Solution method

In the Gravity Flow Module, Equation (28.28) is solved explicitly from the top of the soil column downward.

At the top of the soil column the infiltration rate is first set equal to the amount of water available for infiltration, which is the depth of overland water on the ground surface. This is reduced to the saturated conductivity of the first unsaturated soil cell, which is the maximum infiltration rate for the soil column (Equation (28.27)).

The infiltration rate is further reduced if a leakage coefficient has been speci­fied for the overland-unsaturated zone interface, which may be done in paved areas or under lakes. A leakage coefficient must be explicitly specified for paved areas that are specified as part of the overland flow routing system. That is, paved areas may be defined as part of the overland flow module to route water to streams from parking lots, etc. However, any reduction in the leakage coefficient under such paved areas must be explicitly defined. For example, in an model cell where 25% of the land area is paved, a leakage coefficient may be specified equal to 0.25 times the hydraulic conductivity of the surficial soil.

If the water table is above the ground surface the infiltration is set to zero.

In the special case that the water table is above the top node of the soil col­umn but below the ground surface, the infiltration rate is reduced to an esti­mate of the moisture deficit in the top cell. This is done to reduce or prevent artificial cycling of water between the unsaturated zone and ponded water on the surface.

If there is sufficient water in the top cell at the start of the time step (water content sufficiently above field capacity to satisfy root extraction), or if there is sufficient net infiltration to raise the moisture content above the field capacity, then the flux through the top cell is calculated based on the hydraulic conduc­tivity, which is a function of the moisture content. The flux is first calculated based on the moisture content at the start of the time step and an updated moisture content is calculated. Then the flux is calculated again based on the updated moisture content and another moisture content is calculated. The actual flux through the cell is then set to the average of these two fluxes. Sim­ilarly, the actual updated moisture content is set to the average of the two moisture contents.

This flux is then added the cell below and the calculation repeated down­wards for the remaining cells in the column.

Once the water table is reached, the water contents in the cells are rebal­anced from the bottom up to ensure that no cell is over saturated.

The flux out the bottom of the soil column is accumulated over the UZ time steps and added as a source to the saturated zone calculation at the start of the next SZ time step.