Hydrothermal transfer and bare soil evaporation in surface-groundwater systems in semi-arid areas
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摘要:
地表-地下水系统水、热迁移转化与裸土蒸发机理研究对于水量平衡以及地表能量转化具有重要意义。以鄂尔多斯盆地风沙滩地区为研究区，基于原位蒸渗仪长期观测，结合数值模拟，选择2种地下水位初始埋深分别为80 cm（浅埋深）和290 cm（深埋深）的情景，研究了变饱和带水热迁移转化的动力学过程以及对裸土蒸发的影响。结果表明：变饱和带土壤水的运动规律受水头梯度和温度梯度的共同驱动，且在不同水位埋深条件下呈现不同的运动方式；浅埋深条件下，受水头梯度的作用，土壤的毛细上升高度能够到达地表，蒸发条件下土壤水在毛细力驱动下向上运移，土壤内部不存在零通量面，温度对水分运动的影响较小，发现当地下水位埋深小于毛细上升高度时，地下水在毛细力作用下直接贡献土壤蒸发；深埋深条件下，水头和温度是土壤水运动过程的关键因素，位于地表以下18 cm以浅土壤内部出现孤立的零通量面，阻止了土壤水的向上运移，导致蒸发量减小。当地下水位埋深大于毛细上升高度的1.6倍时，地下水不再直接参与土壤蒸发，但会间接地影响包气带的水分转化；因此模拟期间浅埋深的裸土累积蒸发量约为深埋深累积蒸发量的4倍。
Abstract:The research on the mechanism of hydrothermal transfer and transformation and bare soil evaporation is of great significance for water balance and surface energy transformation in surface-groundwater systems. In this paper, based on long-term observations of in situ lysimeter test and numerical simulation, two cases for lysimeter with initial groundwater depths of 80 cm (shallow groundwater depth) and 290 cm (deep groundwater depth) are used to examine the dynamic process of hydrothermal transformation in the variable saturation zone with its effect on bare soil evaporation in the “wind-blown sand area” of the Ordos Basin in China. The results indicate that soil water movement in the variable saturation zone is driven by the head gradient and temperature gradient, showing different movement modes under different groundwater depths. For the situation of shallow groundwater depth, the capillary height can reach the surface, soil water transports upwards with little temperature effects driven by capillary force under the effects of head gradient under the evaporation conditions, and there is no zero flux plane in the soil. When the groundwater depth is less than the capillary height, groundwater directly contributes to soil evaporation driven by capillary force. For the situation of deep groundwater depth, head and temperature are the key factors in the soil water movement, and there is an isolated zero flux plane located at 18 cm below the ground surface, which prevents the upward soil water movement and leads to a decrease in evaporation. When the groundwater depth is greater than 1.6 times the capillary height, groundwater no longer directly involves in soil evaporation, but indirectly affects the water conversion in the unsaturated zone. Therefore, during the simulation period, the cumulative bare soil evaporation of the shallow groundwater depth was about 4 times that of the deep groundwater depth.
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