Page 127 - Biosystems Engineering

108 Cha pte r T h ree

There are various mechanisms of runoff generation. The most widely
known one is the infiltration excess runoff. As discussed earlier, this
happens when rainfall intensity is higher than soil infiltration capacity.
The portion of rainfall not infiltrating into the soil becomes overland
flow. Runoff can also be generated when rain falls on fully saturated
areas, in which case all the rain falling on these wet areas becomes run-
off. This type of runoff is called saturation excess runoff. This usually
happens near streams. During a rain event, areas near the streams get
saturated first. As the rain continues, the extent of these saturated area
become larger. Once rainfall ceases, these saturated areas recede back
and eventually diminish. The concept of the expansion and contraction
of saturated areas during a rain event is termed the variable source area
concept. Another way runoff can be generated is when shallow ground-
water or water moving horizontally in the shallow soil horizon, that is,
subsurface storm flow intersects the soil’s surface.
Two quantitative measure of runoff are extremely important, both
from soil and water conservation points of view: flow volume and
maximum flow rate or peak flow. Next we summarize two most com-
monly used methods in estimating these quantities.

3.5.3 SCS Curve Number Method
The Soil Conservation Service (SCS) curve number method is probably
the most widely used method in estimating runoff volume. It is simple,
yet it has been shown to be very effective. Many popular watershed scale
hydrologic models employ this method such as the Soil & Water Assess-
ment Tool (SWAT), the Agricultural Non-Point Source Pollution Model
(AGNPS), and the Erosion Productivity Impact Calculator (EPIC).
The SCS curve number (CN) method was developed by the U.S.
Department of Agriculture (USDA) and is described in detail in their
technical report TR-55 (U.S. Soil Conservation Service 1986). Total pre-
cipitation (P) is portioned into components of initial abstractions (I ),
a
excess precipitation (P ), and continuing abstraction (F ). The method
e a
assumes that the ratio of F to the potential maximum retention (S) in
a
a watershed is equal to the ratio of P to the maximum potential runoff
e
(P – I ):
a
F a P e
= − (3.20)
S PI
a
Rearranging this and using the equality P = I + P + F along with
a e a
the assumption I = 0.2S leads to
a
P = Q = ( P − .02 S) 2 (3.21)
S
e
P + .08
Note that the depth of the excess rainfall is equal to runoff volume,
Q (depth and volume are often used interchangeably in hydrology
as they can be easily converted to each other by the relationship