基于热追踪方法的非均质潜流交换机理研究

Hyporheic zone is the transition zone between surface water and groundwater, which is an essential part of hydrologic cycle. Consequentially, hyporheic exchange is of importance for water resources evaluation and limits the development of nearby surface water and groundwater ecosystems. In this project, three subjects will be addressed using the high-density thermal observation and in-situ permeameter test. First, the heterogeneity of hyporheic zone is characterized by the spatial distribution of hydraulic conductivities. In-situ permeameter test is performed to estimate the hydraulic conductivity of streambed sediments located in hyporheic zone. A high-density sampling method is conducted in the study of heterogeneity of hyporheic zone. The heterogeneity and anisotropy of hydraulic conductivity will be obtained through the geostatistics analyses. Secondly, based on the real-time monitoring temperature and water level datasets from streambed at multi-depths, the process of heterogeneous hyporheic exchange is recorded. Inverse modeling using thermal tracing method is applied to extend the insights of the hyporheic exchange in streambed. Hydraulic based method, such as Darcy's Law, is used to calibrate the thermal and hydrogeologic properties of hyporheic zone. Such factors affected the hyporheic exchange are identified by these field observations. Finally, a conceptual model of heterogeneous hyporheic exchange is established. Numerous hypothetical numerical simulations will be involved to distinguish the principal of heterogeneous hyporheic exchange; moreover, the mechanism of such influenced factors will be highlighted based on these results. Based on this study, some novel knowledge of hyporheic exchange under a heterogeneous streambed is expected, leading a prosperous theory system of the interaction between surface water and groundwater.

潜流带作为地表水和地下水之间的相互作用带和过渡带，占据着水文循环过程中的重要位置，潜流交换作用对准确评价和开发利用流域水资源，维持和修复地表水及地下水生态系统健康具有重要意义，因而深入研究潜流交换机理具有重要的科学价值。本项目以原位试验和现场监测为主要研究手段，通过潜流带非均质特征参数的高密度采集，刻画潜流带渗透系数在三维空间上的非均质性和各向异性特征；基于对含水层、潜流带和地表水温度和水位数据的现场实时监测，采用热追踪方法反演非均质潜流交换过程，描述潜流交换过程的非均质特性，确定潜流交换的影响因素；建立非均质潜流交换的概念模型，以数值模型为工具，揭示非均质潜流交换机理以及各要素的影响机制。该项目将在非均质潜流交换机理研究中有所突破，其结论可以丰富和完善地下水与地表水相互作用的理论体系。

潜流带是地表水与地下水的相互作用带和过渡带。潜流带发生的水量交换大小可用潜流通量（q）来表征。本文基于地表水-地下水相互作用的热追踪（温度示踪）方法，选取山东省济宁市汶上县境内大汶河段作为试验场地，主要对河流横断面浅层河床温度进行现场监测，采用渗透实验的方法测定了潜流带三维空间的渗透系数大小，定量描述其非均质性，运用热量运移解析模型求解潜流通量，详细描述潜流通量的空间非均质特征和动态特征，通过建立数值模型确定潜流交换的主要影响因素。.河床渗透系数随机性与结构性并存，渗透系数具有空间变异性，同时存在一些变化规律。渗透系数值随深度增加而减小，而其变异性随深度增加而增大；渗透系数在垂直于河岸的横断面上，有靠近河岸数值偏小，远离河岸数值偏大的趋势；渗透系数大小及其非均质性受潜流交换携带的细颗粒物质沉积的（二次沉积）影响较大；河床地形与潜流带渗透系数存在一定的相关关系，河床地形起伏使得深水区介质的渗透系数要大于浅水区的。河床渗透系数受洪水影响较大，汛期前后两次试验，河床高程下降40 m；汛前试验河床以中粗砂为主，讯后河床20-50cm深度出现大量破碎黏土层；结构性差异较大，汛后试验河床渗透系数在垂直于河岸方向规律性较弱；与汛前相比，汛后河床非均质性更强。.通过对稳态和非稳态潜流通量的分析表明垂向潜流通量的数量大小和交换方向呈现空间非均质特征和动态特征；潜流通量数量上最多相差3个量级；空间分布上潜流通量高值区位于河道中部，低值区多发生在两侧河岸和局部浅层河床；潜流通量呈现动态变化特征，但变幅较小；潜流交换方向以地表水补给地下水为主，但在空间和时间上存在差异。热力学参数灵敏度自大到小排序为：热传导率>孔隙度>固体热容和水的热容。.采用数值模拟方法研究在河床横剖面地形不均匀的条件下，潜流交换量的空间分布以及地下水流场的演变机制 结果表明: 在河床地形起伏不均的情况下，潜流交换量更易发生在河道的深水区域; 地下水流向受河床地形影响较小; 近河床界面处的地下水流速受地形起伏影响剧烈，深水区域的地下水流速远大于浅水区地下水流速; 通过与现场试验结果对比分析，得出河床地形起伏是引起潜流带渗透系数非均质现象的重要原因之一。