鄂尔多斯盆地风沙滩区土壤—地下水蒸散发机理研究

Evapotranspiration (ET) is a dominant discharge item of groundwater in arid and semi-arid area. Restricted by the complex influence factors, highly nonlinear and spatio-temporal variation, mechanism of ET roots from soil-groundwater system in arid and semi-arid area is relatively weak up to now. To make progress, choosing Aeolian sandy desert in Ordos Basin as the research area, the proposed project takes the relationship between ET and depth to groundwater table (GWT) as the main line. Moreover, combining in-door hydraulic parameters, heat parameters test and in-situ observation with physical stimulation and numeric simulation, it tries to launch four incremental research contents. Firstly, evaporation, transpiration and soil water and heat spatio-temporal changes will be analyzed. Secondly, the inner mechanism between evaporation and soil water potential gradient will be discovered, as well as soil temperature gradient to cover the shortage of hydraulic connectivity from the water table up to the surface assumption. Thirdly, interact mechanism of transpiration with soil matrix potential of the plant-soil continuum will be illustrated. Moreover, root uptake mechanism will be revealed and the relationship between transpiration and GWT will be established. Lastly, the numerical model, which coupled liquid water, water vapor and heat movement for partitioning ET, will be constructed and solved. And then, contributions of soil water, groundwater to evaporation as well as transpiration will be calculated respectively. Therefore, the outcomes of the project will guide groundwater resources assessment and vegetation reforestation in arid and semi-arid area.

蒸散发是旱区地下水的重要排泄项。受旱区饱和—非饱和带水热运移影响因素复杂、高度非线性与时空变化大制约，迄今对蒸散发机理的研究比较薄弱。本项目以鄂尔多斯盆地风沙滩为典型研究区，以探索地下水埋深与蒸散发关系为主线，采用原位试验与动态观测、物理模拟与数值模拟相结合的方法，分析蒸发、蒸腾与土壤水热在不同地下水埋深下的时空分布规律，揭示蒸发与土壤水势梯度、温度梯度的内在机制，解决蒸发面处于土壤内部时水力传导度不连续的难题；阐明蒸腾与植物—土壤连续体内水势梯度的互馈机制，揭示蒸腾与根系吸水机理，确定蒸腾与地下水埋深的关系；构建蒸发—蒸腾与饱和—非饱和带水汽热运移的动力学模型并求解，解析土壤—地下水蒸散发的构成比例，为旱区地下水资源评价和植被生态重建提供科学支撑。

蒸散发研究对水资源评价、植被生态环境保护具有重要的理论意义和实际应用价值。本项目以鄂尔多斯盆地风沙滩为研究区，以探索地下水埋深与蒸散发关系为主线，开展了小时监测频率的长期的土壤—地下水蒸散发原位实验，获得了蒸散发、大气、植被、土壤及地下水动态变化重要科学数据。研究了土壤—地下水蒸散发的变化规律及动力学机制，主要成果和结论如下：.（1）在鄂尔多斯盆地风沙滩区，主控蒸散发的大气需水远大于多孔介质输水能力。蒸散发与气象因子的拟合度达80%以上，净辐射和温度为主要影响因素，其次为风速和湿度。.（2）查明了土壤蒸发的动力学过程，厘定了地下水的极限蒸发深度。风积沙土壤蒸发过程可以分为三个阶段：第1阶段毛管流维系蒸发，地下水埋深小于30.7cm；第2阶段薄膜流维系蒸发，地下水埋深介于30.7-60cm左右；第3阶段水汽扩散维系蒸发，极限蒸发深度为71cm。蒸发量应分段计算：潜水埋深小于31cm时，按指数型公式E=0.043*exp（H/10.3）-0.05计算蒸发；潜水埋深介于31-71cm时，应按幂函数E=0.765+0.185/（1+H/81.1）3.716计算蒸发。.（3）厘定了优势植被蒸腾的主控因素，确定了植被蒸腾地下水的临界埋深。优势植被沙柳蒸腾量的变化主要受净辐射驱动，但同时依赖于空气温度、相对湿度和风速。垂向根系发育深度为125cm，能够利用埋深小于215cm的地下水，此时沙柳实际蒸腾与潜在蒸腾的比例为0.5。蒸腾与潜在蒸腾的比值随地下水水位埋深变化为非线性，可以分段用指数函数和幂函数表示。.（4）改进了土壤水力参数方程，构建了考虑蒸发3个阶段的土壤—地下水系统水气热耦合模型，实现了土壤—地下水蒸发的数值仿真模拟。新的方程考虑了薄膜流对蒸发的作用，可以刻画含水率从0至饱和含水率的土壤水力参数。将新建立的方程嵌入至开源代码的Hydrus-1D软件，通过野外实测数据验证发现改进模型计算的含水率、温度变化规律与实测值一致性较好。数值模拟发现，基质势梯度驱动下的水分通量较温度驱动下的水分通量大2-4个数量级，是毛乌素沙地蒸发的主要驱动力。土壤剖面3-20cm 之间是薄膜流占优区。