长江上游地区坡耕地土壤理化性质对土壤侵蚀阻力的影响机理

It is important to study soil erosion process and mechanism of sloping farmland which is the main sediment source in the Upper Yangtze River. WEPP model is a widespread applied process-based model for water erosion prediction, and its results of forecasting depend largely on the calculation accuracy of soil erosion resistance values (soil rill erodibilty and critical shear stress). A set of equations which can be used to estimate soil erosion resistance values were established based on the results of simulated rainfall experiments on the gentle slope plots in the US in WEPP model. Those equations require calibration and validation when applied to sloping farmland in the Upper Yangtze River. In this study, soil samples will be collected from 15 representative positions in four regions with most severe soil loss in the Upper Yangtze River Basin, then soil physical and chemical properties will be measured. Soil sample will be prepared to measure soil detachment capacity using laboratory flume under a wide range of slope gradient and flow shear stress. Values of soil rill erodibilty and critical shear stress will be derived by linear regression of soil detachment capacity against flow shear stresses. Then the relationship between soil physical and chemical properties and soil erosion resistance values, the effects of soil structure reconstructing on the temporal variability of soil erosion will be analyzed and investigated. The results will provide scientific evidences for the prediction of the spatial and temporal distribution of soil loss in sloping farmland using process-based erosion model in the Upper Yangtze River, and the configuration of soil and water conservation measures in small watersheds.

坡耕地是长江上游侵蚀泥沙的主要策源地，研究坡耕地土壤侵蚀过程及其机理具有重要意义。WEPP模型是目前应用较为广泛的土壤侵蚀过程模型，预测效果很大程度上依赖于土壤侵蚀阻力（细沟可蚀性与临界剪切力）的计算精度。而WEPP模型中土壤侵蚀阻力与土壤理化性质间的定量关系，是基于美国缓坡径流小区模拟降雨试验的结果，在长江上游陡坡耕地应用前，需要订正。为此，本研究拟在长江上游四大水土流失重点区域选择15个典型样点，分析土壤理化性质，并在较大坡度和流量范围内，利用变坡水槽，采用径流冲刷方法，测量不同水流剪切力下各样点土壤分离能力，对二者进行线性回归，确定各样点细沟可蚀性与临界剪切力，进一步建立土壤理化性质与土壤侵蚀阻力间的函数关系，并探讨扰动后，坡耕地土壤结构重塑作用对土壤侵蚀季节变化的影响机制。研究结果将为利用土壤侵蚀过程模型预测长江上游地区坡耕地土壤流失量时空分布及小流域水土保持措施配置提供科学依据。

本研究在长江上游四大水土流失重点区域选择10个具有代表性的样点，分析各样点土壤理化性质；利用试验水槽等设备测量各样点土壤在不同水流剪切力下的分离能力，通过线性回归方法确定各样点土壤侵蚀阻力；研究土壤理化性质对细沟可蚀性、临界剪切力等土壤侵蚀阻力指标的影响。此外还以二十天为周期，分别测量雅安地区农地土壤理化性质及侵蚀阻力，研究土壤细沟可蚀性、临界剪切力季节变化特征及其变化规律。结果显示，长江上游地区各地土壤理化性质地区差异明显。各样点土壤细沟可蚀性在0.106～0.250 s/m范围内变化。WEPP模型在长江上游地区坡耕地土壤细沟可蚀性预报时，预报值比本研究实测值偏小。土壤细沟可蚀性与土壤极细沙含量呈显著正相关关系，与土壤有机质含量呈显著负相关关系，土壤细沟可蚀性可通过土壤极细沙含量和有机质含量两个指标来进行预报。土壤临界剪切力与土壤粉粒含量呈显著正相关关系。本研究分别建立了土壤细沟可蚀性、临界剪切力与土壤理化性质的关系表达式。农地土壤翻耕后，在降雨、重力等因素作用下，土壤由松散变得紧实，土壤结构重塑，土壤侵蚀阻力也发生相应变化。翻耕前测得整个试验期间最小细沟可蚀性值0.0003s/m，翻耕后最近一次试验测得整个试验期间最大细沟可蚀性值0.0166s/m。最大临界剪切力值为5.00Pa，翻耕前测得，最小临界剪切力值为1.18Pa，翻耕后最近一次试验测得。土壤细沟可蚀性与土壤前期含水量呈正相关关系、与表土粘结力呈负相关关系；临界剪切力与表土粘结力呈正相关关系。研究还建立了细沟可蚀性、临界剪切力与表征土壤重塑作用的理化性质指标的关系表达式。研究结果可为长江上游地区坡耕地土壤侵蚀预报和治理提供科学依据。