Ground evaporation formula (Water Overlay)

Conceptually, water can evaporate from the ground via crop, tree and plant foliage. This is called evapotranspiration. The amount a plant can evaporate depends on the saturation and ground water level, as well as the root structure. The factor by which roots can extract water from the ground is modeled using the Feddes trapezium.

Feddes trapezium indicating for what water levels and by what factor roots of a plant can extract water from the ground.

The amount a plant, crop or tree evaporates also depends on its growth season. Therefore, the species specific Water evaporation function value can be set as four seasonal values.

Last but not least, the calculated evapotranspiration factor is multiplied with the evaporation amount at time $t$, which is configured for a Weather.

The evaporated amount is first drawn from the unsaturated zone and then, when the expected amount is not yet reached, from the saturated zone.

Formulas

This evaporation is calculated per cell. For all ground evaporation calculation, the ground water depth at time t is used.

$d_{gw,t}=B_{c}-w_{g,t}$

Feddes oxygen stress factor:

$f_{o}={\begin{cases}1.0&{\text{if }}d_{gw,t}{\text{ is }}\geq d_{r,3}\\{\frac {d_{gw,t}-d_{r,4}}{d_{r,3}-d_{r,4}}}&{\text{if }}d_{gw,t}{\text{ is }}\geq d_{r,4}\\0.0&{\text{otherwise}}\end{cases}}$

Feddes saturated zone root update factor:

$f_{s}={\begin{cases}0.0&{\text{if }}d_{gw,t}{\text{ is }}\geq d_{r,1}\\1.0-{\frac {d_{gw,t}-d_{r,2}}{d_{r,1}-d_{r,2}}}&{\text{if }}d_{gw,t}{\text{ is }}\geq d_{r,2}\\1.0&{\text{otherwise}}\end{cases}}$

Seasonal transpiration factor:

$f_{ts,t}=(1.0-f_{se,t})\cdot f_{ts,c}+f_{se,t}\cdot f_{ts,n}$

Max potential evaporation:

$E_{max}=f_{ts,t}\cdot e_{w,t}\cdot f_{o}\cdot \Delta t$

Evaporation unsaturated:

$E_{u}={\begin{cases}0.0&{\text{if }}d_{gw,t}{\text{ is }}\leq 0.0\\min(E_{max},h_{u}\cdot {\frac {d_{r,1}}{d_{gw,t}}})&{\text{otherwise}}\\\end{cases}}$

Potential evaporation saturated:

$d_{s}=w_{g,t}-(B_{c}-d_{b,c})$

$d_{r}=max(0.0,w_{g,t}-(B_{c}-d_{r,1}))$

$E_{p,s}=(E_{max}-E_{u})\cdot f_{s}$

Evaporation saturated zone:

$E_{s}=max(0.0,min(E_{p,s},d_{s}\cdot f_{ws},d_{r}\cdot f_{ws}))$

Evaporation total is the sum of evaporation in the saturated and unsaturated zone:

$E_{g}=E_{u}+E_{s}$

where

$B_{c}$ = the datum height of the surface of cell c, set by the elevation or a Terrain elevation prequel.

$w_{g,t}$ = the ground water level (datum height) at time t.

$d_{gw,t}$ = the depth of the ground water level, which is equal to the column height of the unsaturated zone at time t.

$f_{ts,c}$ = the transpiration factor at current season c, of the buildings (plants) on the surface, or the surface terrain's transpiration factor if none present.

$f_{ts,n}$ = the transpiration factor at coming season n, of the buildings (plants) on the surface, or the surface terrain's transpiration factor if none present.

$f_{se,t}$ = Season fraction. Used for interpolating the transpiration factor.

$f_{ts,t}$ = Can be supplied as four values; it is an interpolated value based on the current and next season.

$e_{w,t}$ = the evaporation rate of the weather at time t.

$f_{o}$ = the calculated feddes oxygen stress factor, calculated using the 3rd and 4th root depths and the feddes trapezium interpolation model and the ground water depth.

$f_{s}$ = the calculated feddes saturated zone root update factor, calculated using the 1st and 2nd root depths and the feddes trapezium interpolation model and the ground water depth.

$\Delta t$ = the computational timestep in seconds.

$d_{b,c}$ = The ground bottom distance of the cell c, defined by a Bottom distance prequel or a general GROUND_BOTTOM_DISTANCE_M of the Water Overlay.

$d_{s}$ = the column height of the saturated zone covered by the roots.

$d_{r,n}$ = the nth root depth value.

$h_{u}$ = the amount of water in the unsaturated zone.

$E_{max}$ = the potential max evaporation.

$E_{u}$ = the evaporation in the unsaturated zone.

$E_{p,s}$ = the potential evaporation in the saturated zone.

$f_{ws}$ = The WATER_STORAGE_PERCENTAGE of the ground terrain type, used here as a fraction.

$E_{s}$ = the evaporation in the saturated zone.

$E_{g}$ = the total evaporation from the ground.

Related

The following topics are related to this formula.

Formulas

Groundwater level formula (Water Overlay)

Models

Ground model (Water Overlay)

Evaporation model (Water Overlay)

Notes

• The ground water level cannot be lowered further than the defined root depth by evapotranspiration.

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