Encyclopedia of Environmental Science and Engineering, Volume I and II

468 HYDROLOGY

has been greatly reduced, perhaps almost to the point where

hygroscopic moisture alone remains. When rain fi rst starts

much is intercepted by the trees and vegetation and this inter-

ception storage is lost by evaporation after the storm. Rain

reaching the soil infi ltrates into pervious surfaces and begins

to satisfy soil moisture defi cits. As soil moisture levels rise,

water percolates downward toward the fully saturated water

table level. If the rain is heavy enough, the water supply may

exceed the vertical percolation rate and water then starts to

fl ow laterally in the superfi cial soil layers toward the stream

0 50 100 150 200 250 300 350 400

Area (square miles)

1 – HOUR

3 – HOUR

24 – HOUR

6 – HOUR

(^30)
(^) –
MI
NU
TE
S
50
60
70
80
90
100
Percent of point rainfall for given area
Duration
Minutes Hours
30 60 2 3 4 6 8 12 24
1
2
3
4
5
6
7
8
9
10
Depth - duration - frequency curves, 41°N 91°W
Rainfall, in.
2
10
100
Return period (years)
FIGURE 2 Rainfall depth-duration and area-frequency curves (US Weather Bureau, after Chow^1 ).
C008_003_r03.indd 468C008_003_r03.indd 468 11/18/2005 10:29:26 AM11/18/2005 10:29:26 AM