# Surface infiltration formula (Water Overlay)

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Surface infiltration is the amount of water that infiltrates from the surface to the unsaturated zone below the surface.

Surface infiltration is calculated per grid cell. First, the infiltration capacities are calculated:

Then the actual infiltration:

Where:

- = The average infiltration capacity based on constructions on top of the surface, based on GROUND_INFILTRATION_MD defined in either the building or function.
- = The average infiltration capacity based on the surface terrain, defined by GROUND_INFILTRATION_MD. This value should be interpreted as the vertical conductivity (Kv) of the sub-soil.

- = The average infiltration capacity on top of the surface.
- = The average infiltration capacity below the surface.
- = The infiltration capacity from surface to the unsaturated zone.

- = The surface infiltration which takes place at time .
- = The amount of water on the surface at time . Also known as the water column.
- = Computational timestep.
- = The interpolated infiltration factor at time . Configured using INFILTRATION_FACTOR_S.

## Related

The following topics are related to this formula.

- Formulas
- Surface water level formula
- Groundwater level formula
- Underground infiltration formula
- Models
- Surface model
- Underground model
- Infiltration model
- Tracer flow model

## Notes

- When Surface infiltration Only is active, the maximum amount of water that can infiltrate into the unsaturated zone can be configured. See Infiltration Model: Surface infiltration Only for more details.