植被恢复与坝库工程对皇甫川流域水土过程的调控与模拟

Vegetation cover changes and dams construction are the two dominant factors influencing the hydrological processes in the watershed. Large amount of soil and water conservation measures have greatly altered the land surface on the Loess Plateau, and led to significant changes in surface runoff and sediment yields. Assessing the variation of hydrological cycles and the driving mechanisms can provide good reference for land and water resources sustainable development and utilization. The project selects the Huangfuchuan watershed as study area, and investigates the relationship among vegetation cover, runoff production and soil erosion through field measurements and rainfall simulation. The total station and check-dam sedimentation drilling will be used for quantify sediment yield dynamics and the dominant factors. Temporal variation in runoff and sediment load and their potential causes will be analyzed using the hydro-climatic and sediment load data at Huangfu gauge. The geographic information data including geology, soil types, vegetation cover, soil and water conservation measures and socio-economic data etc. will be collected as input for model simulation. The semi-distributed hydrological model (Water Availability in Semi-Arid environments - SEDiments, WASA-SED model) is applied to quantify the impacts of vegetation restoration and dams’ construction on hydrological processes in the Huangfuchuan watershed. Both field survey data and gauge observations will be used to cross-verify the model simulation results. Scenario modeling exercises are undertaken to clarify temporal changes in runoff and sediment load and to reveal the driving mechanism of their changes in the study area. The results of the project are great helpful to understand the changes of hydrological cycles disturbed by intensive human activities, and provide scientific basis for soil and water loss control on the Loess Plateau and sediment load adjustment for the Yellow River.

地表覆被变化与坝库工程是影响流域水土过程的主要因素。大规模的水土保持措施极大的改变了黄土高原下垫面，进而改变地表产汇流机制与产输沙过程。科学认识变化环境下流域水土过程演变特征与驱动机制是流域水土资源合理开发利用的前提。本项目以黄土高原皇甫川流域为研究区，通过野外调查、监测与试验，研究植被特征与地表产汇流、侵蚀产沙关系，探讨坡面尺度植被恢复对地表水土过程的影响；采用全站仪测量、淤地坝钻孔等技术反演坝控流域侵蚀产沙强度，阐明坝控流域历史产沙变化特征与主控因素；系统分析流域水沙演变特征及其驱动因素；采用改进的WASA-SED分布式水文模型研究流域水土过程对不同时期下垫面变化的响应，揭示植被恢复与坝库工程对流域水土过程的调控机制。该项目有利于深入认识强烈人类活动干扰下的流域水土过程演变规律，为黄土高原水土流失防治与黄河水沙调控提供科学依据。

近年来，黄河水沙变化特征规律及各要素的影响机制成为研究的热点，但针对黄河水沙锐减原因仍存在较大争议，气候、植被与坝库工程等要素对流域水土过程的影响机制仍不清楚。本研究在系统搜集流域气候、水文、地形、土壤等基础数据的基础上，定量评估了皇甫川流域1950s-2018年水沙多时空演变特征，辨析了流域水沙场次洪水事件的水沙关系、频次、量级变化，揭示了植被盖度、结构对水沙的调控效应，发现60%的植被盖度是控制土壤侵蚀的临界阈值，但并未发现产流的植被盖度阈值。选取典型淤地坝控制流域，研究淤地坝泥沙沉积旋廻特征及淤积过程，获取1958-2014年的坝控流域侵蚀产沙强度，1958-1972年的平均产沙模数为12471.43 t/(km2·a)，1973-1997年为18180.87 t/(km2·a)，1998-2014年为10826.77 t/(km2·a)。对比分析了六种方法区分气候变化和人类活动对皇甫川流域输沙变化贡献。结果表明，人类活动在皇甫川输沙量的减少中起着主导作用，占总减少量的93.6±4.1%，而其余6.4±4.1%的减少归因于气候变化。采用WASA-SED模型对皇甫川流域水土过程进行模拟，模型对日径流输沙模拟效果较好。率定期（1978-1980年）日径流的NS系数为0.87， R2为0.88；验证期（1981-1984年）日径流的NS系数为0.63，R2为0.68。率定期输沙量的NS系数和R2分别为0.74、0.79，验证期的NS系数、R2分别为0.59和0.60。通过不同的情景模拟发现，变化期I（1990-1998年），气候变化、土地利用变化和淤地坝对径流减少的贡献率分别为26.5%、34.3%和 43.5%，对输沙量减少的贡献率分别为32.8%、63.4%和88.9%。各因素协同作用对径流输沙减少的贡献率为-4.3%和-85.0%。变化期II（1999-2015年），对于径流量，气候变化、土地利用、淤地坝的贡献率分别为17.2%、31.7%和32.4%。对于输沙量，气候变化、土地利用、淤地坝对输沙减少的贡献率分别为15.2%、45.1%和63.3%，各因素综合贡献率为-23.6%。