# Ground infiltration formula (Water Overlay)

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The infiltration from the unsaturated zone to the saturated zone is calculated per cell.

First the height of the unsaturated zone is calculated.

Then the ratio of water amount to unsaturated height is calculated.

Then we calculate the distance of the unsaturated zone which can infiltrate.

Finally, we calculate the actual amount of water that infiltrates from the unsaturated zone to the saturated zone.

Where:

- = The groundwater level, relative to datum.
- = the datum height of the surface of cell c, set by the elevation or a Terrain elevation prequel.
- = The height of the unsaturated zone.
- = The amount of water in the unsaturated zone. The height of the water column if the equivalent amount of water was placed on the surface.
- = Ratio of water to height in the unsaturated zone.
- = Computational timestep in seconds.
- = The interpolated infiltration factor at time . Configured using INFILTRATION_FACTOR_S.
- = The GROUND_INFILTRATION_MD of the underground terrain type.
- = The height distance in the unsaturated zone which can be subject to infiltration to the saturated zone.
- = The infiltration which takes place from the unsaturated zone to the saturated zone.

## Related

The following topics are related to this formula.

- Formulas
- Groundwater level formula (Water Overlay)
- Surface infiltration formula (Water Overlay)
- Ground bottom flow formula (Water Overlay)
- Models
- Ground model (Water Overlay)
- Infiltration model (Water Overlay)
- Tracer flow model (Water Overlay)