| Taking Zhifangou Watershed in Pingliang City, Gansu Province as the study area, soil erosion characteristics of rainstorms in loess hilly gully region was studied on different scales. Data of rainfall, runoff and sediment was collected from the historical observation and slope artificial rainfall experiments. Firstly, extracted successive rainstorms from 1959 to 1997 according to the meteorological department standard, described spatial and temporal distribution of rainstorms and characteristics of runoff and sediment, then established watershed rainfall erosion model of one rainstorm. Subsequently, by monitoring soil water content in different slopes, analyzed the change regularity of soil moisture. Finally, analyzed soil erosion regularity and established slope rainfall erosion model by artificial rainfall experiments on different underlying surface conditions.The main conclusions are shown as following:(1) From 1959 to 1997, rainstorms occurred for 97 times on the small watershed, 47.4% of them lasting less than 12 h. The rainstorms distributed mainly between June to September, and 39 times(accounting for 40%) in July. By analysis of the spatial distribution, rainstorm centers occurred in the whole basin. The dispersion coefficient(Cv) was 0.23. Uniformity coefficient(η) was 0.79. Extreme rainfall ratio coefficient(α) was 2.46. Non-uniformity on the space of the long-lasting rainstorms was generally smaller than that of the short-lasting.(2) The interannual rainstorm-runoff change trend was analyzed by utilizing the method of accumulative anomaly and Mann-Kendall. The results showed that because of the change of underlying surface and climate, runoff from 1966 to 1985 had a considerable fluctuation and significantly decreased after 1985. The proportion of rainstorm erosion ratio in a year was between 16.5% ~ 89.8%, average for 62.7%. Soil erosion mainly came from the middle and lower areas.(3) The rainstorm erosion equation of Zhifanggou Watershed was built with three factors of rainfall, runoff depth and average rainfall intensity. The coefficient of multiple correlations was 0.818. The simulation effect was good, and the equation had a good applicability in Zhifanggou Watershed.(4) Dynamics of soil water above 30 cm were strongly influenced by temperature and rainfall. The influence depth of rainfall and temperature to soil water was about 30 ~ 40 cm. Dynamics of soil water below 180 cm were mainly influenced by soil properties and distribution of vegetation roots.(5) Runoff threshold had a positive correlation with vegetation coverage and a negative correlation with slope. Runoff depth and sediment had a negative correlation with vegetation coverage and a positive correlation with slope. Vegetations on slope surface had an obvious effect for runoff and sediment reduction. Within the test atmosphere of rainfall intensity, the highest water reducing rate was 29%, and the highest sediment reduction rate was 87%.(6) When the rainfall intensity was 60mm/h, sediment content on uncovered slopes increased fast. The peak occurred between 5 ~ 10 minutes. About 20 minutes later, it achieved a stable phase fluctuations. Grass had an obvious effect on sediment process regulation, and the sand content had a little increasing trend in accordance with the rainfall.(7) The empirical model of sediment for one rainfall event was built with factors of rainfall, slope and slope length, vegetation coverage and et al. It achieved good simulation results and R2.was equal to 0.91. |
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