| In the upper reaches of Yangtze River, the severe soil loss caused not only the decrease of soil productivity, but also large amounts of sediment to deposit in the main and branch channels, which becomes the hidden trouble of flood in the middle and lower reaches of Yangtze River. The soil erosion and sediment yield is always a concerned problem for the work of soil and water conservation, and also the key problem needs to be solved for developing the dynamic monitoring on soil loss. In the work of soil and water conservation in small watersheds, the quantitative estimation on the temporal and spatial characteristics of soil erosion is very important for the planning and design of soil and water conservation and the evaluation of harness benefit in small watershed.In this paper, we take a typical watershed in Purple Hilly Area in Central Sichuan in the upper reaches of Yangtze River as the study object. Using the nuclear tracer technology, the property and distribution characteristics of 137Cs and 210Pbex in the soil and the laws of soil erosion in this area were studied. The soil erosion distribution and sediment sources in the watershed were evaluated quantitatively in these aspects, including the cause, process and developing trend of soil erosion, which is of significance both in practice and in theory.1. It is the first to study the relationship between the concentration of'37Cs and 2l癙bex in purple soil particles and erosion sediments and the grain size of soil particles.The results show that most of Cs and 2l癙bex are absorbed by clay particles. The finer the soil particle is, the higher the concentration, and vice versa. Compared with the original and on-site soil, the mechanical composition of soil in runoff sediment has no any significant difference in a 25m-long slope, showing that there is no soil particle selectivity when erosion occurred. It's the same to the paddy soil. In addition, the initial distribution of l37Cs and 210PbCx in undisturbed profile has obviously spatial difference, but the spatial difference of the ratio of ' 7Cs to Pbcx reduces significantly, which indicates morereliability by using the two nuclides to estimate soil erosion and sediment sources.2.The spatial distribution pattern of soil erosion.and deposition in a small watershed in Purple Hilly Area was studiedBy analyzing the concentration of b7Cs and 21癙bex in soil profiles in the watershed , we found that spatial variation of soil erosion is extremely different. The erosion intensity decreased from the hilltop to the bottom for the hilly cultivated land. At the foot of the hilly, the deposition occurred. The paddy field is also a deposition area.3.The topographic factors have great relationship with soil erosion in the watershed.The results show that the soil erosion amount in different slope position appeared an exponential decrease in downslope direction in a slope-length range of 40m, and an exponential increase with slope-gradient during 5皛16? Also, the effect of slope aspect on soil erosion is very significant, indicating that the soil erosion amount in north slope is lower than that of in south slope.4 Different land use patterns lead to different loss of137Cs, indicating that the pattern of . land use has close relationship with soil erosion intensity.According to the study results, the soil erosion intensity under different land use pattern is as follows: bare rock>sloping field>forest land. The bare rock, sloping field and forest land belong to extremely intensitive erosion, moderate erosion and slight erosion respectively. Very few paddy fields belong to very slight intensitive erosion. By analyzing the percentage of area of different land use in the small watershed, the bare rock is the heaviest erosion area and sloping field is a main source of runoff sediment.5 It is the first to estimate the average annual sediment delivery ratio in the small watershed in the past 4 decades by I37Cs tracing method.Compared with other researchers' results, the calculated sediment deliv...
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