Surface Runoff from Urban Areas

In urban areas, surface runoff is calculated separately for the directly connected impervious area and the disconnected impervious/pervious area. For directly connected impervious areas, a curve number of 98 is always used. For disconnected impervious/pervious areas, a composite curve number is calculated and used in the surface runoff calculations. The equations used to calculate the composite curve number for disconnected impervious/pervious areas are (Soil Conservation Service Engineering Division, 1986):

$CN_c=CN_p+imp_{tot}*(CN_{imp}-CN_p)*(1-\frac{imp_{dcon}}{2*imp_{tot}})$ if $imp_{tot}<0.30$ 6:3.2.1

$CN_c=CN_p+imp_{tot}*(CN_{imp}-CN_p)$ if $imp_{tot}>0.30$ 6:3.2.2

where $CN_c$ is the composite moisture condition II curve number, $CN_p$ is the pervious moisture condition II curve number, $CN_{imp}$ is the impervious moisture condition II curve number, $imp_{tot}$ is the fraction of the HRU area that is impervious (both directly connected and disconnected), and $imp_{dcon}$ is the fraction of the HRU area that is impervious but not hydraulically connected to the drainage system.

The fraction of the HRU area that is impervious but not hydraulically connected to the drainage system, , is calculated

6:3.2.3

where is the fraction of the HRU area that is impervious (both directly connected and disconnected), and is the fraction of the HRU area that is impervious and hydraulically connected to the drainage system.

Table 6:3-2: SWAT+ input variables that pertain to surface runoff calculations in urban areas.