Ground bottom flow formula (Water Overlay)

Underground seepage from outside the project area is calculated per grid cell.

The water level change due to seepage is calculated using the following formula:

\Delta s_{x,y}={\frac {h_{d}(t)-h_{x,y}}{c_{x,y}}}\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_{x,y}}+h_{v,t}(t)

Finally, the actual amount of water seeping into the ground from the bottom is calculated as followed:

\Delta w_{x,y}={\Delta s_{x,y}}\cdot {ws}

Where:

$\Delta s_{x,y}$ = The water level change due to seepage, in meters.
$\Delta t$ = 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
$c_{x,y}$ = Resistance of the separating layer in days
$ws$ = Water storage fraction of the soil
$\Delta w_{x,y}$ = Seeped in water in m

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.
A seepage head lower than the water level in the project is also allowed, resulting in water seeping out to a region outside the project area.