Prediction of evaporation rates from unsaturated soil surface remains a challenge due to the difficulty in simulating the interactions in the soil-air interface. A physically based analytical model of soil-air interface is critical to predict the evaporation rates from unsaturated bare soils. The conditions of the topmost soil layer (TSL) determine the surface resistance, which is used to predict evaporation rates, when the vaporization plane remains in TSL at initial stage of drying process. The shape of the soil particle in TSL is more similar to sphere, rather than cuboid which has been widely adopted in past studies, thus the sizes of pores between soil particles are gradual. The funnel-shaped pores in TSL is taken into consideration, which can result in the reduction of evaporation area and the increase of thickness of viscous sub-layer during drying process. Through monitoring the distribution and changes of vapor density in TSL by infrared camera, the model is validated against laboratory experiments on the drying process of initially water-saturated soil columns under nonisothermal conditions. The evaporation rates calculated using the surface resistance predicted by model agree better with the laboratory experiments, as the model is based on a more accurate simulation of the soil particle shape in the TSL.
With the consideration of the soil pore size distribution, the model is applicable to different soil types. The model offers a physically based method for predicting evaporation rates from bare soils and provides new insights into the intrinsic links between evaporation rates and soil particle shapes.