Theory
Runoff is the surface water reaching the outlet of the watershed as a result of rainfall. The amount of runoff is the
amount of precipitation less the amount of water that does not become runoff for a variety of reasons. These
reasons or loses include--
Infiltration.
Interception.
Transpiration.
Detention (storage).
These natural losses were discussed in Lesson 1, Part A.
The amount of water remaining after these losses taking a share of the total amount of precipitation is called
runoff. This runoff will flow over the ground and become discharge at the basin outlet.
The Rational Method of estimating this runoff stems from very simple reasoning. To size your structure, you
have to know the maximum rate of flow through resulting from runoff from the watershed. It is important to
remember that we are talking about volume per unit of time expressed in cubic feet per second (cfs), thus talking
about discharge. The Rational Method does not apply to total volume of a parameter that you might use in
designing ponding structures.
The Rational Method says that the rate of flow depends on several factors--
The size of the watershed.
The rate of rainfall.
The proportion of rainfall that actually becomes runoff.
Runoff then equals a coefficient representing the proportion of rainfall that actually becomes runoff times the
volume of water per unit of time that falls on the watershed. Volume of water per unit of time is the intensity of
rainfall in inches per hour times the area of the watershed.
The Rational Method formula is expressed this way--Q = CIA
Where:
Q = Quantity of discharge in cfs.
C = The coefficient of runoff.
I = The rainfall intensity (in/hr).
A = The area of the watershed in acres.
EN5465
2-2
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