Evaporation from Sand and Loess Soils: An Experimental Approach

In arid and semiarid areas, low rainfall and high evaporation make groundwater the main source of soil water in the vadose zone. In order to understand the upward migration rate of soil water and the mechanisms of moisture migration in the vadose zone, evaporation experiments in sand and loess soils were conducted. The evaporation and imbibition of the soil columns were measured in order to analyze the upward migration rate of soil water. Hydrochemical and isotopic methods were applied to investigate the microscopic mechanisms of water movement in the vadose zone. The results show that soil columns with higher loess contents have higher imbibition and evaporation rates. Obvious evaporation occurs only after soil water has reached the surface layer of the soil column, and the evaporation rate is related to soil composition. Salt migrates in the same direction as that of water movement and accumulates after the evaporation of water. The greater the evaporation, the greater the salt accumulation. Only strong hydraulic connections between soil water support the diffusion of salt from areas of higher concentration to those of lower concentration. Before liquid water reaches the surface layer, there are two regions of unsaturated soil. In the lower column, soil water moves in the form of liquid water and hydraulic connections are strong. In the upper column, water vapor from the lower column diffuse in soil pore spaces, and some are absorbed or condensed in the soil.