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:

Δ s = \frac{h_{d} (t) - h_{x, y}}{c_{x, y}} \cdot \frac{Δ t}{24 \cdot 60 \cdot 60}

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

h_{d} (t) = h_{c_{x, y}} + h_{v, t} (t)

Finally, calculate the actual amount of water seeping into (or out of) the ground from the bottom.

Δ w = Δ s \cdot w s

Where:

$Δ s$ = The underground seepage which takes place, in meters.
$Δ t$ = Computational timestep in seconds.
$h_{x, y}$ = Ground water head in the grid cell.
$h_{d, t}$ = additional ground water level (or head) of the external zone causing the seepage at time t, in meters
$h_{c_{x, y}}$ = constant ground water level (or head) of the external zone causing the seepage, in meters
$h_{v, t}$ = additional variation of seepage head over time
$c$ = resistance of the separating layer in days
$w s$ = water storage fraction of the soil

Notes

$h_{c_{x, y}}$ and $c_{x, y}$ can both be provided as spatially variable values.
$h_{d, t}$ can be provided as a set of values, variable over time.