Underground infiltration formula (Water Overlay)

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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


  • 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.


The following topics are related to this formula.

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