When an evaporation demand for soil water exists, SWAT+ must first partition the evaporative demand between the different layers. The depth distribution used to determine the maximum amount of water allowed to be evaporated is:
2:2.3.16
where is the evaporative demand at depth (mm HO), is the maximum soil water evaporation on a given day (mm HO), and is the depth below the surface. The coefficients in this equation were selected so that 50% of the evaporative demand is extracted from the top 10 mm of soil and 95% of the evaporative demand is extracted from the top 100 mm of soil.
The amount of evaporative demand for a soil layer is determined by taking the difference between the evaporative demands calculated at the upper and lower boundaries of the soil layer:
2:2.3.17
where is the evaporative demand for layer (mm HO), is the evaporative demand at the lower boundary of the soil layer (mm HO), and is the evaporative demand at the upper boundary of the soil layer (mm HO).
Figure 2:2-1 graphs the depth distribution of the evaporative demand for a soil that has been partitioned into 1 mm layers assuming a total soil evaporation demand of 100 mm.
As mentioned previously, the depth distribution assumes 50% of the evaporative demand is met by soil water stored in the top 10 mm of the soil profile. With our example of a 100 mm total evaporative demand, 50 mm of water is 50%. This is a demand that the top layer cannot satisfy.
SWAT+ does not allow a different layer to compensate for the inability of another layer to meet its evaporative demand. The evaporative demand not met by a soil layer results in a reduction in actual evapotranspiration for the HRU.
A coefficient has been incorporated into equation 2:2.3.17 to allow the user to modify the depth distribution used to meet the soil evaporative demand. The modified equation is:
When the water content of a soil layer is below field capacity, the evaporative demand for the layer is reduced according to the following equations:
In addition to limiting the amount of water removed by evaporation in dry conditions, SWAT+ defines a maximum value of water that can be removed at any time. This maximum value is 80% of the plant available water on a given day where the plant available water is defined as the total water content of the soil layer minus the water content of the soil layer at wilting point (-1.5 MPa).
Table 2:2-3: SWAT+ input variables used in soil evaporation calculations.
2:2.3.18
where is the evaporative demand for layer (mm HO), is the evaporative demand at the lower boundary of the soil layer (mm HO), is the evaporative demand at the upper boundary of the soil layer (mm HO), and is the soil evaporation compensation coefficient. Solutions to this equation for different values of including for are shown in Figure 2:2-1.
As the value for is reduced, the model is able to extract more of the evaporative demand from lower levels.
when 2:2.3.19
when 2:2.3.20
where is the evaporative demand for layer adjusted for water content (mm HO), is the evaporative demand for layer (mm HO), is the soil water content of layer (mm HO), is the water content of layer at field capacity (mm HO), and is the water content of layer at wilting point (mm HO).
2:2.3.21
where is the amount of water removed from layer by evaporation (mm HO), is the evaporative demand for layer adjusted for water content (mm HO), is the soil water content of layer (mm HO), and is the water content of layer at wilting point (mm HO).
| Definition | Source Name | Input Name | Input File |
|---|---|---|---|
: soil evaporation compensation coefficient
esco
esco
