# Revap

Water may move from the shallow aquifer into the overlying unsaturated zone. In periods when the material overlying the aquifer is dry, water in the capillary fringe that separates the saturated and unsaturated zones will evaporate and diffuse upward. As water is removed from the capillary fringe by evaporation, it is replaced by water from the underlying aquifer. Water may also be removed from the aquifer by deep-rooted plants which are able to uptake water directly from the aquifer.

&#x20;      SWAT+ models the movement of water into overlying unsaturated layers as a function of water demand for evapotranspiration. To avoid confusion with soil evaporation and transpiration, this process has been termed ‘revap’. This process is significant in watersheds where the saturated zone is not very far below the surface or where deep-rooted plants are growing. Because the type of plant cover will affect the importance of revap in the water balance, the parameters governing revap are usually varied by land use. Revap is allowed to occur only if the amount of water stored in the shallow aquifer exceeds a threshold value specified by the user, $$aq_{shthr,rvp}$$.

&#x20;          The maximum amount of water than will be removed from the aquifer via ‘revap’ on a given day is:

$$w_{revap,mx}=\beta_{rev} *E_o$$                                                  2:4.2.15

where $$w_{revap,mx}$$ is the maximum amount of water moving into the soil zone in response to water deficiencies (mm H$$_2$$O), $$\beta_{rev}$$ is the revap coefficient, and $$E_o$$ is the potential evapotranspiration for the day         (mm H$$_2$$O). The actual amount of revap that will occur on a given day is calculated:

&#x20;$$w_{revap}=0$$          if       $$aq_{sh} \le aq_{shthr,rvp}$$                    2:4.2.16

&#x20;$$w_{revap}=w_{revap,mx} - aq_{shthr,rvp}$$   if   $$aq_{shthr,rvp}<aq_{sh}<(aq_{shthr,rvp}+w_{revap,mx})$$       2:4.2.17

$$w_{revap}=w_{revap,mx}$$  if $$aq_{sh} \ge(aq_{shthr,rvp}+w_{revap,mx})$$          2:4.2.18

where $$w_{revap}$$ is the actual amount of water moving into the soil zone in response to water deficiencies (mm H$$_2$$O), $$w_{revap,mx}$$ is the maximum amount of water moving into the soil zone in response to water deficiencies (mm H$$_2$$O), $$aq_{sh}$$ is the amount of water stored in the shallow aquifer at the beginning of day $$i$$ (mm H$$_2$$O) and $$aq_{shthr,rvp}$$ is the threshold water level in the shallow aquifer for revap to occur (mm H$$_2$$O).
