Soil erosion is the important process of the surface material moving, and also is the world environment problem. Intense loss of soil and water would lead lots of problems such as Land degradation, land fertility reduces, agricultural products quality descend, water pollution and so on., and treat the human living environment. In the Loess plateau, the soil erosion is the most serious in china, and each small watershed is an independent unit of soil erosion. It would be great meaning to study the evolution rule of sediment source for establishing erosion forecasting model in basin scale and optimal allocation of comprehensive control measures in small watershed.
A check dam with 34 years deposition (1958-1991) and 11.325 m deposition depth was collected in Wangmaogou watershed in Suide County in Shaanxi Province. The soil samples were collected on the check dam profile and the surface of the slopes including three land use stytes of gully wall, gully slope and cultivated land. The concentration values of 15 fingerprint properties of each soil sample were analyzed using Kruskal-Wallis H-test and multivariate discriminant function analysis, the result showed that the tracer properties of soil χfd, SOM, P, Na and Zn could provide the best composite fingerprint for discriminating source materials on the basis of source type, and combined the time coordinates of the each sediment couplet to research the soil sediment source in the small watershed of the Loess plateau and the relation between sediment yield and rainfall. The main conclusions take follow:
(1) Sediment yield of each couplet that was lead by erosion rainfall in 1958-1991 could be counted by the Reservoir-capacity-curve, the bulk density curve and each depth of the Dam. The result showed that the check dam totally retained 68878 t from 1958 to 1991. Through studying the 137Cs content of each couplet of the check dam, we analyzed the 137Cs distribution characteristics in the check dam, combined the sediment yield and the daily precipitation of 1958-1991 from the local station, and with the principle of strong rainfall erosion power leading more sediment yield, the time coordinates of the each sediment couplet was established.
(2)According to the accumulative sediment yield curve of each sediment couplet yield, the working time of the check dam could be divided into three stages. The result showed that the soil erosion was strong in the initial stage of the dam construction from 1958 to 1964 with the soil erosion modulus of 18732 t/km2.a, then the soil erosion changed smoothly from 1965 to 1983, with the soil erosion modulus of 5899 t/km2.a, but strong again from 1984 to 1991 with the soil erosion modulus of 19127 t/km2.a. From analyzing we found that the rainfall erosion power lead the change of the first two stages, and the third stage was relation to the land police of our country, and was lead by intensify human activity.
(3) According to analysis 15 kinds fingerprint factor in the soil sample of three sources of gully wall, gully slope and cultivated land in the small watershed, χfd, SOM, TP and Mg formed the best fingerprint identification compositions, and calculated the sediment contribution that 23.85～95.00% was from the gully wall, 0～69.97% from the gully slope and 5.00～65.31% from the inter-gully land. And in the hole working time of the check dam, the sediment yield were 47195 t, 3478 t and 18218 t from the gully wall, gully slope, inter-gully land, and the contribute rate were 68.52%, 5.03% and 26.45%. The evolution process of the gully was the main erosion process of the small watershed.
(4)According to researching the relation of the sediment yield(Y) with the precipitation, rainfall kinetic energy, rainfall erosivity(X), accumulative rainfall erosivity, we found that the relation between sediment and accumulative rainfall erosion power was the best, and the R2 was 0.7559. At the same time we researched the relation between the sediment yield and the precipitation, rainfall kinetic energy, rainfall erosivity, accumulative rainfall erosivity of the three stages, we also found that the relation between sediment yield and accumulative rainfall erosion power was the best. The result says that the accumulative rainfall erosion power can be used to forecast the soil erosion of a small watershed in Loess Plateau.
Key words: Loess Plateau; Check Dam; sediment couplet; sediment source;