The Mechanism Of Runoff Generation On The Slopingfarmland Of Purple Soil Based On Hydrogen And Oxygen Isotopes Tracing

Slope runoff, as a key component of water cycle, is not only the primary factor of soil erosion and agricultural non-point source pollution, but also is bounded with rain-fed agriculture and natural disasters caused by water, etc. Slope runoff mechanism should be scientific understanding firstly to dealing scientifically with those environmental problems. Purple soil slope land is160000km2just in the Sichuan basin area, which is the most important of cultivated land resources in the upper Yangtze River, and it is the important influence area for the water environment of the Three Gorges reservoir and the Yangtze rver basin. Due to the limitation of technology, however, the mechanism of runoff generation of this land use type is still not clear. In this paper, through using a traditional hydrological method combined with hydrogen and oxygen isotope technology, under the condition of natural rainfall, slope farmland runoff hydrological processes was monitored in the fixed location, the soil water movement and the runoff process and characteristics of runoff water sources were studied, and the sub-surface runoff mechanisms were analysised. Basically achieved the following results:(1) On the point scale, it is the layer10to30cm of soil profile that its soil moisture content increased the max firstly and fallen firstly, which were the core characteristics of the whole soil profile (0-40cm) of the purple soil slope cultivated land to response to rainfall. On slope scale, soil moisture to response to rainfall showed the depth effect and the slope effect, which reflect the influence of slope-surface hydrological processes on the importance of soil moisture vertical movement. The redistribution of soil moisture of purple soil slope farmland from strongly affected by rain infiltration to the slope flow generation and concentration to the moisture receded and gradually stabilizing process, lasted about40-50h, and the lower slope positions lasted longer. In the soil profile where piston flow occurring, the soil water was hard to be all displacement, what reflected the facts that the soil water have easy mobile part and uneasy mobile part.The way of rainfall infiltration:to the point of time successively, after making the capillary porosity of the soil layer0-10cm to absorb certain moisture, rainfall translated into preferential flow appeared on the layer10-30cm. From soil profile level, the upper was given priority to with pistonal infiltration, the deeper was given priority to with preferential flow. To the point of the influence of earlier water content, From early on. the influence of water content, high moisture condition on the soil profile decreased the loss of precipitation in the process of infiltration caused by filling the capillary pore and ensured that the amount of water flowing in the soil structure of big pore space, thus promoting the preferential flow occurred more quickly.(2) For purple soil slopingfarmland, the discharge hydrograph ofltssubsurface flow (interflow, underflow) generally presents three stages:the first is fast rising, second is fast fading and third is slow fading; however, the third stage is not obvious for interflow. The peak formed by the former two stages is the most distinct characteristic of the discharge hydrograph ofthe subsurface flow. Under4rainfall events monitored in this paper, flow generation started firstly from the deeper layer of the vadose zone of purple soil sloping farmland and then developed from the deep to the shallow.(3) Based on isotopic concentration data, it was be quantified that the contribution rate of a pre-event soil water and an event precipitation to the subsurface flows in two land parcel scale using the two-component model. At peak time, the mean proportion of the pre-event soil water were56.4%for interflow in plot scale,33.5%for underflow in plot scale,58.2%for underflow in slope scale, respectively. Thus, it indicates that the pre-event water plays an important role in subsurface flow.The subsurface flow of farmland inpurple soil area is well-developed. From the point of rainfall process, the vadose zone structure (large pore and fracture) significantly affect the different stages of flow. It shows that vadose zone of purple soil slope land is given priority to preferential flow which also can be named heterogeneous flow during the fast rising and fast fading stages of discharge hydrograph; but it turns to bematrix flow which named homogeneous flow when flow slowly descend down after the rain stopping; Both homogeneous flowheterogeneous flowcoexist during that two phases, and the importance of preferential flow promoteswhen the rain intensity increases. From perspectives of rain intensity and infiltration, that is to be mainly the runoff yield underexcess infiltration and saturated storage. However, in the shallow vadose zone (soil layer) of purple soilarea with high permeability, the excess infiltration does not always observed.