# Underground infiltration formula (Water Overlay)

Underground infiltration (from the unsaturated zone to the saturated zone) is calculated per cell.

First the height of the unsaturated zone is calculated.

Hunsat = Hsurface - WLunderground

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

S = Wunsat/Hunsat

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

Cinf = min( Hunsat , Δt * Iund )

Finally, calculate the actual amount of water infiltrating.

Δw = Cinf * S

Where:

• Δw = The underground infiltration which takes place.
• Δt = Computational timestep.
• Hunsat = The height of the unsaturated zone.
• S = Ratio of water to height in the unsaturated zone.
• Cinf = The height in the unsaturated zone which can be subject to infiltration to the saturated zone.
• Wunset = 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.
• WLunderground = The groundwater level, relative to datum.
• Hsurface = The terrain height in the cell, relative to datum.
• Iund = The GROUND_INFILTRATION_MD of the underground terrain type.

## Related

The following topics are related to this formula.

Formulas
Groundwater level formula
Surface infiltration formula
Models
Underground model
Infiltration model
Substance flow model