Peak Runoff Rate

The peak runoff rate is the maximum runoff flow rate that occurs with a given rainfall event. The peak runoff rate is an indicator of the erosive power of a storm and is used to predict sediment loss. SWAT+ calculates the peak runoff rate with a modified rational method.

The rational method is widely used in the design of ditches, channels and storm water control systems. The rational method is based on the assumption that if a rainfall of intensity $i$ begins at time $t=0$ and continues indefinitely, the rate of runoff will increase until the time of concentration, $t=t_{conc}$, when the entire subbasin area* is contributing to flow at the outlet. The rational formula is:

$q_{peak}=\frac{C*i*Area}{3.6}$ 2:1.3.1

where $q_{peak}$ is the peak runoff rate ($m^3 s^{-1}$), $C$ is the runoff coefficient, $i$ is the rainfall intensity (mm/hr), Area is the subbasin area (km$^2$) and 3.6 is a unit conversion factor.