Relationships between vapor condensation, evaporation and salinization near ground surfaces in arid and semi-arid areas.

通过对我国西部荒漠区实测气象资料的沙柱蒸发试验和数值模拟，从大气水汽凝结的角度探讨了干旱半干旱地区土壤表面蒸发与土壤盐渍化的关系，试验采用风干沙丘砂（A）和细硅砂（B），在天平上设置沙柱，其中仅在后一种情况下，设置具有高浓度的NaCl溶液的浅水位。将柱交替暴露于控制在65%湿度（对应于夜间）和20%湿度（白天）的环境空气中，其中空气温度保持恒定（25℃），仅在干燥空气阶段对砂表面施加550 W/m^2的热辐射。利用湿杯测量的沙柱中的水蒸气信息，分析了两种情况下蒸发区和冷凝区的位置和运动。对于情况B，湿润阶段的凝结量与干燥阶段的蒸发量基本相等，而对于情况A，蒸发量约为凝结量的10倍，其剩余蒸发量来源于下伏含水层。结果表明，B锋面前土壤水分处于化学饱和状态，锋面后土壤水分浓度随深度呈指数衰减，而蒸发锋面后土壤盐渍化程度较低。结果表明，该沙漠的蒸发源主要是降雨，在距地表10 cm处，通过降雨入渗和土水蒸发的相互作用，出现了一个溶质浓度剖面尖峰。

By paying attention to the condensation of atmospheric vapor, the relation between soil-surface evaporation and salinization in arid/semi-arid areas was explored through an evaporation experiment from a sand column and a numerical simulation to meteorological data observed in a desert area in West China (HEIFE).Using air-dried dune sand for HEIFE (Case A) and a fine silica sand (Case B), sand columns were set up on the balance, where only in the latter case, a shallow water table of NaCl solution with a high concentration was set. The columns were exposed to ambient air controlled at 65% humidity (corresponding to nighttime) and at 20% (daytime) alternately, where the air temperature is kept constant (25℃) and 550 W/m^2 of heat radiation was applied on the sand surface only at the dry air stage. Locations and movements of the evaporation and condensation zone for both cases were analyzed using information on vapor in the sand columns measured by humid-cups. For Case B the condensation amount at the wet stage was almost equal to the evaporation one at the dry stage, while for Case A, the evaporation was about ten times larger than the condensation and source of its surplus evaporation was the underlying aquifer. Soil water ahead of the evaporation front for Case B was chemically saturated and its concentration behind the front decayed exponentially with depth, while salinization for Case A was much lower.A numerical simulation using a model of heat-moisture-solute movement in the atmosphere-land system was successfully undertaken. And it was known that source of evaporation in that desert was almost rainfall and a spike of solute concentration profiles observed at 〜10 cm deep from the ground surface appeared through the interaction between rain-infiltration and soil-water evaporation.