A storm with a rainfall pattern as shown below occurred over your watershed and you are
asked to determine the runoff from the rainstorm. The rainfall pattern is shown: 30 min of
rainfall, 30 minutes of no rain (infiltration recovery), followed by 60 minutes of rain at different
amounts for each 30 min period. The infiltration rates are sketched in. They are not accurately
plotted and are diagrammatic only.
Depth, in
2
0.5
Data are as follows:
0
1.5 in
Rainfall
Item
Infiltration - not to scale
Rainfall period duration, each, min
fo, in/hr
fc, in/hr
K, hr-¹(decay)
Kr, hr¹ (recovery)
Partial Recovery
Interception Losses in 1st rainfall period only, in
Depression Storage in 1st rainfall period only, in
0.75 in
Rainfall
Time, minutes
90
2.0 in
Rainfall
120
Infiltration Rate, in/he
Value
0.10
0.15
30
1.50
0.50
3
2
a) Calculate the actual infiltration rates, f, in/hr, at each 30 minute period, taking into
account recovery and decay
b) Calculate the amount of infiltration that occurred in each 30 minute rainfall period, F, in.
c) Taking into account the interception and depression storage losses and the infiltration
from b), determine the direct runoff in each of the 3 rainfall periods, in.
Fig: 1