西藏纳木错曲嘎切流域冻融侵蚀水文泥沙过程观测与模拟研究

Freezing-thawing erosion accounts for about one-third of the total soil erosion area in China. The erosion is most concentrated and strongest in the Tibetan Plateau, and becomes one of the main problems threatening the local ecological environment safety. The climate features such as low temperature and large temperature variation in the Tibetan Plateau make freezing-thawing alternate frequently and provide the suitable conditions for the type of erosion. Influenced by glaciers, snow cover and permafrost, soil erosion is driven by multi-forces including melting snow, melting ice and precipitation. As a result, the freezing-thawing erosion mechanism is quite different from other regions. In the Namco area of Tibet with strong freezing-thawing erosion, this project selects the Qugaqie catchment as the study area where mountain glaciers and permafrost coexist and water is supplied by snow and ice melt and precipitation. Through field observation of runoff and sediment load at the end of the glacier and the outlet of the catchment and monitoring of soil physical and chemical properties, soil temperature and humidity, hill slope runoff and sediment dynamics, the study will reveal the affecting mechanisms of meteorological factors, topography, vegetation, soil freezing and thawing on the melt water and rainfall erosion. Parameterization schemes of freezing-thawing soil erosion forced by melt water will be improved based on the field investigation. Using distributed, physically-based hydrological model, the hydrological and sediment processes in the catchment will be quantitatively analyzed.Through model sensitivity analysis and scenario prediction,this study will provide basis for soil erosion control and soil and water conservation planning in Tibet.

冻融侵蚀约占我国水土流失总面积的三分之一，在青藏高原地区分布最集中、侵蚀最强烈，是威胁当地生态环境安全的主要问题之一。气温低、温差大等气候特点使得青藏高原冻融交替频繁,为冻融侵蚀创造了条件；受冰川、积雪、冻土等影响，水土流失呈融雪、融冰与降水多源驱动的表现形式，冻融侵蚀机理与其它地区有较大差异。本项目在西藏冻融侵蚀强烈的纳木错地区，选取高山冰川与冻土并存、冰雪融水和降水混合补给的曲嘎切流域为研究区，通过实地监测流域内冰川末端和流域出口处的径流量、输沙量以及流域内土壤理化性质、土壤温湿度、坡面产流产沙的动态变化，揭示气象要素、地形、植被、土壤冻融等因素对冰雪融水和降雨侵蚀的影响机制。并在观测研究的基础上，完善冻融土壤冰雪融水侵蚀的参数化方案，利用基于物理机制的分布式水文模型定量研究流域的水文泥沙过程，通过模型敏感性分析和情景预估，为西藏土壤侵蚀防治和水土保持规划提供依据。

冻融侵蚀约占我国水土流失总面积的三分之一，在青藏高原地区分布最为集中且侵蚀最强，是区域面临的重要生态环境问题之一。青藏高原地区土壤可蚀性受频繁的冻融交替影响，且冰雪融水和降水多源侵蚀力共存，土壤侵蚀机理与其它地区有很大差异。本项目选取青藏高原中部高山冰川与冻土共存的、冰雪融水和降水混合补给的冬克玛底流域为研究区，分别对冰川末端和流域出口处的径流量和输沙量进行连续监测，并对流域的气象要素、地形条件、植被指数、土壤理化性质、土壤冻融过程进行了调查，同时开展了野外和室内的冻融侵蚀模拟实验，初步揭示了青藏高原中部冰川流域的土壤侵蚀机理，并提出了土壤冻融和冰雪融水与降雨多源驱动力影响下的参数化方案与输沙模型，完成了研究区水文泥沙过程的模拟与情景预测。主要研究发现和成果包括：1）研究区近50年的降水量和气温均呈显著上升的趋势，上升速率分别为7.9 mm/10a和0.32℃/10a；但近40年植被指数NVDI无显著变化趋势。2）每年的消融初期，流域径流主要来源于冰川融水，之后受降雨汇流和地下水等补给，冰川融水比例逐渐降低。3）流域径流悬移质含量最大值出现在径流量最高的雨季，但消融初期冰川区输沙量占流域总输沙量的比例为全年最高。4）消融初期径流输沙由解冻土壤提供的丰富可蚀性物质与融水的共同作用产生，而雨季的径流输沙则以降雨侵蚀为主。5）考虑土壤冻融和多源侵蚀力的影响，提出了基于洪峰次序和累积降雨变化阶段的水沙关系曲线分区方法。与传统分区方法相比，新方法有效地提高了悬移质含量和输沙量的模拟精度。6）敏感性分析结果表明，研究区的径流量和输沙量主要受气温控制的冰川融水的影响。情景预估结果显示，在当前气候环境变化背景下，研究区未来径流量和输沙量还将持续增长。本项目的研究成果为西藏土壤侵蚀防治和水土保持规划提供了理论依据。