紫色土坡耕地地表与近地表水流交互作用侵蚀机理研究

Purple soil is one of the important soil types of China, distributed widely in 11 southern provinces to cover an area of about 290,000 km2. Relatively high infiltration rate of sloped farmland, combined with preferential flow makes it easy to form subsurface above low permeable layer in the cultivated layer. Presence of subsurface flow, when coupled with surface water flow can cause severe soil erosion. The research on its erosion mechanism is of great significance. In this study, the infiltration rate of surface soil and the preferential flow will be measured for determining water flowing into the subsurface layer. Flow velocities of both surface and subsurface flows are to be measured for determining the hydraulic power to erode soil under the interactive impacts of the two flows. Laboratory experiments are designed, which use controlled flow rates at the soil surface and flow depths in the subsurface soil layer, to study the critical condition and evolution of soil erosion under the impact of the increasing subsurface flow and under different slope gradients of 5°, 15°, 25°. The condition of soil erosion occurrence and the soil erosion evolution process are studied under the interactive influences of surface flows of 2, 4 and 8 L/min and subsurface flows of 0/4 (control), 1/4, 2/4, and 3/4 depth of the cultivated layer. Upon the determination of the soil erosion processes, the detachment rate, sediment transport csapacity as well as the soil erodibility and critical shear stress are computed.

紫色土是我国的重要土壤类型之一，分布于南方15个省市区，面积约29万km2。坡耕地紫色土结构松散，具有较高的入渗性能，并存在优先流，在耕层弱透水层上方容易形成近地表水流。地表和近地表水流耦合作用常会引起强烈的土壤侵蚀，其过程和机理研究具有重要意义。本项目拟通过系列试验，研究紫色土的入渗性能和优先流通量，确定近地表水流的强度；研究地表径流和近地表水流的流速，确定二者单独和交互作用的侵蚀动力条件。发展室内试验方法，通过定量控制地表径流流量与近地表水流深度的试验方法，采用不同坡度（5°、15°、25°），研究近地表渗流汇集、延程增大过程中触发土壤侵蚀的临界条件和演变规律；研究不同地表流量（2、4、8 L/min）与近地表水流深度（0/4（对照）、1/4、2/4、3/4耕层土壤厚度）交互作用下，土壤侵蚀发生的条件，侵蚀产沙过程、土壤侵蚀速率、土壤临界剪切力、水流输沙能力及土壤可蚀性参数。

项目以紫色土坡耕地为研究对象，研究紫色土坡耕地土壤在地表径流与近地表壤中流交互作用下，土壤侵蚀随沟长的变化过程，探索土壤侵蚀机理及动力机制。经课题成员4年研究，完成课题预期目标。获得如下主要研究成果：(1)利用表置环式土壤入渗仪，定量测定研究了紫色土坡耕地土壤基质入渗性能和优先流入渗性能，为降雨条件下近地表壤中流和地表径流预测提供基础。(2)构建了近地表壤中流和水流深度控制的试验设备和试验方法，并研究了地表径流和近地表水流的流速，为研究水流侵蚀动力提供基础。(3)发展了近地表水流影响下坡面水流侵蚀过程的试验方法，在不同坡度、地表径流流量与近地表水流深度交互作用下，研究紫色土坡耕地侵蚀产沙过程。(4)利用试验数据计算得到水流输沙能力、土壤侵蚀速率、土壤可蚀性和临界剪切力。成果有助于深入认识地表与近地表水流交互作用下土壤侵蚀机理、提供侵蚀模型参数体系，为紫色土坡耕地土壤侵蚀预报模型构建提供支撑。