The rate and quantity of evaporation from a soil surface is a complicated process affected by many soil characteristics, tillage, and environmental interactions. However, it is known that energy and water availability largely dominate the process, thus on the average these broad principles can be used to estimate direct soil water evaporation. This evaporation is not to be confused with that considered as intercepted soil surface water since water from deeper that a few soil grains must be moved toward the surface to interact with eh climatic energy, thus it encounters variable resistance due to gradient flow characteristics and latent soil heat.
Soil water evaporation is represented by defining a thin (0.5-1.0 inch) upper boundary layer (evaporation layer) of the soil profile which is included in the soil profile incrementation. This upper boundary layer has all of the same functions as other layers (except no roots), plus the water is readily evaporated and limited only by PET. The lower limit of soil water content in the evaporative layer is set as just below wilting point.
Upward water movement from the second layer into the evaporation boundary layer and its evaporation is estimated by a modified Darcy equation using a reduced unsaturated conductivity rate for the current soil water content. The conductivity reduction by a small percentage represents the fact that evaporation is largely vapor flow rather than liquid and the effective conductivity is significantly less. This upward flow is obviously also dependent on the soil water content in the second and deeper soil layers. Effects such as tillage or deep soil cracking can be estimated by increasing the evaporation percentage.
This soil water evaporation routine was developed in an attempt to represent the effect of soil characteristic, and yet maintain SPAW's balance of complexity and accuracy. The programmed routine approximates the traditional observed three-stage drying process. The upper boundary layer evaporation is limited only by the PET rate (stage 1), upward movement and evaporation from a wet soil remains rapid at a decreasing rate with drying (stage 2), and evaporation from a relatively dry soil becomes very restricted (stage 3). Soil surface evaporation can accumulate to several inches over a year depending on the crop canopy, precipitation pattern and amount and PET. This soil surface water evaporation makes up the second component of actual ET.
For dry soil conditions with a partial canopy, there is some portion of the radiation energy (PET) which impinges on the soil surface but is not utilized in water evaporation. This energy heats the soil, adjacent air, and canopy, and is then reflected or absorbed and reradiated. The result is that the crop canopy has this available as a second source of PET in addition to the directly intercepted energy. To represent this effect, a linear relationship of canopy versus percent of unused energy or sensible heat absorbed by the canopy is included as shown in Figure 6. Based only on intuitive reasoning, it was assumed that when the canopy value reaches 60 percent, all soil surface unused energy is re-captured by the canopy and it becomes a part of the potential transpiration.
Figure 6: Unused soil water evaporation energy transfer to plant canopy.