Recharge

Water that moves past the lowest depth of the soil profile by percolation or bypass flow enters and flows through the vadose zone before becoming shallow and/or deep aquifer recharge. The lag between the time that water exits the soil profile and enters the shallow aquifer will depend on the depth to the water table and the hydraulic properties of the geologic formations in the vadose and groundwater zones.

An exponential decay weighting function proposed by Venetis (1969) and used by Sangrey et al. (1984) in a precipitation/groundwater response model is utilized in SWAT+ to account for the time delay in aquifer recharge once the water exits the soil profile. The delay function accommodates situations where the recharge from the soil zone to the aquifer is not instantaneous, i.e. 1 day or less.

The recharge to both aquifers on a given day is calculated:

$w_{rchrg,i}=(1-exp\lfloor-1/\delta_{gw}\rfloor)*w_{seep}+exp\lfloor-1/\delta_{gw}\rfloor*w_{rchrg,i-1}$

2:4.2.2

where $w_{rchrg,i}$ is the amount of recharge entering the aquifers on day $i$ (mm H$_2$O),$\delta_{gw}$ is the delay time or drainage time of the overlying geologic formations (days), $w_{seep}$ is the total amount of water exiting the bottom of the soil profile on day $i$ (mm H$_2$O), and $w_{rchrg,i-1}$ is the amount of recharge entering the aquifers on day $i-1$ (mm H$_2$O). The total amount of water exiting the bottom of the soil profile on day $i$ is calculated:

$w_{seep}=w_{perc,ly=n}+w_{crk,btm}$ 2:4.2.3

where $w_{seep}$ is the total amount of water exiting the bottom of the soil profile on day $i$ (mm H$_2$O), $w_{perc,ly=n}$ is the amount of water percolating out of the lowest layer, $n$, in the soil profile on day $i$ (mm H$_2$O), and $w_{crk,btm}$ is the amount of water flow past the lower boundary of the soil profile due to bypass flow on day $i$ (mm H$_2$O).

The delay time, $\delta_{gw}$, cannot be directly measured. It can be estimated by simulating aquifer recharge using different values for $\delta_{gw}$ and comparing the simulated variations in water table level with observed values. Johnson (1977) developed a simple program to iteratively test and statistically evaluate different delay times for a watershed. Sangrey et al. (1984) noted that monitoring wells in the same area had similar values for $\delta_{gw}$, so once a delay time value for a geomorphic area is defined, similar delay times can be used in adjoining watersheds within the same geomorphic province.