Improvements to the calculation of actual evaporation from bare soil surfaces

Evaporation of water from a saturated soil surface with ample free water is known as potential evaporation, PE, and the associated physical processes are quite well understood. However, evaporation of water from an unsaturated soil surface is known as actual evaporation, AE, and the associated physical processes are more complex and less understood. The calculation of actual evaporation is important for many geotechnical engineering applications. Soil suction and the corresponding water content at which the AE rate begins to depart from the PE rate during a drying process are re-assessed using a series of laboratory tests (i.e., thin soil section drying tests and soil column drying tests). Laboratory results show that the suction at which the actual rate of evaporation begins to depart from the PE rate for soil columns (or thick soil layers) may be different than for thin soil layers. Suction at the “evaporation-rate reduction point” (ERRP) appears to be approximately 3000 kPa for thin soil layers, but is between the air-entry value and residual soil suction for thick soil layers or soil columns. The analyses presented in this paper have resulted in the development of a methodology for the estimation of suction corresponding to the ERRP in soil columns. Equations are also proposed to calculate the coefficient of surface moisture availability, vapour pressure, and “surface resistance” at the ground surface. The paper also presents an equation (i.e., new soil–atmosphere moisture flux equation) for predicting evaporation rate from a soil surface using “surface resistance” to vapour water diffusion from the soil to the atmosphere. The proposed soil–atmosphere model is verified using evaporation rate data collected from various drying tests on thin soil layers and soil columns. Reasonably good agreement was found between the computed and measured rates of evaporation. The findings and recommendations in this paper contribute to an improved understanding of the prediction of AE from unsaturated soil surfaces.