Evapotranspiration

MIKE SHE uses the Crop Reference ET rate for all calculations of ET. The Crop Reference ET rate is

(20.15)   

where ET_ref is the Reference Evapotranspiration (V1 p. 234) and k_c is the Crop Coefficient specified in the Vegetation Development Table (V1 p. 405) that adjusts the Reference ET rate for different vegetation types. The maxi­mum amount of ET that can be removed in one time step is

(20.16)   

ET_max is satisfied in the following order:

1.       ET is first removed from snow storage.

2.       If the snow storage cannot satisfy ET_max, water is evaporated from the interception storage.

3.       If the interception storage cannot satisfy ET_max, water is evaporated from the ponded water until the ponded water is exhausted.

4.       If ET_max has not yet been satisfied, water is removed from the unsatu­rated zone until ET_max is satisfied or the water content of the upper UZ layer is reduced to qmin.

5.       If ET_max has still not yet been satisfied, water is extracted from the satu­rated zone if the water table is above the ET extinction depth.

However, if Transpiration during ponding (V1 p. 751) is allowed, then the order of the last three calculations above are changed and ET is removed from the ponded water after being removed from the UZ and SZ.

ET from Snow

ET from snow is a sum of the ET from wet snow and and dry snow storages.

(20.17)   

ET will be first removed from the wet snow storage, if it exists,

where ET_ref is the Reference Evapotranspiration (V1 p. 234) before being reduce by the Crop Coefficient, k_c, that is specified in the Vegetation Develop­ment Table (V1 p. 405).

However, if there is insufficient wet snow storage, then ET will be removed from dry snow as sublimation using

(20.18)   

where S_f is the sublimation reduction factor found on the Snow Melt (V1 p. 239) dialog, which reduces the amount of ET that can be removed from dry snow due to the extra energy required to sublimate snow.

If there is not enough snow storage then ET_snow will reduce the snow storage to zero.

ET from the Canopy

After the ET is removed from any available snow storage, ET is removed from the canopy storage until the canopy storage is exhausted or ET_max is satisfied using

(20.19)   

If there is not enough canopy storage then ET_canopy will reduce the canopy storage to zero.

ET from Ponded Water

ET is removed from the ponded storage until ET_max is satisfied or the ponded storage is exhausted. Thus,

(20.20)   

If there is not enough ponded storage then ET_ponded will reduce the ponded storage to zero.

ET from the Unsaturated Zone

ET is only removed from the upper UZ layer. However, as the water content of the root zone decreases, plants will find it harder to remove the water. Finally, when the wilting point is reached ET will stop. However, the reduction in ET does not occur immediately. Plants will remove ET from the root zone at the maximum rate until a plant specific water content is reached, at which point the rate of ET removal will be restricted. In the 2-Layer Water Balance method this phenomena is accounted for by a plant specific deficit fraction. ET will be removed at the maximum rate until this deficit fraction is reached, then linearily reduced to zero as the water content falls to the wilting point.

Thus, the ET_rate is reduced by

(20.21)   

where q_p is water content when ET begins to be restricted given by

(20.22)    

q_act is the current water content in the upper layer, q_min and q_max are the mini­mum and maximum water contents in Figure 20.9 and F_p is the user-speci­fied, plant-specific deficit fraction when ET begins to be restricted by a lack of soil water. From Equations (20.21) and (20.22) q_p is variable if the ET extinc­tion depth is above the bottom of the root zone. If the ET extinction depth is below the root zone, then q_p represents the minimum water content for full ET. Crop specific values for q_p can be found in the literature or from the FAO.

ET is removed from the upper UZ layer until ET_max is satisfied or the average water content is reduced to the wilting point. Thus,

(20.23)   

However, if Transpiration during ponding (V1 p. 751) is allowed, then F_ETUZ is multiplied by the anaerobic tolerance factor to account for the decreased capacity for ET when there is ponded water on the ground surface. Also, the maximum remaining amount of ET that can be removed is also reduced by the anaerobic tolerance factor, F_antol.  That is,

(20.24)   

ET from the Saturated Zone

If the water table is above the extinction depth, then ET is removed from SZ until ET_max is satisfied using

(20.25)   

where F_ETSZ is 1.0 when the water table is in the root zone and decreases lin­early from 1.0 to zero when the water table is below the root zone, but above the extinction depth.

If the water table is below the extinction depth, then F_ETSZ is zero, and no ET is removed from the saturated zone.

In the case where Transpiration during ponding (V1 p. 751) is allowed, then, F_ETSZ is multiplied by the anaerobic tolerance factor to account for the decreased capacity for ET when there is ponded water on the ground sur­face. Also, the maximum remaining amount of ET that can be removed is also reduced by the anaerobic tolerance factor, F_antol.  That is,

(20.26)   

Actual ET

Finally, the actual evapotranspiration is the sum of the above contributions

(20.27)   

remembering that ET_actual cannot be greater than ET_max and that the ET is calculated in a specific order until ET_max is reached.