Surface Runoff Lag

In large subbasins with a time of concentration greater than 1 day, only a portion of the surface runoff will reach the main channel on the day it is generated. SWAT+ incorporates a surface runoff storage feature to lag a portion of the surface runoff release to the main channel.

Once surface runoff is calculated with the Curve Number or Green & Ampt method, the amount of surface runoff released to the main channel is calculated:

$Q_{surf}=(Q'_{surf}+Q_{stor,i-1})*(1-exp[\frac{-surlag}{t_{conc}}])$ 2:1.4.1

where $Q_{surf}$ is the amount of surface runoff discharged to the main channel on a given day (mm H $_2$ O), is the amount of surface runoff generated in the subbasin on a given day (mm H $_2$ O), $Q_{stor,i-1}$ is the surface runoff stored or lagged from the previous day (mm H $_2$ O), $surlag$ is the surface runoff lag coefficient, and $t_{conc}$ is the time of concentration for the subbasin (hrs).

The expression $(1-exp[\frac{-surlag}{t_{conc}}])$ in equation 2:1.4.1 represents the fraction of the total available water that will be allowed to enter the reach on any one day. Figure 2:1-3 plots values for this expression at different values for $surlag$ and $t_{conc}$ .

Figure 2:1-3: Influence of $surlag$ and $t_{conc}$ on fraction of surface runoff released.

Note that for a given time of concentration, as $surlag$ decreases in value more water is held in storage. The delay in release of surface runoff will smooth the streamflow hydrograph simulated in the reach.