Ground evaporation formula (Water Overlay)

Underground evaporation is calculated per cell.

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

H_unsat = H_surface - WL_underground

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

C_sat = max( 0 , min( RD , GBDM ) - H_unsat ) * WSP

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

C_unsat = max( 0 , min( RD , H_unsat ) ) * ( W_unsat / H_unsat )

Finally, the actual evaporation is calculated:

Δw_unsat = min( C_unsat , Δt * E_weather * E_top )

Δw_sat = min( C_sat , (Δt * E_weather * E_top) - Δw_unsat )

Δw = Δw_unsat + Δw_sat

Where:

• Δw = The total amount of evaporation which takes place.
• Δt = Computational timestep.

• Δw_unsat = The amount of evaporation which takes place from the unsaturated zone.
• Δw_sat = The amount of evaporation which takes place from the saturated zone.
• C_sat = The amount of evaporation that can take place from the saturated zone.
• C_unsat = The amount of evaporation that can take place from the unsaturated zone.
• H_unsat = The height (column) of the unsaturated zone.

• W_unsat = 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.
• WL_underground = The groundwater level, relative to datum.
• H_surface = 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 of the Water Overlay. (effectively available height in the underground).
• E_weather = The evaporation rate of the weather.
• E_top = The WATER_EVAPORATION_FACTOR of the construction if present, the WATER_EVAPORATION_FACTOR of the surface terrain otherwise.

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