黄土丘陵区小流域土地利用/景观格局演变与侵蚀产沙的响应关系

The interaction between landscape pattern and soil loss represents a key topic in exploring the pattern-process interplay in integrative physical geography and landscape ecology studies. Runoff yielding in the semi-arid Loess Hilly Region is dominated by excess infiltration, and regional soil loss is predominantly influenced by land use/landscape pattern, which has been altered considerably in the context of regional soil conservation and ecological restoration, leading to continuous changes in the processes of catchment sediment yield. It is thus of great theoretical and practical significances to investigate the processes of sediment yield in typical catchments in the Loess Hilly Region and their response to changes in land use/landscape pattern. This study attempts to reveal the runoff and sediment dynamics under current catchment surface condition and the coupling of these two processes during storm events through field monitoring，and apply CSSI and geochemical fingerprints which are capable of indexing the vegetation type to quantify the relative contribution of land use-based source units to within-storm sediment yield; combine stratigraphy differentiation, Cs-137 dating, Pb-210 model to reconstruct the historical sedimentation processes in an outlet check-dam and catchment sediment yielding, and infer the historical changes in sediment source composition through CSSI and geochemical fingerprinting; calculate landscape metrics using historical land use maps and reveal its impact on catchment sediment yield through statistical analysis.

景观格局与水土流失的关系是综合自然地理学和景观生态学格局-过程研究的关键问题。半干旱黄土丘陵区以超渗产流为主，土地利用/景观格局是影响水土流失的重要因素，其在区域水土保持和生态恢复背景下发生重大变化，驱动流域侵蚀产沙过程持续演变。开展黄土丘陵区典型流域侵蚀产沙过程及其对土地利用/景观格局变化的响应研究具有重要理论和现实意义。本研究通过野外监测揭示当前流域下垫面条件下次暴雨过程的水、沙动态过程和耦合特征，运用能表征地表覆被状况的CSSI和地球化学元素示踪，定量估算典型覆被单元对次降雨过程流域产沙的贡献；结合淤地坝沉积物旋回识别、Cs-137断代和Pb-210沉积模型，重构坝地泥沙沉积和流域侵蚀产沙过程，采用CSSI和地球化学元素示踪反演坝地沉积物来源变化；根据历史土地利用计算景观格局指数，结合统计分析阐明其对流域侵蚀产沙的影响机制。

黄土高原生态环境脆弱，人类活动强烈，土地利用粗放，是我国典型的水土流失区和黄河泥沙主要来源。黄土高原经历了复杂的人类活动过程，实施了水土保持和生态修复措施，导致区域土地利用和覆被格局发生重大变化，驱动流域侵蚀产沙过程持续演变。黄土高原生态治理突出表现在治理模式简单粗放，措施实施范围局限在小区域尺度，治理效益缺乏稳定性和可持续性，治理成果可推广性差等方面，缺乏充分的理论认识支撑。开展黄土高原侵蚀产沙过程对人类活动的响应研究，探明各项水土保持和生态修复措施的效益及潜在机理，为当前措施的优化调整提供理论支撑。本研究选取黄土丘陵区典型恢复小流域，通过坝地沉积序列重构及来源复合指纹示踪，揭示淤地坝泥沙沉积过程及基于土地利用类型的来源构成。研究结果表明：（1）降雨是半干旱黄土丘陵区侵蚀产沙的主要驱动力，降雨特征（强度和频率）决定侵蚀产沙的时间动态过程和强度。侵蚀性降雨主要发生在雨季6～9月，侵蚀产沙和坝地淤积主要发生于频次低、历时短、强度大的次暴雨过程。因此，历史降雨资料能有效指示流域侵蚀产沙和坝地沉积事件，为坝地沉积反演提供重要前提。（2）坝地沉积物和流域内来源土壤的粒度、有机质和Cs-137比活度存在差异，反映了侵蚀产沙过程的差异和土地利用/覆被类型的影响。（3）通过沉积剖面粒度和Cs-137的分布，辨识出沉积旋回层结构，结合建坝历史和降雨资料，筛选出有效的暴雨事件，作为反演沉积旋回的年代标记，重构出坝地泥沙淤积时间序列过程。（4）基于泥沙来源复合示踪框架，根据地形分异和土地利用划分出五类来源类型，筛选出9种具有诊断识别能力的指纹因子，估算了各来源对坝地沉积物的相对贡献。