plants - at a
maximum

Other conditions that affect evapotranspiration - temperature, wind,
relative humidity…
Dangerous rainfalls

Summer storms (short time, high intensity)
 are significant for smaller watersheds (outflow volume, peak)
 flash floods
 enhance erosion and transport processes in the watershed (bad quality
of water in streams, smaller potential flow in channels and smaller
volumes of reservoirs)

Regional rainfalls (long duration of rainfall)
 high amount of water (area + time period)
 regional floods

Spring rainfalls (snow cover present)
 + sharp rise in temperature  quick thaw  sharp increase in overland
flow
 + frozen ground underneath the snow  rapid flow on ice  increasing
discharge
Physical-geographic factors
(natural, non-manageable)
Characteristics of watershed
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Watershed area – volume and culmination of
total runoff
Shape of watershed – time of concentration to
the outlet
Elevation
Slope of the area
– The steeper the slopes, the lower the rate of
infiltration and faster the rate of run-off when the
soil is saturated (saturated overland flow)
– Strong influence on erosion and transport
processes

Length of slope and length of valley – lag time
to the valley and to the outlet
Physical-geographic factors
(natural, non-manageable)
Geological and soil characteristics
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Bedrock permeability - Run-off will occur quickly where
impermeable rocks are exposed at the surface or quickly when
they underlay soils (limited amount of infiltration).
Soil permeability - Soils with large amounts of clay do absorb
moisture but only very slowly - therefore their permeability is low.
Thickness - The deeper the soil the more water can be absorbed.
Infiltration capacity - Soils which have larger particle sizes (e.g
those derived from the weathering of sandstones) have larger
infiltration capacities.
The infiltration capacity is among others dependent on the porosity
of a soil which determines the water storage capacity and affects
the resistance of water to flow into deeper layers.
Initial conditions (e.g. the degree of saturation of the soil and
aquifers)
Anthropogenic factors (manageable)
Land use (e.g. agriculture, urban development, forestry operations)


Direct influence on retention capacity, hydrologic balance of watershed a the
volume of direct runoff)
Measures:
– Increasing afforestation
– Increasing meadow area at the expense of arable land
– Limit of impervious surface
– Prefer pervious road construction (forest and field)
Vegetation type and cover
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Interception reducing initial surface flow
Evapotranspiration
Infiltration (the root systems)
Velocity of overland flow
Preferable vegetation cover to increasing retention capacity: forests a meadows
a close-seeded a grains a row crops
Agriculture
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Irrigation and drainage ditches increasing the speed of water transfer
contour tillage
Tillage on wet land compresses the subsoil - creating a "plough pan" a
decreasing water holding, infiltration and increasing run-off/erosion.
Human activities - development and
urbanization:
imperviousness - natural landscape is replaced
by impervious surfaces (roads, buildings, parking
lots) - reduce infiltration and accelerate runoff to
ditches and streams
 removal of vegetation and soil
 constructing drainage networks and
underground sewer increase runoff volumes
and shorten runoff time into streams -> the
peak discharge, volume, and frequency of floods
increase in nearby streams

River network


Routing and detention
Drainage density
– This ratio is the length of river course per area of land. The larger the amount of
streams and rivers per area the shorter distance water has to flow and the faster the
rate of response.
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River conditions
Surface depressions, marchlands, wetlands

Storage , hydrologic balance
Reservoirs, ponds etc.
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Important storage volumes a retention capacity
Ecological balance - ecosystems
Prevent or delay runoff from continuing downstream
Decrease the peak discharge
Protection of the low-lying land downstream.
Water extracted for industry, irrigation, and domestic use, also reduce
discharge.
! reservoir aggradation (storage volume) - erosion control measures
Dry reservoirs – polders a temporary storage during higher discharge
(floods) a usually used as meadows
Precipitation – spatial
variability

is measured in gauges or by radar
Representation of precipitation depth
spatial variability in a catchment:

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Arithmetic mean
Polygons (Thiessen)
Isohyets
Isohyet or isohyetal line

line joining points of equal
precipitation on a map.
Isohyetal map
P
1
 Pj .Aj
A j
Precipitation – temporal variability

