Excess Rainfall Reading for today’s material: Sections 5.3- 5.7 Slides prepared by V.M. Merwade Quote for today (contributed by Tyler Jantzen) "How many times it thundered before Franklin took the hint! Nature is always hinting at us. It hints over and over again. And suddenly we take the hint.“ Robert Frost
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Excess Rainfall Reading for today’s material: Sections 5.3-5.7 Slides prepared by V.M. Merwade Quote for today (contributed by Tyler Jantzen) "How many.
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Excess Rainfall Reading for today’s material:
Sections 5.3-5.7
Slides prepared by V.M. Merwade
Quote for today (contributed by Tyler Jantzen)
"How many times it thundered before Franklin took the hint! Nature is always hinting at us. It hints over and over again. And suddenly we take the hint.“
Robert Frost
Excess rainfall
• Rainfall that is neither retained on the land surface nor infiltrated into the soil
• Graph of excess rainfall versus time is called excess rainfall hyetograph
abstractions• Abstractions/losses – difference between total
rainfall hyetograph and excess rainfall hyetograph
-index
-index: Constant rate of abstraction yielding excess rainfall hyetograph with depth equal to depth of direct runoff
• Used to compute excess rainfall hyetograph when observed rainfall and streamflow data are available
-index method
M
mmd tRr
1
• Goal: pick t, and adjust value of M to satisfy the equation
• Steps1. Estimate baseflow2. DRH = streamflow
hydrograph – baseflow3. Compute rd, rd =
Vd/watershed area4. Adjust M until you get a
satisfactory value of 5. ERH = Rm - t
interval time
runoffdriecttongcontributi
rainfallofintervals#
indexPhi
rainfall observed
runoffdirect ofdepth
t
M
R
r
m
d
ExampleTime Observed
Rain Flow
in cfs
8:30 203
9:00 0.15 246
9:30 0.26 283
10:00 1.33 828
10:30 2.2 2323
11:00 0.2 5697
11:30 0.09 9531
12:00 11025
12:30 8234
1:00 4321
1:30 2246
2:00 1802
2:30 1230
3:00 713
3:30 394
4:00 354
4:30 303
0
2000
4000
6000
8000
10000
12000
7:30 PM 9:00 PM 10:30 PM 12:00 AM 1:30 AM 3:00 AM 4:30 AM 6:00 AM
Time
Str
eam
flo
w (
cfs)
0
0.5
1
1.5
2
2.5
No direct runoff until after 9:30And little precip after 11:00
Have precipitation and streamflow data, need to estimate losses
Basin area A = 7.03 mi2
Example (Cont.)
• Estimate baseflow (straight line method)– Constant = 400 cfs
0
2000
4000
6000
8000
10000
12000
7:30 PM 9:00 PM 10:30 PM 12:00 AM 1:30 AM 3:00 AM 4:30 AM 6:00 AM
Time
Str
eam
flo
w (
cfs)
baseflow
Example (Cont.)
• Calculate Direct Runoff Hydrograph– Subtract 400 cfs
Total = 43,550 cfs
Example (Cont.)
• Compute volume of direct runoff
37
3
11
1
11
1
ft10*7.839
/sft 550,43*hr5.0*s/hr3600
n
nn
nd QttQV
• Compute depth of direct runoff
in80.4
ft4.0
ft5280*mi03.7
ft10*7.83922
37
A
Vr dd
Example (Cont.)
• Neglect all precipitation intervals that occur before the onset of direct runoff (before 9:30)
• Select Rm as the precipitation values in the 1.5 hour period from 10:00 – 11:30
)5.0*3*08.220.233.1(80.41
M
mmd tRr
in27.0t
in54.0
in80.4dr
Example (Cont.)
0
2000
4000
6000
8000
10000
12000
7:30 PM 9:00 PM 10:30 PM 12:00 AM 1:30 AM 3:00 AM 4:30 AM 6:00 AM
Time
Stre
amflo
w (c
fs)
0
0.5
1
1.5
2
2.5
t=0.27
SCS method• Soil conservation service (SCS) method is an
experimentally derived method to determine rainfall excess using information about soils, vegetative cover, hydrologic condition and antecedent moisture conditions
• The method is based on the simple relationship that Pe = P - Fa – Ia
PPee is runoff volume, P is is runoff volume, P is precipitation volume, Fprecipitation volume, Faa is continuing is continuing abstraction, and Iabstraction, and Iaa is the is the sum of initial losses sum of initial losses (depression storage, (depression storage, interception, ET)interception, ET)