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, $imp_{dcon}$, is calculated

$imp_{dcon}=imp_{tot}-imp_{con}$ 6:3.2.3

where $imp_{tot}$ is the fraction of the HRU area that is impervious (both directly connected and disconnected), and $imp_{con}$ 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.

Variable Name	Definition	File Name
CN2	$CN_p$: SCS moisture condition II curve number for pervious areas	.mgt
CNOP	$CN_p$: SCS moisture condition II curve number for pervious areas specified in plant, harvest/kill and tillage operation	.mgt
URBCN2	$CN_{imp}$: SCS moisture condition II curve number for impervious areas	urban.dat
FIMP	$imp_{tot}$: fraction of urban land type area that is impervious	urban.dat
FCIMP	$imp_{con}$: fraction of urban land type area that is connected impervious	urban.dat