紫色土坡耕地顺坡垄沟侵蚀分异及其动力机制研究

More and more studies have begun to investigate traditional farming knowledge to better conserve soil and water, and several traditional tillage practices have been discovered to have an effect on soil conservation, especially at the microscale. However, much less is known about the specific details of traditional knowledge, and very little scientific analysis has been done on them. Downslope ridge tillage is one of the traditional farming experiences, used to develop a drainage system on slope land. Before crops are planted, farmers prepare the land with downslope furrow ditches spaced at 0.5-1.0 m, depending on the cultivator's practice. The furrow ditches are dug by hoes, generally 15-25 cm deep, and are connected with level trenches. Although the anti erodibility of plow pan is more than ridge surface soil, the toe of the furrow erosion is serious, because the downslope furrow is prepared random so long that the shear stress of flow concentrated in the furrow which is more than critical shear stress of rill initiation. And restricted by the anciently measure methods to survey variables in microscale, the mechanism and spatial distribution of furrow erosion is dimness yet. In order to investigate critical condition and renovate the traditional downslope ditch tillage practice, this research will utilize microtopography scan technology and nuclide tracing technology and simulation rainfall to investigate the depth distribution of runoff and the critical shear stress of runoff required for rill erosion on the surface plow pan with different gradients according to local rainstorm conditions.

顺坡垄作是长江上游特别是紫色土坡耕地上农民普遍采用的耕作模式，有着广泛的实践基础和深厚的历史传承，是在长期的自然选择中"优胜劣汰"存在下来的传统经验。传统顺坡垄作通过顺坡起垄形成耕种土垄和垄沟，顺坡垄沟与横坡边沟、背沟共同实施形成完善的地块排灌沟道网络。农民长期耕种过程中形成的犁底层具有较强的抗蚀能力，但由于在布设顺坡垄沟时坡长随意，垄沟中下部侵蚀强烈；同时由于以往观测手段的限制，紫色土顺坡垄作的侵蚀分异及其动力过程尚不清楚。本研究采用核素示踪、三维激光扫描与模拟试验相结合的方法；定量评价顺坡垄作的垄沟、垄坡土壤侵蚀强度，试验分析不同降雨特性下顺坡垄沟内径流切应力沿程变化规律，查明紫色土垄沟侵蚀临界抗剪切力及其影响因素，阐明顺坡垄沟侵蚀分异及其动力机制；以期优化顺坡垄作模式更好的发挥其水土保持功能，同时也为进一步完善坡面流侵蚀动力理论提供参考。

顺坡垄作是在长江上游特别是紫色土坡耕地上农民普遍采用的耕作模式，以往由于主流思想的影响，没有得到应有的重视。.本研究采用野外调查与小区原位实验相结合，分别采集垄沟和垄坡表层土壤，测试顺坡垄作地块沟、垄土壤容重、粒径分布、有机质含量、TN、TP、土壤含水量等指标，小区实验调查顺坡垄作条件下的土壤流失特征，进而探讨顺坡垄作导致的土壤特性差异与顺坡垄作的水土保持机制。结果表明：与垄沟内土壤相比，垄坡土壤平均含水量、容重分别降低29.74 %和16.41 %，土壤总孔隙度高20.56%，说明垄坡土壤具有更大的吸持水分的潜力，降雨产流阈值相比于垄沟土壤也更大。相比于垄沟内土壤，粘粒在垄沟内发生富集，垄坡土壤中的有机质、总氮（TN）和总磷（TP）指标分别高18.79 %、17.30 %和9.31 %。侵蚀泥沙中沙粒含量出现明显富集，几乎是垄沟土壤沙粒含量的1倍，而粘粒和粉粒含量则远小于径流小区土壤成分，说明顺坡垄作有利于<50 μm土壤细颗粒的保持。侵蚀泥沙中有机质和TN远小于径流小区土壤成分，而TP则高于径流小区土壤成分，与侵蚀泥沙颗粒分选现象近似，说明顺坡垄作的肥力指标流失差异同样主要由侵蚀分选导致，也说明顺坡垄作有利于紫色土有机质和氮肥的保持。.本项目在四块坡降分别为17.63%、26.79%、36.40%和46.63%的2*8m2小区上实施模拟降雨与三维激光扫描试验，初步结果如下：与不起垄耕作坡面相比，顺坡垄作坡面径流系数低7.5个百分点，径流含沙量低14.03g/l。从扫描的数据提取垄沟和垄上坡面侵蚀数据来看，随着坡降的增大，垄沟与垄坡面侵蚀产沙量迅速增大；小坡降以垄沟侵蚀为主，随着坡降增大，垄沟与垄坡面侵蚀量比例逐渐变小。从空间分布来看，除坡降为46.63%坡面外，其它坡面垄沟内侵蚀与淤积在顺坡方向上交替出现，坡降为46.63%坡面由于坡降太大，主要以侵蚀为主。