GROUNDWATER
Fig.56: Two zone Groundwater model (Source: User Manual for EPA-SWMM 5.1)
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:
fI | Infiltration from the surface |
fEU | Evapotranspiration from the upper zone which is a fixed fraction of the un-used surface evaporation |
fU | Percolation from the upper to lower zone which depends on the upper zone moisture content & depth dU |
fEL | Evapotranspiration from the lower zone, which is a function of the depth of the upper zonedU |
fL | Percolation from the lower zone to deep groundwater which depends on the lower zone depthdL |
fG | Lateral groundwater interflow to the drainage system, which depends on the lower zone depth dL 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.