覆被格局演变对流域水沙过程的影响机理与定量解析

Based on the relationship between landscape pattern, structure and function, ecosystem restoration by remolding watershed landscape pattern has changed the spatial structure of runoff and sediment source and sink areas (e.g. connectedness) and optimized the spatial allocation of land use, and that contributed to soil and water conservation. Currently, the response mechanism and quantitative analysis of runoff and sediment process to watershed landscape pattern change is a hot issue. However, a critique of conventional landscape metrics for lack of consideration on the runoff and sediment process, there is still some limitations in quantifying the variation of water-sediment transport in watersheds; meanwhile, there is great uncertainty to determine the variation of relationship between runoff and sediment due to the difference of time scales. Therefore, we take the Northern Luo river basin in Loess Plateau region for research objects, the main objectives of this study are to (i) develop landscape hydrological metrics based on the runoff and sediment process, (ii) quantitatively analyze water-sand regime changes and factors which contribute to the changes, and (iii) simulate water and sediment yield under climate and land-use scenarios to understand the response mechanism of the runoff and sediment process to watershed landscape pattern based on a distributed hydrological model. Overall, the development of landscape hydrological metrics in this study is an exploration and practice in landscape ecology, and the establishment of the differential equation for simulating the water and sediment yield is an innovative approach to determine the variation of runoff-sediment relationship. The research on the response mechanism of the runoff and sediment process to watershed landscape pattern will have an important scientific and practical significance in improving water and natural disaster management.

根据景观格局-结构-功能的关系，通过重塑流域覆被格局，生态恢复改变了泥沙源、汇功能区的空间结构（如连通性），土地利用类型空间上的有效组合发挥了水土保持的功能。覆被格局演变下流域水沙过程的响应机制与定量解析是目前的热点问题。由于缺乏水沙过程基础，常规景观指数在水沙过程变化研究中存在局限；同时，流域水沙关系因时间尺度的差异存在巨大不确定性。因此，本项目以黄土高原地区北洛河流域为研究对象，①建立基于侵蚀产沙过程的水文响应景观指数，②定量分析流域水沙情势变化并解析各贡献因子，③基于分布式水文模型研究历史气候和土地覆被情景下流域产水产沙，从而认识流域水沙过程对覆被格局演变的响应机制。本项目建立的水文响应景观指数是景观生态学理论的重要实践，流域输沙变化微分方程的提出是水沙变化研究的方法创新。流域水沙过程对覆被格局演变的响应机制有助于提高区域水资源管理和灾害防治的能力，具有重要的科学价值与实践意义。

系统认知覆被格局演变对流域水沙过程的影响机理是黄土高原开展生态恢复遏制水土流失的理论和现实需求。然而，流域水沙过程的不确定性、高度非线性关系、时空分异和数据的稀缺性对目前水沙变化的科学评估带来巨大挑战。本项目提出了流域输沙变化微分方程；提出输沙率曲线新的表现形式；求得其解析解和数值解。为准确评估极端降水空间分布及其时空演变，本研究通过集成通用多重分形理论和分割算法，提出了基于非线性理论的极端降水阈值判定方法，具有明显的方法论优势。结果表明，黄土高原中部是本区极端降水强度最高的区域，是该地区土壤侵蚀最严重地区的气候因素。黄土高原91%的极端降水发生在6-8月，极端降水中心日早于降水中心日11天，揭示了本区沙峰早于洪峰出现的气候原因。主导产流的气候因子依降序分别为年降水量（PT）和有效降水量（R5），年最大极端降水事件（MPE）、大雨降水量（R25）、大雨天数和年最大连续降水天数（CWD）。主导产沙的5个因子依次是MPE，R25，侵蚀性降水量，PT和CWD。主导气候因子的产水产沙动力受流域特征影响，其中耕地是正效应主导因子，LAI、林地和降水梯度是负效应主导因子。黄土高原泥沙调节存在两种途径：①调节径流和改善水-沙关系，②减少径流。在水土保持综合治理期（1980-1999）和退耕还林期（2000-2015），生态恢复的减沙作用由63%增加到81%；减水贡献由55%增加到75%。泥沙调节的主导水土保持措施由工程措施（淤地坝）转变为植被措施（林地）。退耕还林期的调水减沙效率显著高于水土保持综合治理期，但随着输沙模数的持续下降，生态恢复的调水减沙效率降低，减沙水代价由5.2 m3/t提高到6.4 m3/t。随着生态恢复面积比例的持续增加，通过增大生态恢复面积以进一步调节径流和泥沙的效率下降，在水资源趋于短缺的背景下，流域精准管理是黄河流域生态保护与高质量发展的未来方向。