Study On The Relationship Between Soil Erosion And Sedimentation Sediment Source Of Dashiba Reservoir In Yunnan Province

The Dianchi lake watershed is one of the most potential agricultural areas in China. However, in recent decades, due to the population expansion, rapid economic development and the long-term unreasonable utilization of cultivated land resource, the Dianchi lake watershed suffered from serious soil erosion. Soil erosion could lead to soil degradation and soil fertility decline, nutrient loss and a series of problems, which seriously impact the status of the ecological environment. Therefore, studies of soil erosion and quantitative analysis of sediment source are conducive to the healthy and stable development of the ecological environment. The technique of 137Cs tracer, as a new approach to the study of soil erosion, injects new vitality to the relationship between LUCC and soil erosion. On the other hand, sediment sources discrimination with composite fingerprinting pcedures has been established as a novel technique in investigating catchment soil erosion and sediment distribution. Thus, in this paper, choosing a typical watershed, the Dashiba reservoir in Dianchi watershed as study area, we studied the distribution law of soil erosion on different utilization soils upon different time scale by using137Cs technique and composite fingerprinting procedures together. The results showed that:(1) Of all the sampling sites, the movement of 137Cs with soil particles was obvious, the soils suffered different degrees of erosion. For eroded sample points of the cultivated land, the average 137Cs content of the northern slope was far less than the south slope while the coefficient variation of the 137Cs was on the contrary, therefore position was the key factor for the differentiation of 137Cs.On the contrary of the cultivated land, for eroded sample points of forest land, the average 137Cs content of the ecological public welfare forest in the north slope was equal to the commercial forest in the south slope, while the coefficient variation of the 137Cs of ecological public welfare forest in the north slope was less than the commercial woodland in the south slope, which indicated that the type of the forest played a great role in soil erosion. The vertical distribution of 137Cs also suggested that land use affected the redistribution of 137Cs. The relationship between 137Cs and the TOC、TN and TP indicated that soil nutrient content was closely related to soil erosion and in the process of the eutrophication, we should pay attention to the governance of farmland.(2) We calculated the soil erosion rates under different land utilization ways via the appropriate Cs soil erosion model and concluded that land use influenced the intensity of soil erosion. Forest land was given priority to micro erosion and mild erosion, and the maximum value of erosion modulus was less than the minimum of cultivated land. So soil erosion in cultivated land was more severe than that in the uncultivated land. We also found the soil erosion rates was influenced by position in the landform. The spatial differentiation of soil erosion intensity in Dashiba watershed showed that soil erosion of different sites varied largely. The erosion of southwest part of the south slope is serious, while for the northwest part of the northern slope, the soil erosion is light. On the whole, the erosion of the northern slope was serious than the southern slope. We analyzed the spatial distribution of soil nutrients and heavy metal and discussed the relationship among soil erosion, soil nutrients and heavy metal. And we found that soil erosion was easy to cause the loss of TOC and TN and soil erosion was also easy to cause TP loss. However, artificial fertilization can make the negative correlation between soil erosion and TP not obvious. Change of Cu element in the space was relatively small. The distribution of Fe and Mn elements in the space was complex and was similar. Soil erosion has little effect on Fe and Mn, and the reason for spatial changes was mainly man-made factors. (3) We calculated the contribution rates of different source of silt to the sediment by using composite fingerprinting procedures. And we got the following conclusion:Among the whole depositional stage, two-thirds of the sediment silt was from cultivated land, and only one-third was from woodland. From this, we got that the erosion rate of farmland was larger than that of the non-cultivated land, which matched the previous conclusion we made. Among the whole depositional stage, the average contribution rate of the commercial forest land to the sediment was 18.48%, and for ecological public welfare forest land the value was 14.13%. Especially in the first 20cm in depth, the average contribution rate of the commercial forest land to the sediment was 12.12% while for ecological public welfare forest land the value was almost 0. Namely, nearly no silt was from ecological public welfare forest land. The type of the forest indeed affected soil erosion rate. Thus in the ecological fragile district, in the process of returning farmland to forest, we must give full consideration to forest land types, in order to achieve the best effect of soil and water conservation.(4) We calculated and analyzed the contribution rates of different source of silt to the sediment upon different time scale by using composite fingerprinting procedures. The result showed that among the whole depositional stage the sediment source of the Dashiba reservior appeared two obvious phases. Before 1984, the sediment was mainly from farm land. However, after 1984, the sediment source changed dramatically. The sediment silt came from farmland was little and mainly came from the surrounding forest land, which indicated that in the middle 1980s was significant to effectively reduce soil erosion. At the same time, the engineering of returning farmland to woodland also played a vital role in improving the ecological environment of the Dashiba reservoir region.