# Underground seepage formula (Water Overlay) Hoe is kwel geïmplementeerd in het Tygron Geodesign Platform? (Dutch only)

Underground seepage from outside the project area is calculated per grid cell. The formula is based on the principles used in the formula of Pankow for estimating seepage.

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

$\Delta wl_{x,y}={\frac {h_{d}(t)-h_{x,y}}{c_{x,y}}}\cdot {\frac {\Delta t}{24\cdot 60\cdot 60}}$ The external seepage head is variable over time by the introduction of $h_{v}(t)$ :

$h_{d}(t)=h_{p_{x,y}}+h_{v}(t)$ Finally, the actual amount of water seeping into the ground from the bottom is calculated as followed:

$\Delta s_{x,y}={\Delta wl_{x,y}}\cdot {ws}$ where:

$\Delta wl_{x,y}$ = 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
$h_{p_{x,y}}$ = 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
$\Delta s_{x,y}$ = Seeped in water in m