Groundwater

Fig. 56 is a definitional sketch of the two-zone groundwater model that is used in GeoSWMM. The upper zone is unsaturated with a variable moisture content of θ. The lower zone is fully saturated and therefore its moisture content is fixed at the soil porosity. The fluxes shown in the Fig., expressed as volume per unit area per unit time, consist of the following:

f_I

Infiltration from the surface 

f_EU

Evapotranspiration from the upper zone which is a fixed fraction of the un-used surface evaporation

f_U

Percolation from the upper to lower zone which depends on the upper zone moisture content & depth d_U

f_EL

Evapotranspiration from the lower zone, which is a function of the depth of the upper zoned_U

f_L

Percolation from the lower zone to deep groundwater which depends on the lower zone depthd_L

f_G

Lateral groundwater interflow to the drainage system, which depends on the lower zone depth d_L as well as the depth in the receiving channel or node

 

After computing the water fluxes that exist during a given time step, a mass balance is written for the change in water volume stored in each zone so that a new water table depth and unsaturated zone moisture content can be computed for the next time step.