2:4.2.6 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 is the water table height on day (m), is the water table height on day (m), is the baseflow recession constant, is the time step (1 day), is the amount of recharge entering the aquifer on day (mm HO), and is the specific yield of the shallow aquifer (m/m).

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