# 2:2.3.3.2 Soil Water Evaporation
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:
$$E\_{soil,z}=E''\_s\*\frac{z}{z+exp(2.374-0.00713\*z)}$$ 2:2.3.16
where $$E\_{soil,z}$$ is the evaporative demand at depth $$z$$ (mm H$$\_2$$O), $$E''\_s$$ is the maximum soil water evaporation on a given day (mm H$$\_2$$O), and $$z$$ 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:
$$E\_{soil,ly}=E\_{soil,zl}-E\_{soil,zu}$$ 2:2.3.17
where $$E\_{soil,ly}$$ is the evaporative demand for layer $$ly$$ (mm H$$*2$$O), $$E*{soil,zl}$$ is the evaporative demand at the lower boundary of the soil layer (mm H$$*2$$O), and $$E*{soil,zu}$$ is the evaporative demand at the upper boundary of the soil layer (mm H$$\_2$$O).
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.
Figure 2:2-1: Soil evaporative demand distribution with depth.
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:
$$E\_{soil,ly}=E\_{soil,zl}-E\_{soil,zu}\*esco$$ 2:2.3.18
where $$E\_{soil,ly}$$ is the evaporative demand for layer $$ly$$ (mm H$$*2$$O), $$E*{soil,zl}$$ is the evaporative demand at the lower boundary of the soil layer (mm H$$*2$$O), $$E*{soil,zu}$$ is the evaporative demand at the upper boundary of the soil layer (mm H$$\_2$$O), and $$esco$$ is the soil evaporation compensation coefficient. Solutions to this equation for different values of $$esco$$ including for $$esco=1.0$$ are shown in Figure 2:2-1.
Figure 2:2-2: Soil evaporative demand distribution with depth
As the value for $$esco$$ is reduced, the model is able to extract more of the evaporative demand from lower levels.
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:
$$E'*{soil,ly}=E*{soil,ly}*exp(\frac{2.5*(SW\_{ly}-FC\_{ly})}{FC\_{ly}-WP\_{ly}})$$ when $$SW\_{ly}\
