Groundwater Height

Although SWAT+ does not currently print groundwater height in the output files, the water table height is updated daily by the model. Groundwater height is related to groundwater flow by equation 2:4.2.6.

$Q_{gw}=\frac{8000*K_{sat}}{L^2_{gw}}*h_{wtbl}=\frac{8000*\mu}{10}*\frac{10*K_{sat}}{\mu * L^2_{gw}}*h_{wtbl}=800*\mu*\alpha_{gw}*h_{wtbl}$

2:4.2.19

where $Q_{gw}$ is the groundwater flow into the main channel on day $i$ (mm H$_2$O), $K_{sat}$ is the hydraulic conductivity of the aquifer (mm/day), $L_{gw}$ is the distance from the ridge or subbasin divide for the groundwater system to the main channel (m), $h_{wtbl}$ is the water table height (m), $\mu$ is the specific yield of the shallow aquifer (m/m), and $\alpha_{gw}$ is the baseflow recession constant. Substituting this definition for $Q_{gw}$ into equation 2:4.2.9 gives

$h_{wtbl,i}=h_{wtbl,i-1}*exp[-\alpha_{gw}*\Delta t]+\frac{w_{rchrg}*(1-exp\lfloor-\alpha_{gw} *\Delta t\rfloor)}{800*\mu*\alpha_{gw}}$ 2:4.2.20

where $h_{wtbl,i}$ is the water table height on day $i$ (m), $h_{wtbl,i-1}$ is the water table height on day $i-1$ (m), $\alpha_{gw}$ is the baseflow recession constant, $\Delta t$ is the time step (1 day), $w_{rchrg}$ is the amount of recharge entering the aquifer on day $i$ (mm H$_2$O), and $\mu$ is the specific yield of the shallow aquifer (m/m).

Table 2:4-1: SWAT+ input variables used in shallow aquifer calculations.

Variable Name	Definition	File Name
GW_DELAY	$\delta_{gw}$: Delay time for aquifer recharge (days)	.gw
GWQMN	$aq_{shthr,q}$: Threshold water level in shallow aquifer for base flow (mm H$_2$O)	.gw
ALPHA_BF	$\alpha_{gw}$: Baseflow recession constant	.gw
REVAPMN	$aq_{shthr,rvp}$: Threshold water level in shallow aquifer for revap (mm H$_2$O)	.gw
GW_REVAP	$\beta_{rev}$: Revap coefficient	.gw
RCHRG_DP	$\beta_{deep}$: Aquifer percolation coefficient	.gw
GW_SPYLD	$\mu$: Specific yield of the shallow aquifer (m/m)	.gw