退耕驱动近地表特性变化对侵蚀过程的影响及其动力机制

The implement of the “Grain-for-Green” project has caused great changes in near soil surface characteristics (e.g. soil physical and chemical properties, vegetation stem, litter cover, biological soil crust, and root system), which likely lead to considerable responses in soil erosion processes. However, the degrees and their dynamic mechanisms are not clear. Therefore, this project is planning to quantify the effects of near soil surface characteristics driven by farmland abandonment on soil erosoin processes and demonstrate the responsed mechanisms of overland flow hydro-dynamics, soil detachment, sediment transport, and sediment deposition to those changes via field survey, monitoring, simulation, and modeling. The regression equations between the adjustment coefficients of rill erodibility, critical shear stress and near soil surface characteristics will be developed and soil detachment process of vegetation covered hillslope will be simulated. The functions between sediment transport coefficient and near soil surface characteristics will be proposed and the sediment transport capacity equation of overland flow will be developed. The quantitative effects of vegetation belt on sediment deposition process will be analyzed and the changes in soil and water conservation benefits with slope gradient will be revealed. The sediment deposition model will be developed for vegetation hill slope. The results will be helpful for quantifying the effects and their dynamic mechanisms of the “Grain-for-Green” project on soil erosion processes, evaluating their soil and water conservation benefits, analyzing the evolution of regional erosion environment, and developing the process-based soil erosion model for vegetation covered hillslopes.

退耕还林（草）工程的大面实施，必然会引起近地表特性（土壤理化性质、植被茎秆、枯落物、生物结皮、根系系统）的显著变化，进而导致土壤侵蚀过程发生响应，但其程度及动力机制尚不清楚。本项目拟采用调查分析、原位监测、模拟试验、数值计算、模型模拟等技术与方法，系统研究退耕驱动的近地表特性变化对侵蚀过程的影响，揭示坡面流水动力学特性、土壤分离过程、泥沙输移过程和泥沙沉积过程对近地表特性变化的响应机制。发展细沟可蚀性和临界剪切力修正系数与近地表特性间的数学模型，实现退耕坡面土壤分离过程模拟；提出泥沙输移系数与近地表特性间的函数关系，建立退耕条件下坡面流挟沙力方程；明确泥沙沉积与植被带参数间的定量关系，揭示植被拦沙效益随坡度的变化规律，建立植被坡面泥沙沉积模型。为阐明退耕还林（草）工程对土壤侵蚀过程的影响及其动力机制、评价其水土保持效益、分析区域侵蚀环境演变规律和构建退耕条件下土壤侵蚀过程模型提供科学依据。

植被恢复驱动的植物群落近地表特性变化，可能会显著影响土壤分离、泥沙输移和泥沙沉积过程，本项目系统研究了植被恢复驱动近地表特性变化对坡面径流水动力学特性、土壤分离过程、径流挟沙力和泥沙沉积过程的影响及其动力机制。发现植被茎秆盖度和排列方式、枯落物盖度、生物结皮类型和盖度，显著影响坡面径流水动力学特性。随着茎秆、枯落物和生物结皮盖度增大，地表随机糙率增大，径流阻力增强，流速下降，侵蚀动力减弱，这是植被恢复影响土壤侵蚀过程重要的动力机制之一。随着生物结皮盖度增大，土壤侵蚀阻力呈指数函数增大，苔藓结皮对土壤分离过程的影响显著大于藻类结皮；生物结皮盖度的季节变化导致土壤侵蚀阻力呈现显著的季节波动。随着枯落物与表土混合密度增大，土壤侵蚀阻力呈指数函数增大，随着表土中枯落物的逐渐分解，枯落物对土壤侵蚀阻力的影响逐渐降低；随着根系密度增大，细沟可蚀性呈指数函数减小，须根系对土壤侵蚀阻力的影响显著大于直根系，根比表面积是刻画根系影响土壤侵蚀阻力的最佳根系参数。随着退耕年限增大，土壤抗蚀性能逐渐增强并趋向稳定，灌木是梁峁坡上提升土壤抗蚀性能的最佳退耕模式，而草本是陡峻沟坡上强化土壤抗蚀性能的有效模式。坡面径流挟沙力随泥沙粒径增大呈幂函数减小，可用水流功率和泥沙中值直径有效模拟裸露陡坡径流挟沙力；随着茎秆盖度增大，挟沙力呈指数函数下降；茎秆排列方式会改变径流和泥沙连通性，显著影响挟沙力；随着枯落物盖度增大，挟沙力呈指数函数减小；形态特征差异是引起枯落物类型影响挟沙力的根本原因；可用坡度、泥沙中值直径和流速有效模拟植被覆盖坡面径流挟沙力。降雨显著降低草地坡面泥沙沉积速率，在泥沙净沉积条件下，降雨对泥沙输移的影响主要来自雨滴动能打击；将草带布设于坡面下部，可有效抑制细沟发育、拦截泥沙，发挥较强的土壤侵蚀阻控功能；随着时间的延长草带拦沙效益明显下降，缓坡上草带瞬时拦沙效益和拦沙能力较高，随着含沙量增大坡度对草带拦沙能力的影响会被放大；构建了草带拦沙过程模型，实现了草带拦沙动态过程模拟。研究成果对于阐述植被恢复影响土壤侵蚀过程动力机制、模拟植被覆盖条件下的土壤侵蚀过程，具有重要的理论和实践意义。