花岗岩红壤优先流及其与崩岗侵蚀发育的关系

Benggang is a serious erosion in granite red soil area in south China. This compound soil erosion is initiated by many factors that interacts on each other, resulting in several co-existing erosion mechanisms which are not fully understood. In a Benggang erosion watershed, numerous fractures or macropores are often found in the soil and granite parent material, hence, preferential flow in macropore is prone to occur in the slope. The macropore flow can rapidly change the spatial-temporal distribution of soil water, thus change the soil swell and shrink, soil hydraulic properties, soil weight, and then change soil shear strength and destructive force in the local zones, and finally influence on Benggang erosion development. On the other hand, Benggang erosion and preferential flow can promote macropore forming and developing in the soil. They constitute a positive feedback ring, which is a reciprocal causation between Benggang erosion, preferential flow, and soil macropore. This research is based on the hypothesis to study the mechanism of Benggang erosion promoted by preferential flow in soil. In order to investigate the preferential flow in soil profile-scale in an active Benggang watershed, it is planned to survey soil macropore in the slope, to perform dyeing solution infiltration on-the-spot and simulate rainfall infiltration in soil box. Then the soil water content and soil temperature will be monitored at high frequency to study the law of preferential flow in slope-scale and soil water spatial-temporal distribution in the watershed. At the same time, the in-situ monitoring will be periodically performed in the watershed to study the Benggang erosion event and development. All the experiments are aimed to demonstrate the relationship between “soil macropore - preferential flow - soil water spatiotemporal variation - Benggang erosion”. This research is designed to investigate the mechanism of Benggang erosion caused by soil water movement in the watershed. The research also provides an opportunity to apply and develop the theory of preferential flow in Benggang watershed.

崩岗是我国南方花岗岩红壤地区严重的侵蚀类型，各种影响因素相互作用，多种侵蚀机理并存，目前研究还不充分。花岗岩母质和土壤存在大量裂隙与大孔隙，坡面容易发生优先流，能迅速改变土壤水分时空分布，进而改变土壤胀缩状况、水力性质、土体自重，从而改变局部抗剪强度与剪切破坏力，影响崩岗侵蚀发育；反过来，崩岗侵蚀和优先流也促进土壤大孔隙发育，构成正向反馈环，互为因果。本项目基于这一学术假设，在活动期崩岗的集水坡面，通过现场调查、不同位点染色入渗、室内模拟降水入渗，研究土壤大孔隙状况与剖面尺度优先流规律；通过高频监测降水过程中土壤温度与含水量，研究坡面尺度优先流规律与水分时空变异；通过定期监测，研究崩岗侵蚀发育状况与崩岗侵蚀事件；由此探明“大孔隙状况－优先流特征－水分时空变异－崩岗侵蚀”之间的关系。研究结果有望从坡面壤中水分运动角度揭示崩岗侵蚀发育的机理，也为土壤优先流理论的应用提供样本和契机。

崩岗是我国南方花岗岩红壤地区严重的侵蚀类型，风化残积母质和土壤存在大量裂隙与大孔隙，坡面的优先流能迅速改变土壤水分时空分布，影响崩岗侵蚀发育。本项目在活动崩岗的集水坡面，通过现场调查、不同位点染色入渗、室内模降水入渗，通过现场高频测量不同坡位不同土层的含水量和温度，初步探明了坡面水分状况和优先流特征及其与崩岗侵蚀发育的关系。（1）坡面土壤饱和导水率存在明显的空间分布规律，红土层导水率最大，砂土层次之，而红-砂过渡层最小；坡面水平方向，越靠近崩壁土壤饱和导水率越大。（2）测量饱和导水率的双环的内径必须大于30 cm，说明土壤存在大孔隙；越靠近崩壁，坡面土壤大孔隙数量越多。（3）坡面土壤含水量与年降水量呈正相关；红土层含水量高于砂土层，上坡位高于下坡位，土壤含水量总体呈现头重脚轻空间格局，并且下坡位波动比上坡位大。（4）降水入渗过程中，土壤水分存在“替换”和“绕过”现象，可以据此根据含水量的瞬时变化来判断是否发生优先流；雨水温度和土壤温度不一致，导致土壤升温模式在有优先流和无优先流下不一样，本项目据此提出了同步监测含水量和温度识别优先流的方法。（5）强降雨下坡面土壤普遍发生优先流，靠近崩壁发生更频繁，这利于崩岗侵蚀启动（崩壁崩塌）。（6）崩岗侵蚀启动需要同时满足两个标准，第一，土壤前期含水量足够高（下坡位土壤含水量>0.32 m3/m3或饱和度>0.72），第二，降水足够大（24h降水量>100 mm并且降水历时>8 h）。（7）崩岗侵蚀启动瞬间发生特殊的土壤水分动态，可以指示崩岗侵蚀是否启动，即土壤含水量继续增加但剖面土壤不同层次的含水量空间变异（标准差）突然降低，这种含水量和变异系数的相反变化趋势指示了崩岗侵蚀启动。本项目揭示了崩岗坡面优先流发生规律以及其与崩岗侵蚀发育的关系，证实了壤中流对崩岗侵蚀起重要作用，在一定程度上揭示了崩岗侵蚀机理，对崩岗侵蚀防治也具有指导意义。