Ground evaporation formula (Water Overlay)

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Underground evaporation is calculated per cell.

Evaporation capacities:

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.
  • Wunsat = The amount of water in the saturated 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.
  • RD = The ROOT_DEPTH_M of the construction if present, the ROOT_DEPTH_M of the surface terrain otherwise.
  • GBDM = The GROUND_BOTTOM_DISTANCE_M (effectively available height in the underground model).
  • Eweather = The evaporation rate of the weather.
  • Etop = The WATER_EVAPORATION_FACTOR of the construction if present, the WATER_EVAPORATION_FACTOR of the surface terrain otherwise.

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