西南喀斯特坡地降雨-产流阈值时间变化特征及其形成机制

With the features of the well-developed surface-subsurface dual spatial structure, thin and discontinuous soil layer, poor soil water storage capacity and relatively high hydraulic conductivity of bedrock, the hydrological processes in karst region of southwest China are rapid, and flood disaster occurs frequently. Revealing the temporal variation characteristics and the formation mechanism of rainfall-runoff threshold are the foundation of flood disaster warning and hydrological forecast. Nevertheless, the “fill and spill” theory applied in other regions cannot completely explain the threshold phenomenon on karst hillslopes. In this project, a typical karst hillslope is selected as our study object, through monitoring the soil water contents in different depths and slope positions, the formation and expansion of the saturation area at the soil-bedrock interface, as well as runoff (including surface flow, subsurface flow and epikarst seepage flow) characteristics, based on the combination of rainfall simulation and natural rainfall events in situ, this study aims to determine the mean infiltration rate of the bedrock, reveal the temporal variation of threshold is different time scales (rainfall events, seasons, and years), clarify the effect of soil thickness and the infiltration rate of bedrock on threshold, as well as illustrate its formation mechanism. Lastly, the Hydrus-3D model will be employed and validated based on the above experiments to predict the threshold variation by simulating varieties of hillslope characteristics (slope gradient, soil thickness, antecedent soil water contents, and soil and bedrock hydraulic conductivity) as well as extreme rainfall events scenario. The obtained results will be useful for the understanding of the hydrological processes on karst hillslope in southwestern China and will help provide scientific basis for the prevention of flood disaster in return.

西南喀斯特山区二元结构发育，土层浅薄且不连续、土壤储水能力低且基岩渗漏性强，使得水文过程迅速、洪涝灾害频发。明晰坡地降雨—产流阈值时间变化特征及其形成机制是洪涝灾害预警的基础，但实际观测表明其他地区适用的“充填和溢出”理论并不能完全解释喀斯特坡地的阈值现象。本项目拟以典型喀斯特坡地为研究对象，采用模拟降雨和自然降雨监测相结合的手段，通过观测坡地剖面土壤水分动态、土壤-基岩界面饱和区的产生和扩展，及产流特征，确定基岩入渗率，探明阈值在不同时间尺度的变化，阐明土层厚度和基岩入渗率对阈值的影响，并揭示阈值形成机制。在上述研究基础上，用观测数据率定Hydrus-3D模型参数，设置多种坡地特征（坡度、土层厚度、前期土壤含水量、土壤和基岩入渗率等）和极端降雨情景组合，基于数值实验进行情景模拟，阐明阈值对典型山坡特征和极端气候事件的响应。结果将有助于理解喀斯特坡地水文过程，并为防治洪涝灾害提供科技支撑。

我国西南喀斯特山坡二元结构发育，土层浅薄且不连续、土壤储水能力低且基岩渗漏性强，使得水文过程迅速、洪涝灾害频发。然而目前喀斯特山坡多界面产流机制尚不明确，降雨-径流关系也缺乏研究，阐明产流机制和降雨-径流关系是洪涝灾害防治的基础。本项目首先基于喀斯特坡地三维水土过程小区开展模拟降雨实验，通过连续监测土壤-表层岩溶带水分动态、土壤-表层岩溶带界面瞬时地下水位时空格局，揭示了西南喀斯特山坡地表径流（超渗-蓄满，也即Dunton产流机制）与壤中流（充填-溢出）产流机制；根据水量平衡原理和地表径流产流机制，确定了土壤-表层岩溶带界面的稳定入渗速率（约为30-50 mm/h），最后提出了二元结构喀斯特坡地多界面产流的概念模型。其次，基于2018-2019年43场自然降雨事件中12个坡地径流小区（5m×20m）的降雨、径流（地表径流、壤中流及表层岩溶带侧渗产流）及土壤含水量（10cm、30cm及土壤-表层岩溶带界面处）数据，量化了喀斯特山坡不同径流成分的径流系数（地表径流、壤中流及表层岩溶带侧渗产流的径流系数分别为2.2%、3.8%和31.2%）；基于结构方程模型发现土层厚度控制地表径流量，二者为正相关关系；地表径流量和壤中流量与降雨量之间存在明显的阈值关系，阈值分别为35.7±1.8 mm和43.4±7.4 mm；且存在多种降雨-径流阈值关系类型，如曲棍球型、阶梯型及S型，表明喀斯特坡地水文系统的复杂性，阈值的确定方法需根据降雨-径流阈值关系的形状来选择。最后，基于不同降雨强度模拟降雨实验，阐明了降雨强度对产流的影响，发现随雨强度的增加，水分由垂向（深层渗漏）转为侧向（多种径流成分）运移；探讨了径流产生与消退过程和土壤水分之间的滞后效应，表现为顺时针（壤中流）或逆时针（表层岩溶带径流）的“绳套关系”。本项目研究成果可为西南喀斯特地区水资源管理、洪涝灾害防治以及石漠化治理提供科学依据。