黄土坡耕地秸秆覆盖地表与犁底层上方水流交互作用侵蚀机理研究

Sloped loess farmlands are the most sever soil erosion places and one of primary sediment sources of river systems. Straw mulch can increase infiltration, which when encounter with less permeable plow pan layer (PPL) can induce subsurface water flow along the cultivated slopes. The coupled impacts of the runoffs at soil surface and at the PPL can induce sever soil erosion. The study on the mechanism is of great importance. This project aims at the study of the PPL-affected cultivated field, to study infiltration process and flow intensity transformed from rainfall into water flows at soil surface and above PPL, as function of time, under different mulch rates of 0, 1, 2, 4 t/ha, slope gradients of 5°,15°,25° and rainfall intensities of 50, 100, 150 mm/h. Flow velocities at surface as well as at the PPL are measured and used to determine the hydrodynamic conditions of soil erosion as affected by one factor alone or by the interactive effects of the two. The erosion and sedimentation processes affected by surface runoff alone during the initial stage of rainfall-runoff, those induced by the surface and subsurface runoffs as well as the interactive effects of the two factors are to be quantified through the study process. This study will supply a base for understanding soil erosion processes of farmland, as impacted by the interactions of water flows at the soil surface and above the PPL.

坡耕地是黄土区土壤侵蚀重要部位和河流泥沙的来源地之一。秸秆覆盖可以增加土壤入渗，入渗水流到达渗透性差的犁底层时，在其上方容易形成顺坡渗流。地表和犁底层上方水流耦合作用下可能引起强烈的土壤侵蚀，其机理研究具有重要意义。本项目以受犁底层影响的黄土坡耕地为研究对象，通过模拟试验，研究不同秸秆覆盖量（0、1、2、4 t/ha）、坡度（5°、15°、25°）、降雨强度（50、100、150 mm/h）下，土壤入渗的过程，确定降雨转化为地表和犁底层上方水流的强度和随时间变化的过程；研究地表径流和犁底层上方水流的流速，确定二者单独和交互作用的侵蚀动力条件；并研究不同覆盖量、雨强、坡度对产流过程的影响。通过采集径流样品测量得到径流含沙量，研究降雨初期降雨产流单独影响下地表径流的侵蚀产沙过程、犁底层上方水流与地表径流共同作用阶段的侵蚀产沙过程、研究二者交互作用对侵蚀产沙的贡献。

研究采用室内模拟试验和野外调查的方法，开展了地表水流与犁底层上方土壤中水流交互作用下的黄土坡耕地侵蚀过程研究。研究结果为我国黄土坡耕地土壤侵蚀估算，覆盖耕作法在坡耕地的应用及农地可持续利用提供计算及理论基础。.（1）饱和土壤水流速测量方法：开发了单探针热脉冲技术测量壤中流流速的装置，得出水流流速与探针温度变化之间的解析关系式。试验验证了该流速测量方法的可靠性及精度。（2）发展了降雨条件下坡面水流流速的测量方法，得到坡面流速分布规律，建立了基于坡度和降雨强度的水流流速的预报模型；结果表明坡面水流流速随坡度、降雨强度和坡长增加而增加，且增加速率随坡长逐渐减小，至稳定流速。 （3）首次采用电解质示踪法，测量秸秆覆盖条件下坡面水流流速及其沿坡面的演变规律。结果表明，秸秆覆盖量8 t/ha处理的水流流速小于覆盖量为2 t/ha的处理，覆盖坡面水流流速为0.016-0.117 m/s，远小于裸土坡面水流流速，为裸土坡面流速的27.8%。（4）试验研究了覆盖条件下犁底层影响下的土壤入渗过程。结果表明，覆盖耕作层土壤先于裸土饱和；此后，受犁底层低入渗率控制，土壤入渗率降低速率大于裸土坡面，快速达到其最小值。（5）研究了覆盖条件下犁底层上方土壤水流和地表径流共同作用下的土壤侵蚀过程。结果表明，秸秆覆盖增大了径流和对饱和耕作层土壤的冲刷，导致陡坡耕地更严重的土壤侵蚀。在陡坡上，秸秆覆盖坡面产流后，径流流量及径流含沙量增加速率大于裸土坡面。秸秆覆盖坡面次降雨的总土壤流失量大于裸土坡面。.在极端降雨条件下，秸秆覆盖可能加剧陡坡水土流失。在缓坡上，覆盖坡面的水流流速较小，输沙能力较弱；因此，秸秆覆盖更能起到保持水土的效果。