Agriculture Reference
In-Depth Information
where
Q
=
runoff (mm)
P
=
rainfall (mm)
I a =
initial abstraction (mm)
S
=
potential maximum retention after runoff begins (mm)
The I a is approximated as (USDA-SCS, 1985 )
I a =0.2 S
Thus, Q = ( P
0.2 S ) 2
(6.10)
P
+
0.8 S
The potential retention ( S ) can range from zero on smooth, impervious surface to
infinity in deep gravel. For convenience, the “ S -values” were converted to runoff
curve numbers (CN) by the following transformation.
1, 000
10
CN
=
(6.11)
+
S
1, 000
CN
Or, S
=
10
The curve number is based on the area's hydrologic soil group, land use, treatment,
and hydrologic condition.
From the above equation, it is revealed that for S
100, and when S
approaches infinity, CN approaches zero. But for practical applications, the CN val-
ues are almost limited to the range of 40-98. The CN values for most US watersheds
are tabulated in Chapter 9 of NEH-4 (National Engineering Handbook, Section
4) (USDA-SCS, 1985 ; USDA-SCS, 1993). The amount of runoff can be found
if the rainfall amount and curve number is known. The runoff curve number, an
index/indication of runoff producing potential, represent the combined hydrological
effect of soil type, land use, agricultural land treatment, hydrologic condition, and
antecedent soil moisture. For agricultural land (grassland and well-developed crop-
field), the curve number for low-, moderate-, well-, and high-runoff potential soils
usually range 40-68, 60-79, 65-86, and 80-89. Here, low-, moderate-, well-, and
high-runoff potential soil indicate high infiltration rate and well-drained; moderate
infiltration rate and moderate well to well-drained; slow infiltration rate and imped-
iment of downward water movement; and, very slow infiltration rate and having
claypan/clay-layer at or near surface; respectively.
Weighted curve number for an area can be obtained as
=
0, CN
=
(CN i ×
A i )
CN w =
A i
(6.12)
 
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