A hyetograph
– a graphical representation of the amount of precipitation that falls
through time
– is used in hydrology to illustrate the temporal variability of
precipitation
Characteristics
– Intensity (depth / time interval)
20.0
20.0
 maximum
 average
17.4
17.4
18.0
18.0
100.0
81.8
14.0
14.0
80.0
12.0
12.0
10.0
10.0
60.0
8.0
8.0
40.0
6.0
6.0
4.0
4.0
3.2
3.2
20.0
2.0
2.0
0.0
0.0
0.0
Time
Time tt[min]
[min]
Cumulative rainfall intensity [mm]
16.0
16.0
Rainfall
Rainfall intensity [mm/10min]
– Cumulative intensity
– Maximum depth
– Time period
120.0
Intensity
Intensity
Cumulative intensity
Average
Average
15
:5
0:
16 0 0
:0
0: 0
00
16
:1
0:
0
16
00
:2
0:
16 000
:3 0
:0
0
16
:4
0:
0
00
16
:5
0: 0
00
17
:0
0:
0
00
17
:1
0:
1 :0000
7:
20
:
1 7 000
:3
0:
17 000
:4
0: 0
00
17
:5
0:
0
00
18
:0
0:
18 000
:1
0
:
1 8 000
:2
0:
00
1
8: 0
30
: 00
0
18
:4
0:
0
0
18
:5
0:
19 000
:0
0
:
1 9 000
:1
0::0
00
1
9: 0
20
::000
0
19
:3
0:
0
19
00
:4
0:
19 000
:5
0:
2 0 000
:0
0::0
000

Extreme of rain events
I
[mm/min]
Return
period
[years]
Statistic analysis of maximum rainfall events
IDF curves:
relations between intensities, duration and
frequency of rain events



•
•
Intensity – I (mm/min)
Duration – D (min)
Frequency – F (1/years)
probability of different rain event intensities for
different durations (5, 10, 15, 30 … minutes, …
24 hours)
an each curve represents a certain frequency of
occurrence or a certain return period expressed
in terms of years.
N value:
•
•
•
the average over a number of years of
observation
Value that is exceeded ones per N years (return
period)
Rainfall depth (mm) of certain duration (e.g. 24
hours) whose probability of appearance is 1/N =
Frequency (1/years)
D [min]
100
90
80
70
H1d,N [mm]
•
60
50
40
30
20
10
0
0
20
40
60
80
100
120
N [years]
N [years]
2
10
20
50
100
H1d,N
[mm]
36.3
60.6
70.4
82.6
92.1
Hydrograph

hydro- water, -graph chart

plots the discharge of a river over time
a representation of how a watershed responds to rainfall.
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Characteristics:
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Peak discharge Qmax (m3.s-1)
– The highest point when there is the greatest amount of water in the
river.
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Time of peak (min)
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Volume V (m3)
Rising limb
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– The part up to the point of peak
discharge.
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Falling limb
– The part after the peak discharge.
Extreme discharge
25
Extreme values - the average over a
Flood frequency curve
20
Maximum (N value)
QN (m3/s)
number of years of observation
QN(m3/s):
• Value that is exceeded ones per N years
(return period) - statistically
15
10
0
• Discharge
whose probability of
appearance is 1/N = Frequency (1/years)
(m3/s)
0
60
80
100
120
N (years)
1
2
QN (m3/s)
6
8
5
10
20
10.9 13.2 15.6
50
100
18.8 21.5
Discharge Qm [l/s]
500
Minimal Qm(l/s):
• Value (discharge) that is exceeded m-days
per a year – statistically
Qm [l/s]
40
600
• important for assessing risk for highly
unusual events, such as 100-year floods.
m [day]
20
N (years)
• Are required for the design of dam,
spillways, nuclear power stations, major
bridges…
• Important for dry seasons, ground water
storage
probability distributions
5
400
300
200
100
0
0
100
200
300
Time m [day]
30
60
90
120
150
180
210
240
270
300
330
335
364
507
350
270
218
180
150
125
104
85
68
50
47
35
The surface runoff process
Rainfall excess = rainfall - losses =
= rainfall - interception - surface retention - infiltration
Direct runoff = surface runoff + interflow
Interception
+ retention
Direct
runoff
Rainfall event – flood Český Krumlov
Hydrograph 11.-13.8. 2002 (Polečnice catchment)
220
0
Rainfall Hs=133,2mm
200
outlet of Polečnice
160
Outlet Chvalšinský stream
1
Outlet Polečnice stream above
the confluence
1.5
3
140
120
2
100
2.5
80
3
60
3.5
40
4
20
0
12:45
4.5
20:45
4:45
12:45
20:45
Time (h:min)
4:45
12:45
20:45
Intensity of rainfall (mm/10min)
The confluence
180
Discharge (m /s)
0.5
Curve Number Method (SCS-CN)
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A method for simulating rainfall-runoff processes
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Developed by SCS (Soil Conservation Service – 1972)
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Widely used and efficient method
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Determines the approximate amount of direct runoff
from a rainfall event in a particular area.

