紫色土横垄侧渗流驱动下垄面细沟沟头溯源侵蚀量化研究

Contour ridging is an effective soil conservation practice used on sloping land of purple soil. Contour ridges can pond water in the furrows, which may induce seepage generation. Seepage plays an important role in rill headcut erosion process, which is likely caused by increased soil pore water pressures and decreased soil shear strengths. Understanding seepage and its effects on rill head advancing process will improve our knowledge on seepage erosion mechanisms in contouring ridge system. In this study, a series of rainfall simulation experiments in laboratory are conducted, with three ridge geometry indices (ridge height, width and slope), two levels of rainfall intensity and two levels of slope gradients. The hydraulic flume experiments with pre-made initial headcuts (3 cm high) are conducted to investigate the effects of seepage, inflow rate, slope gradient on rill head advancing process. Photogrammetric technique is applied in the experiment to detect changes of headcut advancing process. The fractal interpolation and multifractal measures are used in characterization of headcut microtopography. The objective of this study is to analyze the influences of rainfall intensity, slope and ridge structure on seepage generation and quantify the change of seepage and its effects on headcut advancing rate, headcuts developed on rill channel and soil erosion process. The results can improve the knowledge regarding soil erosion and provide concrete technical support for soil conservation practices using contour ridge systems.

横坡垄作是紫色土坡耕地主要的耕作方式之一。横垄因汇水而具有“沟渠效应”，但同时垄沟内汇集的雨水会诱发侧渗流产生，加速垄面细沟沟头溯源侵蚀过程，从而减弱横垄的水土保持作用。因此，揭示垄作系统中侧渗流产生机制，定量识别其对垄面细沟沟头溯源侵蚀的贡献，是横垄水土保持机理研究需要解决的关键科学问题。本项目拟采用人工模拟降雨实验和数值模拟方法，通过建立紫色土横垄坡面土壤水分运动模型，定量分析横垄蓄水作用对土壤水分运动的影响及其贡献，阐明横垄侧渗流产生机制；结合室内放水冲刷实验、立体摄影测量技术，基于分形插值方法和多重分形理论，量化垄面细沟沟头跌坎微地貌形态，探讨侧渗流作用下沟头跌坎形成与发育过程对侵蚀产沙的影响，揭示紫色土横垄侧渗流驱动下垄面细沟沟头溯源侵蚀机理。研究结果有助于深入认识紫色土横垄坡面土壤侵蚀过程，为合理布设横垄措施提供科学依据。

横垄具有蓄水保土效益，但是水蚀过程中横垄坡面微地貌的演化可导致细沟发育和横垄垮塌，从而减弱横垄的水土保持作用。本项目采用人工模拟降雨实验、野外原位监测和数值模拟方法，通过建立紫色土横垄坡面土壤水分运动模型，定量分析横垄蓄水作用对土壤水分运动的影响及其贡献，阐明横垄侧渗流产生机制；结合室内放水冲刷实验、立体摄影测量技术，基于分形插值方法理论，量化垄面细沟沟头跌坎微地貌形态，探讨侧渗流作用下沟头跌坎形成与发育过程对侵蚀产沙的影响，揭示紫色土横垄侧渗流驱动下垄面细沟沟头溯源侵蚀机理。结果表明：（1）细沟沟长随时间和流量呈线性变化，放水流量为3、4、5、6 L/min条件下沟头溯源侵蚀速率分别为9、12.2、16.92、18.64 cm/min，细沟深度分别在5~6.4、6~8.6、8~10.6、9~11.8 cm之间，流量越大溯源侵蚀速率越快，细沟深度越深。沟头下方水刷窝深度和溯源速度受放水流量和坡度制约，产沙率与沟头跌坎高度和溯源侵蚀速率呈极显著相关；（2）随着沟头溯源侵蚀进程的推进，细沟横断面形态指标η在0.400～0.852之间变化，从坡脚到坡顶横断面指标η呈现出先减小后增大的趋势，细沟断面形态由宽深状逐渐变窄再变宽，细沟发育形态以沟头溯源和沟底下切为主。随着流量和坡度的增加，一级沟头下方发育二级沟头的概率明显增加，二级沟头数量与产沙率也存在显著相关（R=0.65，P=0.002），A型沟头溯源速率随着径流量增加而增加，A型与B型沟头之间的间距随径流量增加而减小；（3）耕作层侧向渗漏水量占整个土壤剖面渗水量的21.6 %，说明结构疏松的耕作层及与犁底层的交界面是侧渗的主要通道。一方面耕作层自身透水性好及优先流通道的存在，使水分易于侧向运动，另一方面犁底层阻滞水分下渗，促使水分沿着透水性强的耕作层及层间位置运动。入渗量与时间呈指数函数关系。