Surface-Subsurface Leakage Coefficient Surface-Subsurface Leakage CoefficientConditionswhen Overland

Surface-Subsurface Leakage Coefficient


Surface-Subsurface Leakage Coefficient
Conditions	when Overland Flow + the Finite Difference method is selected in the Simulation Specification dialogue AND the Reduced vertical exchange in specified areas option selected in the main Overland Flow dialogue
dialogue Type	Stationary Real Data
EUM Data Units	Leakage Coeff./Drain Time Const.

The Surface-subsurface leakage coefficient reduces the infiltration rate at the ground surface. It works in both directions. That is, it reduces both the infiltration rate and the seepage outflow rate across the ground surface.

Conceptually, the leakage coefficient is used to account for soil compaction and fine sediment deposits on flood plains in areas that otherwise have similar soil profiles.

If the groundwater level is at the ground surface, then the exchange of water between the surface water and ground water is based on the specified leakage coefficient and the hydraulic head between surface water and ground water. In other words the UZ model is automatically replaced by a simple Darcy flow description when the profile becomes completely saturated.

If the groundwater level is below the ground surface, then the vertical infiltration is determined by the minimum of the moisture dependent hydraulic conductivity (from the soils database) and the leakage coefficient.

This option is often useful under lakes or on flood plains, which may be permanently or temporarily flooded, and where fine sediment may have accumulated creating a low permeable layer (lining) with considerable flow resistance. It is also used to simulate soil compaction and paved areas.

The value of the leakage coefficient may be found by model calibration, but a rough estimate can be made based on the saturated hydraulic conductivities of the unsaturated zone or in the low permeable sediment layer, if such data is available.

The leakage coefficient is also applied to the vertical leakage from ponded water to the UZ model. In this case, the vertical leakage is a series calculation based on the harmonic mean of the UZ hydraulic conductivity and the leakage coefficient.

The specified leakage coefficient is used wherever it is specified. In areas where a delete value is specified, it is ignored.

In the processed data, the item, Surface-Subsurface Exchange Grid Code, is added, where areas with full contact are defined with a 0, and areas with reduced contact are defined with a 1.

Note: A time-varying value can be specified by using a dfs2 file and checking ON the time-varying option to support reduced infiltration due to frozen soil in the winter.