青藏高原典型高山冻土流域融雪侵蚀机理研究与过程模拟

Increasing ice and snow melting runoff discharges during ablation period under global changing induces aggravated soil and water loess problem on the Tibetan Plateau, and has seriously threaten to the ecological security shelter function of the Plateau. However, due to the harsh observation conditions, complex water and heat transfer processes, and runoff and sediment generation mechanisms of freezing-thawing soil at the snow-melting slope, some shortages existed in the studies of snowmelt erosion mechanism and process simulation. This project selects a typical snowmelt erosion catchment in the central Plateau as the study area. 1) runoff and sediment processes observation will be conducted and their components will be divided to clarify snowmelt erosion process with the combination of soil temperature & moisture and snowmelt monitoring. 2) Slope snowmelt runoff and sediment generation processes will be observed. Besides, field in-situ simulation experiments under different slope gradients, snowmelt discharge rates, depths of melting soil layer and vegetation conditions will be carried out to reveal the affecting mechanisms of meteorological factors, topography, soil, vegetation coupled with soil freezing and thawing effect on snowmelt erosion and its water and heat transfer processes. 3) Parameterization schemes of freezing-thawing soil erosion forced by snowmelt water will be improved. Snowmelt erosion forecasting model will be established by the combinations of snowmelt model, soil erosion model and hydrological model thereafter. At last, the proposed snowmelt erosion model will be validated and evaluated with filed observation data. This study aims to predict snowmelt erosion process in the high mountain permafrost regions and provides guidelines for soil and water conservation planning in the Tibetan Plateau and similar regions.

全球变化背景下，青藏高原消融期融冰融雪径流增加引起的水土流失问题日益加剧，严重危害了高原的生态安全屏障作用。受恶劣观测条件和融雪坡面冻融土壤复杂的水-热传输过程和产流产沙机制的影响，相关融雪侵蚀机理与过程模拟研究存在不足。本项目拟选取高原中部融雪侵蚀强烈的典型流域：1）结合流域土壤温湿度过程与积雪变化监测，通过流域径流泥沙过程观测与径流泥沙组分辨识，明确流域融雪侵蚀过程；2）通过坡面融雪产流产沙观测与田间不同坡度、融雪流量、土壤解冻深度、植被条件下的模拟融雪侵蚀机理试验，揭示气象、地形、土壤、植被和土壤冻融的耦合作用对融雪产流产沙的影响机制和坡面水-热传输的影响；3）完善冻融土壤融雪侵蚀的相关参数化方案，结合融雪模型、土壤侵蚀模型和水文模型建立融雪侵蚀预报模型，利用观测数据对模型进行验证与适用性评价。本研究拟实现高山冻土流域的融雪侵蚀过程预报，为青藏高原等相似区域的水土保持规划提供依据。

快速升温背景下青藏高原高寒河源区径流增加，尤其是春季融水增加引起水土流失加剧，对区域生态和水环境产生威胁。受多源径流补给、水力叠加冻融作用复合侵蚀等复杂驱动机制以及观测手段的限制，河源区融雪侵蚀规律与模拟研究存在一定不足。本项目选取高原中部融雪侵蚀强烈的长江源典型高山冻土小流域为研究区，在开展气象、水文、土壤等要素监测的基础上，结合同位素水化学径流分割、泥沙指纹识别、核素示踪等手段，明确了融雪期解冻坡面的产流机制、泥沙主要来源以及主要侵蚀沉积区，发展了基于温度的解冻土壤可蚀性估算方案，开展了融雪径流与基于融雪侵蚀的泥沙过程模拟，揭示了流域融雪侵蚀输沙特征。主要研究发现如下：1）流域春季融雪侵蚀主要发生在5月中至6月上旬，此阶段始终伴随着表层土壤的昼夜冻融；2）融雪期冻结坡面融雪径流以地表产流为主，随着解冻深度的增加入渗比例增加产流系数减小；3）从融雪侵蚀事件的泥沙来源来看，来自坡面的泥沙平均占比为50.8%，其中草甸和裸地坡面分别占18.3%和32.5%，另外47.5%来源于冰川冰碛物。从位置来看，随着融雪径流的逐渐汇集以及低海拔较早解冻可蚀性物质相对较多，侵蚀量从坡顶向坡底逐渐增加，但在坡脚缓坡处发生部分沉积；4）基于遥感影像和气象数据发现流域积雪覆盖率与气温呈负相关，基于模拟冲刷实验发现土壤可蚀性K与冻结强度（温度）呈指数函数关系；5）通过流域总径流量以及融雪径流组分等径流分量变化模拟，进而得到了融雪径流与基于融雪侵蚀的含沙量变化，由此得到冬克玛底流域年融雪侵蚀产生的输沙量为3400±1400 t/a，占全年流域总输沙量的14.2±2.9%，年融雪侵蚀的输沙模数为108.0±43.4 t/km2 a，其中6 月最大单月模数为82.5±38.5 t/km2，高于欧洲阿尔卑斯和北美地区，与研究区土壤发育年轻、冻融作用强烈等因素密切相关。