2:1.3.4 Modified Rational Formula

The modified rational formula used to estimate peak flow rate is obtained by substituting equations 2:1.3.15, 2:1.3.16, and 2:1.3.17 into equation 2:1.3.1

$q_{peak}=\frac{\alpha_{tc}*Q_{surf}*Area}{3.6*t_{conc}}$ 2:1.3.20

where $q_{peak}$ is the peak runoff rate ($m^3 s^{-1}$), $\alpha_{tc}$ is the fraction of daily rainfall that occurs during the time of concentration, $Q_{surf}$ is the surface runoff (mm H$_2$O), $Area$ is the subbasin area (km$^2$), $t_{conc}$ is the time of concentration for the subbasin (hr) and 3.6 is a unit conversion factor.

Table 2:1-5: SWAT+ input variables that pertain to peak rate calculations.

Definition	Source Name	Input Name	Input File
Area of the subbasin (km$^2$)	SUB_KM		.sub
Fraction of subbasin area contained in HRU	HRU_FR		.hru
$L_{slp}$: Average slope length (m)	SLSUBBSN		.hru
$slp$: Average slope steepness (m/m)	HRU_SLP		.hru
$n$: Manning’s “n” value for overland flow	OV_N		.hru
$L$: Longest tributary channel length in subbasin (km)	CH_L(1)		.sub
$slp_{ch}$: Average slope of tributary channels (m/m)	CH_S(1)		.sub
$n$: Manning’s “n” value for tributary channels	CH_N(1)		.sub