| Loess Plateau is a region with the most serious water and soil erosion in China. The change of global climatic and intensification of human activities, particularly the large-scaled implement of soil and water conservation measures such as conversion of cropland to forest and grassland, have great impact on the underlying surface. Yanwachuan watershed, which is the representative watershed of Loess Tableland, was selected as research object in this thesis. Based on meteorological and hydrological observation(1981-2009), and the land use data of 1985, 1996 and 2009, and relied of mathematical statistic and spatital analysis technology of RS&GIS and theory of landscape ecology and so on, by the method of combining of qualitative description and quantitative analysis, the digital watershed model of Yanwachuan watershed was established, the characteristics of climate and hydrology and evolution of land use pattern were analyzed. The effects of water and sediment reduction after the comprehensive management were carried out and the impacts of climatic variable change and human activity on runoff and sediment were quantitative evaluated. Further analysis on runoff simulation of individual rainfall and soil erosion intensity were carried out. The main conclusions were as follows:(1) The annual mean temperature increased significantly during 1981-2009 at the rate of 0.081℃/a. All the four seasonal temperature(spring, summer, autumn and winter) increased significantly at the rate of 0.125℃/a,0.065℃/a,0.067℃/a and 0.082℃/a, respectively. The relatively cold and warm periods were seperated by the year 1997. The extreme maximum temperature increased significantly at the rate of 0.094℃/a. The extreme minimum temperature decreased significantly at the rate of-0.019℃/a. Annual precipitation exhibited a deceasing trend in the studied period at the rate of-1.521mm/a. The decreasing trend was significant in spring, but not in autumn. While the increasing trend in summer and winter was not significant. The annual daily maximum precipitation and the annual total amount, the frequency and intensity of exteme precipitation all didn’t show significant decrease at the rate of-0.064 mm/a,-0.864 mm/a,-0.080 times/a and-0.008 mm·d-1/a. The frequency of severe drought occurrence increased significantly at the rate of 0.085 times/a. As spatial distribution concerned, the precipitation was high in the west and low in the east. The relative humidity in studied period showed indistinctive decreasing at the rate of-0.178%/a. And the potential evapotranspiration increased significantly at the rate of 5.672 mm/a.(2) The space-time characteristics of land use were analyzed by using the land use data in 1985, 1996 and 2009. The result showed that all the land use types were relatively stable from 1985 to 1996. The area of woodland increased 258.06% from 1985 to 2009, and the dynamic degree was 10.75%, while the area of grass decreased 38.22% and the dynamic degree was-1.59%. Analyzed from the transfer matrix of land use, woodland, grassland and cultivated land were more active. Analyzed from the gravity center shift, the gravity center of woodland and grassland moved southwest on the whole from 1985 to 2009, while the gravity of residential land moved northwest and the spatial centroid of water moved northwestward. From the perspective of landscape ecology, the landscape heterogeneity increased, the landscape type developed towarded multi-function direction, the average level and stability of landscape increased and the possibility of soil erosion decreased. Analyzed from the vegetation coverage, the NDVI in growing season increased significantly from 1981 to 2009 at the rate of 0.0042 per year.(3) Significant downward trend was observed in annual time series for runoff from 1981 to 2009, at the rate of-9.573×104 m3/a. The rainy season also had significant decreasing trend, at the rate of-6.643×104 m3/a. The decreasing trends were significant in spring and summer, but not in autumn and winter. The decreasing rates in four seasons were-3.583×104 m3/a,-2.607×104 m3/a,-1.213×104 m3/a and-0.786×104 m3/a. The significant decreasing trend was observed in annual time series for sediment from 1981 to 2009, at the rate of-2.458×104 t/a. The rainy season also had significant decreasing trend, at the rate of-2.389×104 t/a. Significant decreasing trends were observed in four seasons for sediment at the rate of-0.736×104 t/a,-16.22×104 t/a,-0.702×104 t/a and-0.006×104 t/a. The flood frequency decreased significantly and mostly occurred from July to September. And the maximum flood peak mostly occurred in July and August.(4) The runoff and sediment in the watershed had significantly decreased since an abrupt(4) The runoff and sediment in the watershed had significantly decreased since an abrupt change in 1997, which was identified using three methods, liked the double mass curve. Thus, the whole period was divided into two sub-periods, the base period(1981-1997) and the management period(1998-2009). The benefit value of runoff reduction and sediment reduction in management period compared with base period in the control watershed of Yanwachuan hydrologic station were 27.2% and 75.9%, respectively. The precipitation in management period would increase the runoff and sediment because the annual average precipitation, rainfall erosion, extreme precipitation, extreme precipitation intensity increased 2.61%, 12.47%, 3.19% and 5.05%. The improvement of underlying surface conditions would decrease the runoff and sediment because the value of NDVI increased 20.59%. The contribution rates to runoff and sediment reduction were 108.9% and 105.1% from human activities, and-8.9% and-5.1% from precipitation. Analyzed from the relationship characteristic of individual rainfall between runoff depth and modulus of sediment yield from 1981 to 2009, the sediment concentration of tended to be stable when the flow discharge exceeded a certain critical value, which was 3 mm. Whether at annual time scale or inter-event time scale, the flow-sediment relationship had significantly varied in the management period, compared with the base period. The mean sediment discharge per unit runoff in management period was significantly lower than that in the base period.(5) SCS model combined with GIS was applied to analyze the spatial distribution of individual rainfall runoff in base period and management period. The result showed that the runoff decreased caused by the transform of underlying surface that the woodland increased, wild grassland decrease and vegetation coverage increased significantly. The runoff in plateau area decreased less and the runoff in gully region decreased more. Under the same precipitation condition and taking the individual rainfall of 20090816 as a reference, the runoff depth and runoff amount would decrease 0.34 mm and 124.53×104m3, respectively, when the grassland was changed to woodland. And the runoff depth and runoff amount would decrease 0.66 mm and 241.74×104m3, respectively, when the vegetation coverage changed from medium to high. The sub-watersheds, which would be influenced more greatly, mainly distributed on gully region in the middle of watershed.(6) RUSEL model combined with GIS was applied to analyze the soil erosion modulus. The result showed that the soil erosion intensity of Yanwachuan watershed was strong erosion with average erosion modulus of 6474.82 t·km-2·a-1 in base period, and was medium erosion with average erosion modulus of 3414.43 t·km-2·a-1 in management period. The soil erosion intensity in the plateau area was mainly slight, while it was mainly strong intensity and more intense in the gully area. Analyzed from land erosion intensity transformation from base period to management period, the areas of slight, light, medium and strong erosion would increase 7.87%, 55.88%, 1.99% and 18.66%, respectively. And the areas of extremely strong and violent strong would decrease 9.86% and 58.59%, respectively. Each of the erosion grades mostly change to the lower degree and the soil erosion was preliminary controlled. Average erosion modulus in Yanwachuan watershed decreased with the rise of slop. Average erosion modulus order in different aspect was sunny slope>half-sunny slope> half-shady slop>shady slop. The contributions of precipitation and human activities on soil erosion reduction in management period were-12.36% and 112.36%. Under the same precipitation condition, the soil erosion amount would decrease 41.71% and 38.19%, respectively, when the grassland changed to woodland and the vegetation coverage changed from medium to high. |
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