Evaluation of Surface Evaporation Considering the Effect of Soil Water Gradient

Evaporation of unsaturated loess refers to the phenomenon that, under the vertical driving force, the water contained in soil moves to the evaporation surface and diffuses into the atmosphere as water vapor. This lowers the water content of unsaturated loess. The existing calculation methods of soil evaporation mainly consider environmental factors and soil water content. However, the soil water gradient (SWG), which has been ignored by previous studies, can also significantly affect loess evaporation. The objective of the present study was to establish an SWG-dependent evaporation prediction model for evaluating the SWG-affected evaporation of loess. First, the parameters of SWG-affected water evaporation of loess samples with changed densities and water contents were assessed by indoor and outdoor evaporation experiments. The results indicated that the ratio of soil evaporation to water evaporation (i.e., the evaporation coefficient) was mainly related to the water supply capacity of soil. The SWG exerted a large effect on loess surface evaporation, especially when the loess had high dry density and low surface water content. For example, the evaporation intensity of a soil sample with 6% (20%–26%) SWG and $1.3\mathrm{g}/{\mathrm{cm}}^3$ dry density was found to be 62.92% higher than that of a soil sample with 20% uniform water content; the evaporation intensity of a soil sample with 6% (15%–21%) SWG and $1.5\mathrm{g}/{\mathrm{cm}}^3$ dry density was found to be 114.24% higher than that of a soil sample with 15% uniform water content. Using both soil water content and SWG as parameters to characterize the soil water supply capacity can establish a model for the prediction of the unsaturated loess surface evaporation coefficient. Moreover, comparison of predicted and measured data ensured that the model can precisely predict the unsaturated loess surface evaporation coefficient.