Ground bottom flow formula (Water Overlay)

Underground seepage (from (or to) outside the project area) is calculated per cell.

The seepage is calculated using the following formula:

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The external seepage head is variable over time by the introduction of h_v,t

h_d(t) = h_c + h_v,t(t)

Finally, calculate the actual amount of water infiltrating.

Δw = Δs/ ws

Where:

Δs = The underground seepage which takes place, in meters.
Δt = Computational timestep in seconds.
h_d = ground water level (or head) of the external zone causing the seepage, in meters
h_{v> = additional variation in head of h_d over time}
c = resistance of the separating layer.
ws = soil water storage fraction

H_unsat = The height of the unsaturated zone.
S = Ratio of water to height in the unsaturated zone.
C_inf = The height in the unsaturated zone which can be subject to infiltration to the saturated zone.

W_unset = The amount of water in the unsaturated zone. The height of the water column if the equivalent amount of water was placed on the surface.
WL_underground = The groundwater level, relative to datum.
H_surface = The terrain height in the cell, relative to datum.
I_und = The GROUND_INFILTRATION_MD of the underground terrain type.

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