黄土高原小流域退耕还林（草）对切沟侵蚀的影响及其演变

The sediment discharge of the Yellow River has been markedly decreasing since the implementation of the “Grain-for-Green” Project, which gives rise to increasing concern on the response mechanism of soil erosion to revegetation in the Loess Plateau. Gully erosion is one of the major erosion forms in the Loess Plateau. However, owing to the lack of suitable monitoring technique, the quantitative research on gully erosion as affected by revegetation has been limited for a long time. Instead, many studies have been conducted to analyze the impact of revegetation on rill and interrill erosion. Therefore in the current study, typical small catchments with diverse years of revegetation are selected. The SfM (Structure from Motion) photogrammetry is used to monitor the 3D dynamics of gullies during the study period to explore the response of gully erosion to revegetation. For a couple catchments has been revegetated for more than 15 years, high resolution remote sensing imageries are applied to analyze the temporal trends of gully erosion since the revegetation. In addition, based on the historical meteorological data, the contributions of revegetation and precipitation to gully erosion are quantified. The results will help to illuminate the temporal evolution of gully erosion under the background of the “Grain-for-Green” Project in the Loess Plateau, to reveal the quantitative effects of revegetation and climate change on gully erosion, thereby to provide scientific reference for the planning of soil and water conservation in the Loess Plateau.

退耕还林（草）工程大面积实施以来，黄河输沙量明显减少，越来越多的研究开始关注黄土高原土壤侵蚀对退耕的响应机制。切沟侵蚀是黄土高原土壤侵蚀的主要形式之一。然而，囿于监测手段的欠缺，长期以来，关于退耕对土壤侵蚀的影响研究多集中于坡面的细沟和细沟间侵蚀，对切沟的定量研究相对薄弱。本项目拟在黄土高原地区选择具有不同退耕年限的典型小流域，采用SfM（Structure from Motion）摄影测量法监测小流域内切沟的短期三维动态变化，探讨小流域切沟侵蚀对退耕年限的响应；基于高分辨率遥感影像，揭示退耕以来小流域切沟侵蚀各形态参数的长期演变特征；同时，结合小流域降水资料，分析退耕还林（草）和降水变化对切沟侵蚀演变的定量贡献。通过本项目的研究，可阐明黄土高原退耕背景下小流域切沟侵蚀的演变规律，揭示退耕还林（草）和气候变化对切沟侵蚀的定量影响，为黄土高原地区的水土保持规划提供科学依据。

目前，黄土高原退耕还林（草）工程在遏制水土流失方面的重要作用有目共睹，但其对土壤侵蚀过程的影响及其机制尚不清楚。切沟侵蚀是黄土高原土壤侵蚀的主要类型之一，囿于监测手段的欠缺，切沟侵蚀的定量研究相对薄弱。亟需选择适当的方法和手段监测黄土高原的小流域切沟发育，深入探讨退耕还林（草）这一大背景下切沟侵蚀的发展与演变。本项目的研究内容包括：（1）评估新兴发展的Structure from Motion（SfM）摄影测量法在切沟测量中的精度，在此基础上探讨退耕年限对小流域切沟侵蚀的影响；（2）开展不同时期遥感影像的切沟解译，评估其精度并进一步剖析退耕条件下小流域的切沟发育速率及其影像因素；（3）收集黄土高原典型小流域的长时序水沙资料，探究气候变化与人为活动对小流域土壤侵蚀的定量贡献。研究结果发现：（1）以实时动态差分（Real-time Kinematic， RTK）GPS与三维激光扫描仪的测量数据为基准，SfM在切沟DEM重建与形态参数提取方面均表现出较高的精度，可用于野外切沟侵蚀的快速监测；随着退耕年限的增长，植被覆盖与枯落物增加，切沟发育速率总体变慢。（2）以陕西省神木市六道沟小流域为研究对象，通过与RTK GPS的实地测量结果对比，WorldView-2遥感影像解译较为精确地反映了切沟的轮廓与形态；对比2012与2018年的WorldView-2解译结果，发现小流域各集水区的切沟总长度、总最大表面宽度、总周长与总面积的变化率平均为-0.02、0.06、0.01和0.2 % a-1；在退耕还林（草）条件下，小流域内的切沟扩张速率主要与集水区地形条件和植被覆盖有关。（3）黄土高原15个小流域的逐年径流深与其中9个小流域的逐年输沙模数均未出现显著的周期性波动，年降水量与潜在蒸散发量对年径流深的影响相当；径流深年均减少0.24~0.71 mm a-1，严重威胁小流域地表径流的可持续利用与社会经济发展；各小流域水沙持续减少，主要与人类活动（退耕还林（草））有关。本研究对于探索黄土高原切沟侵蚀过程与机制、构建切沟侵蚀模型及水土保持规划具有重要意义。