Technical information

Calculating Runoff | 145
Calculating Runoff
Runoff is the water that flows out of a watershed subarea as a result of a storm
event. It is typically expressed as a flow rate in cubic feet per second, or as a
volume in cubic feet or acre-feet. The runoff volume is equal to the volume
of rainfall that occurs on the area, minus the volume of rainfall that is infil-
trated by the ground, is intercepted by foliage, or is held in small depressions.
Runoff is calculated by examining the following:
■ Rainfall intensity, duration, and distribution
■ Soil conditions
■ Antecedent moisture conditions (how much moisture is already present in
the soil before the storm occurs)
■ Land use
Sometimes a runoff volume within a specified time period is adequate for
design purposes, but a peak flow rate is generally needed also. In many cases,
a hydrograph is required to show a flow-versus-time relationship.
The civil engineering tools provide the following methods for calculating
peak runoff flow rates from watershed areas:
■ Rational
■ TR-55 Graphical Peak Discharge
■ TR-55 Tabular Hydrograph
■ TR-20
NOTE It is important that you have some familiarity with the methods and
terminology described above. For more information about NRCS (Natural
Resources Conservation Service) methods, you can obtain documents from your
local NRCS or county Soil & Water Conservation District office, most college
libraries, or the National Technical Information Service in Washington, D.C.