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:

\Delta s={\frac {h_{d}(t)-h_{x,y}}{c}}\cdot {\frac {\Delta 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}+h_{v,t}(t)

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

\Delta w={\frac {\Delta s}{ws}}

Where:

$\Delta s$ = The underground seepage which takes place, in meters.
$\Delta t$ = Computational timestep in seconds.
$h_{x,y}$ = Ground water head in the grid cell.
$h_{d,t}$ = ground water level (or head) of the external zone causing the seepage at time t, in meters
$h_{c}$ = 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
$ws$ = water storage fraction of the soil

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