> For the complete documentation index, see [llms.txt](https://swatplus.gitbook.io/io-docs/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://swatplus.gitbook.io/io-docs/~/changes/GiVD5ID0WVyRIAumaMy2/groundwater/shallow-aquifer/groundwater-height.md).

# 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       |