# Underground evaporation formula (Water Overlay)

Underground evaporation is calculated per cell.

For all underground evaporation, the height of the unsaturated zone is used.

Hunsat = Hsurface - WLunderground

First the capacity for saturated evaporation is calculated, based on how much of the saturated area is in contact with the roots.

Csat = max( 0 , min( RD , GBDM ) - Hunsat ) * WSP

Next the height of the unsaturated zone, and based on that the capacity for unsaturated evaporation is calculated.

Cunsat = max( 0 , min( RD , Hunsat ) ) * ( Wunsat / Hunsat )

Finally, the actual evaporation is calculated:

Δwunsat = min( Cunsat , Δt * Eweather * Etop )
Δwsat = min( Csat , (Δt * Eweather * Etop) - Δwunsat )
Δw = Δwunsat + Δwsat

Where:

• Δw = The total amount of evaporation which takes place.
• Δt = Computational timestep.
• Δwunsat = The amount of evaporation which takes place from the unsaturated zone.
• Δwsat = The amount of evaporation which takes place from the saturated zone.
• Csat = The amount of evaporation that can take place from the saturated zone.
• Cunsat = The amount of evaporation that can take place from the unsaturated zone.
• Hunsat = The height (column) of the unsaturated zone.

## Related

The following topics are related to this formula.

Formulas
Groundwater level formula
Models
Underground model
Evaporation model