Used for small catchments
CN
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An empirical parameter for predicting direct runoff.

Developed from empirical analysis of runoff from small
catchments and hillslope plots monitored by the SCS.
Curve number (CN) depends on…

Soil
– 4 classes (A – D) according to infiltration rate

Cover and hydrologic condition of the land surface
– Various types of vegetation and crops, land treatments
and crop practices, paving and urbanization

Antecedent wetness
– 3 classes of antecedent moisture condition (AMC) – dry,
average, wet
The bigger CN the higher runoff volume
CN catalog
Land
ID
Land name
AMC I
AMC II
AMC III
SCS Soil type SCS Soil type SCS Soil type
A B C D A B C D A B C D
3
"Paved parking lots, roofs, driveways,
etc. (excl. ROW)"
94 94 94 94 98 98 98 98 99 99 99 99
7
"Dirt streets"
53 66 73 76 72 82 87 89 86 92 95 96
18
"Developing urban area, newly graded
(no vegetation)"
59 72 80 85 77 86 91 94 89 94 97 98
22
"Meadow - continuous grass, no
grazing"
27
"Woods-grass combination - orchard Fair"
25 45 58 66 43 65 76 82 63 82 89 92
31
"Woods - Good"
15 35 51 59 30 55 70 77 50 74 85 89
37
"Cultivated agr. - row - straight
row(SR) - Good"
47 60 70 76 67 78 85 89 83 90 94 96
53
"Cultivated agr. - small grain - C Good"
41 54 64 68 61 73 81 84 78 87 92 93
61
"Cultivated agr. - close-seeded - SR Good"
38 53 64 70 58 72 81 85 76 86 92 94
15 38 52 60 30 58 71 78 50 76 86 90
The bigger CN the higher runoff volume
18
Volume of runoff
Oph  1000 * Ho * F
Ho
2

H s  0.2 * A

H s  0.8 * A
 1000

A  25.4 * 
 10
 CN

for
H s  0.2 * A
Oph – volume of direct
runoff in m3,
Ho – depth of runoff
in mm,
F – watershed area
in km2,
Hs – depth of rainfall
in mm,
A – potential retention in
mm
0.2*A – initial abstraction
in mm
Hs - 0.2*A – effective
storm rainfall in mm
CN – curve number
CN method
Rainfall depth
Volume of
direct runoff
• Hydrologic soil
groups
• Land use
• AMC

Temporal
distribution of runoff
(hydrograph)
Unit Hydrograph - UH
UH is a hypothetical response of a catchment to
unit rainfall excess - empirical
Original concept - Leroy Sherman (1932)
It’s been developed since and applied in many
versions.
The use: hydrologic models – known rainfall depth
→ runoff volume → temporal distribution
Unit hydrograph method


Response function
– input - rainfall excess (unit volume, constant
intensity, uniform distribution over a
catchment)
– output – direct runoff
– assumptions – principle superposition or
linearity and temporal invariance
Superposition
– Output rate is dependent linearly on input rate
– Temporal distribution is not influenced by input rate
– Result output equals sum of outputs resulting from unit inputs

Temporal invariance
– Starting time of input has no influence on rate or temporal
distribution of output
Unit hydrograph – 1 pulse
Q(t)
Q(t)=Pef.u(t)= u(t)
..pro Pef =1
Q(t)=Pef.u(t)= I. Dt.u(t)
..pro Pef ≠ 1
t [h]
Composite UH
Q(t)
t [h]
Composition of
runoff hydrograph
from unit
hydrographs
Thank you for your attention