黄土丘陵沟壑区退耕植被对坡沟系统侵蚀产沙的阻控机制与定量评估

The project of "grain for green" has strengthened vegetation recovery of slopelands and gullies with different classes in the loess hilly-gully area. However, little information on vegetation impacts on slope-gully soil erosion processes especially gully erosion. Based on the simulation experiments and field observations on slope-gully erosion processes, from the perspectives of slopeland, gully and slope-gully system, the vegetation trapping upslope runoff and sediment, blocking mechanism of vegetation on gully erosion, and the resist and control mechanism of vegetation spatial distribution on the runoff and erosion transport processes in slope-gully system will be focused on. Based on a series of experiments and observations on different vegetation types and spatial distribution characteristics, the calculation method of the sediment trapping capacity will be proposed and improved; from the respective of controlling gully erosion, the effective mode of vegetation recovery and critical value of silt-laden inflows will be suggested; the blocking and control mechanism of vegetation on slope-gully chain erosion processes will be clarified; and the quantitative assessment model on slope-gull erosion and sediment yield will be establish. These results and model can help to assess the benefit of the vegetation recovery in water and soil conservation and optimize the vegetation spatial layout at the watershed scale in the loess hilly-gully area.

退耕还林（草）工程明显改善了黄土丘陵沟壑区不同微地形（包括浅沟、切沟）的植被覆盖，但植被对有侵蚀沟分布的坡沟系统侵蚀影响机制尚不明确。本研究采用模拟试验与野外原位观测相结合的手段，从坡面、沟道和坡沟系统3个层面分别开展不同植被配置方式下的上方来水来沙和人工降雨试验，同时选择野外典型退耕坡沟系统布设不同坡长径流小区进行原位对比观测，探讨坡面植被拦沙，植被控制沟道侵蚀以及植被对坡沟系统侵蚀水、沙和侵蚀能量传递过程的影响。以期提出植被坡面拦蓄径流泥沙量和拦沙能力表达式，从沟道侵蚀控制角度提出浅沟、切沟沟槽植被恢复的模式和临界水流条件；从径流能量出发系统阐明植被配置对坡沟侵蚀链发育的阻控机制；进而构建退耕背景下的坡沟系统侵蚀产沙定量评估模型，为黄土丘陵沟壑区的退耕还林还草水土保持效益评估和植被措施优化提供依据。

本项目以退耕还林还草背景下黄土丘陵沟壑区坡沟系统为研究对象，通过坡面、浅沟等部位植被恢复对泥沙沉积、沟道侵蚀以及坡沟系统侵蚀链发育与沟道演化的系统研究，取得的主要进展有：1）草地恢复能够更有效的拦截坡面部位的侵蚀产沙。与裸土坡沟系统相比，草地恢复后浅沟、坡面和坡沟系统的侵蚀产沙分别减少了73%、98%和90%。裸坡条件下浅沟侵蚀量仅占总侵蚀量34%，草地恢复后上浅沟部位侵蚀贡献率达57%。草地的恢复能够有效的减缓浅沟发育速度，特别是浅沟的下切侵蚀。与裸土坡沟系统相比，草地恢复后浅沟沟道的平均宽度和深度分别减少了37%和50%。2）裸土和草地坡沟系统发生泥沙沉积的上方来沙临界含沙浓度均为120 g L-1。当含沙浓度小于临界值时，净侵蚀速率随着含沙浓度的增加而增大；而大于临界值时，净侵蚀率显著下降。含沙水流流量增加加剧裸坡浅沟的下切侵蚀。不同流量条件下浅沟沟道形态发育存在明显的差异，放水流量增加后浅沟沟道平均深度从7.72 cm增加到9.42 cm，增加了25％，浅沟沟道平均宽度从18.18 cm减少到16.39 cm，减少了10％。）影响坡沟系统植被带侵蚀泥沙输移的因素的强弱顺序为坡度 > 草带宽度 > 单位流量 > 含沙浓度。含沙浓度越高会引起更低的草带瞬时拦沙效率，但是得到更多的次拦沙总量。植被带上部拦蓄泥沙作用显著，而坡度和流量的作用在整个草带均有体现，且高入流含沙浓度会增强坡度对草带拦沙效果的作用。4）前期沉积的泥沙会使坡沟系统植被带草带拦沙性能逐渐降低，即草带拦沙过程具有自反馈（自抑制）特征，并基于这一特征构建草带拦沙过程建立草带拦沙时间变化过程模型STPMOD-VFS。模型能够较好地预测各条件下（不同入流输沙率、泥沙粒径组成和坡度）草带拦沙时间变化过程。