Underground 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.
Related
The following topics are related to this formula.
- Formulas
- Groundwater level formula
- Models
- Underground model
- Evaporation model