| There are a lot of channels in the loess plateau; each small watershed is a independent units of erosion and sediment yield. Check Dam is not only could intercept sediment in the source of erosion, but also could record the every time of the erosion of the event. The purpose of our study is building sediment time series through collect sample of soil and sediment. Based on the measured value in wild and daily rainfall over years, the time when sand deposited was confirmed and the time order of sand sedimentary cycle was built. At the same time, the technology of compound fingerprint identification was used to calculate the main source of sand sedimentary, several results were shown below:(1) 2001.7 t/km2·a erosion and sediment yield rate was impounded in this silt dam built from 1974 to 2003. According to the analysis of the amount of sand sedimentary cycle and physical and chemical properties of profile soil, the fifth sedimentary cycle(under 67cm) was ascertained, which formed from the cumulative plough layer in 1999. Combining with daily rainfall, the time order of sedimentary cycle was built. The history of the silt dam could be marked off four parts: from 1974~1977, the rate of sand production was 827.3t/km2·a and there was the weak water and soil loss; from 1978~1981, the rate of sand production was 5229.7 t/km2·a and there was the serious water and soil loss; from 1982~1999, the rate of sand production was 1939.7 t/km2·a; from 2000~2003, the rate of sand production was 712.4 t/km2·a and there was a decrease of water and soil loss due to returning farmlands to forests.(2) Through analyzing 11 materials in these soil samples. Firstly, according to the four sources which include farmland, grassland, forest, gully walls using motile-distinguish analysis, Ca, P, Mg, Fe was chosen as the best fingerprint group. With this group, contribution rates of sand yield form sedimentary cycle of every layer was calculated by the best mixed model. It was shown that the four sources has different contribution rate, respectively were: 27%, 17%, 25% and 31%. The gully walls are the main source of soil erosion. The second, the source is divided into upland and gully land. In this situation, using motile-distinguish analysis, TN, Ca, Mg, Zn was chosen as the best fingerprint group. The contribution rate of gully land and upland was 40.7%, 59.3%. The gully land is the main source of soil erosion.(3) The measures of returning farmland to forest(grassland) was started in this watershed since 1999. Before 1999, the erosion and sediment mainly comes from farmland and gully walls, their contribution rate are 29%. After 1999, the contribution rate of farmland reduced to 16%. At the same time, the gully walls is main source of erosion and sediment, they contribution rate is 42%. The rainfall of the continuous heavy rainfall in July 2013 in this watershed over numbered the standard of once-in-a-century rain. In this rainfall, the rate of contribution to erosion in slope cropland, grassland, forestland and gully walls were 31.94%, 12.98%, 18.31% and 36.77%, and the sand tiled were 21.48 × 103 t, 8.37 × 103 t, 12.31 × 103 t and 24.37 × 103 t, respectively. The gully walls were the main source of erosion. From the contrast of the contribution rate to erosion, sand yield and year’s average value, the rate of contribution to erosion from furrow bank and slope cropland were higher than the corresponding average values under storm, while that from grassland and forestlands were lower than the corresponding average values under extreme storm. This result shown that returning farmland to grassland and forestlands was essential in loess Hilly. |
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