实验流域风化基岩结构与径流形成机制研究

The weathered bedrock zone is an important component of the critical zone. The weathered bedrock not only controls the processes of soil formation and landform evolution but also is an important part of both the terrestrial hydrological cycle and ecosystem. However, the weathered bedrock layer is often covered by soil layers and thereby the information about weathered bedrock is hard to acquire, resulting in an insufficient understanding of the structural characteristics of weathered bedrock, such as bedrock topography, thickness and permeability, and how weathered bedrock layer impacts on runoff generation. For example, the weathered bedrock layer is often treated as a relatively impermeable or low-permeability layer, or as residual in the water balance equation, which limits our understanding of runoff generation processes in the hilly area and the accuracies of hydrological simulation results. In this research, we selected Hemuqiao experimental station as the study site. Through surveying the structure of the weathered bedrock and its hydraulic characteristics, the surface hydraulic properties and deep leakage of the weathered bedrock are revealed. We are also trying to investigate the interaction between soil and weathered bedrock water, the contribution of weathered bedrock to stream runoff, and the role of weathered bedrock on hillslopes, valleys, and streams hydrological connectivity. This research is the basic hydrological work, and also the frontier of hydrological science. The results of this study will enhance our understanding of runoff generation processes and will improve the watershed flood prediction ability.

风化基岩是流域关键带的重要组成部分，不仅控制着风化成土、地貌演化等过程，也是陆地水文循环及生态系统的重要组成部分。然而，由于风化基岩通常被土壤层覆盖而难于直接探测，导致风化基岩的结构特征（如基岩地表地形、风化基岩厚度、渗透性等）及其对径流形成的影响尚未引起足够的重视。现有研究通常假设土壤-基岩界面为相对不透水层，或是将风化基岩渗漏量笼统地当作水量平衡方程中的残余项处理，这不仅限制了对山丘区小流域径流形成的认知，也不利于模型理论的进一步发展。本研究以和睦桥实验流域为对象，通过对实验流域风化基岩结构探测及其水力学特征分析，揭示风化基岩表层水力性质及深层渗漏规律，分析不同降雨场次下土壤、风化基岩水流的交互过程，量化风化基岩对河道径流的贡献，揭示风化基岩在山坡-谷地-河道水文连通性中的作用。本研究是基础水文工作，也是水文科学的前沿问题，研究成果将增强人们对产流过程的认知，提高山丘区洪水预测能力。

本项目选取太湖源头区睦桥流域为研究对象，开展了水文气象要素的强化观测以及风化基岩结构对径流形成机制的影响研究。在强化观测山坡上，改造了原有的量水堰，堰内增设风化基岩井1口，使之对堰下风化基岩内水位波动有持续监测；在山坡其他点位钻取风化基岩钻井12口，收集13口钻井的岩芯样本分析风化程度及裂隙发育状况，揭示了山谷处的风化程度低、边坡处的风化程度高的特点。超出钻孔深度外的风化基岩结构（通常位于土壤-基岩界面以下2.5m），采用100MHz频率的探地雷达（GPR）进行了勘探。结果显示，基岩风化层的厚度可深至4m。另外，基于表层关键带探测及水文过程强化监测的结果，研究强调了风化基岩-土壤界面对水文连通性产生的重要性（90%降雨场次），发现了基于土壤层间流的山坡-谷地-河道水文连通方式（仅10%降雨场次）。在山坡由湿变干的过程中，发现水分依然可以通过非饱和侧向流的形式从边坡运动至谷地，并且在风化基岩与土壤的界面处产生了“阻尼振荡”式下降规律，该发现补充了一般认为仅湿润状态下地形对水分再分配起决定性作用的不足。通过无人机航拍、深度学习等方法对植被进行识别，发现了陡坡诱发的非饱和侧向流叠加骤旱对水分分配的作用，揭示了地形-水分分布-植被分布的协同关系。此外，在风化基岩中发现了毛竹根系，这一结果突出了风化基岩水对植物生长的重要作用。综合降雨、蒸散发、产流量、植物截流量以及山坡蓄量的变化，计算出约12.8%的降雨以风化基岩径流的形式流出。对比风化基岩井、土壤井、地表/壤中径流数据发现，风化基岩井对降雨的响应最频繁，退水也比土壤井退水过程缓慢的多（>10 d）。随后，本项目研发了数字水系提取技术，推导出了表征山坡地貌结构特征与径流响应关系的解析函数，并搭建了模拟原型实验的物理模型。项目研究成果在水文水资源领域如Water Resources Research、Journal of Hydrology，土壤学领域顶级刊物如Geoderma上发表。