# Underground seepage formula (Water Overlay)

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^{[1]}.

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

The external seepage head is variable over time by the introduction of :

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

where:

- = The water level change due to seepage, in meters.
- = Computational timestep in seconds.
- = Ground water head in the grid cell.
- = Additional ground water level (or head) of the external zone causing the seepage at time t, in meters
- = Constant ground water level (or head) of the external zone causing the seepage, in meters
- = Additional variation of seepage head over time
- = Resistance of the separating layer in days
- = Water storage fraction of the soil
- = Seeped in water in m

## Notes

- The calculated is essentially the change in groundwater level, inherently taking into account the porosity of the soil. The final is representative of the actual amount of water, i.e. the height of the water, if the relevant quantity of water wasn't in the underground but sat on the surface instead.
- and can both be provided as spatially variable values.
- 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.

## Related

The following topics are related to this formula.

## See also

- Ground last seepage result type (Water Overlay)
- Seepage GeoTiffs (Water Overlay)
- Seepage fluctuation (Water Overlay)
- How to configure constant seepage (Water Overlay)

## References

- ↑ Estimating seepage, http://grondwaterformules.nl/index.php/formules/ontwatering/kwel-berekenen-pankow, last visited on 2-9-2020