Ground bottom flow formula (Water Overlay)

Underground seepage (from (or to) outside the project area) is calculated per 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, calculate the actual amount of water seeping into (or out of) the ground from the bottom.

\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