Analysis On The Topographical Characteristics And Vegetation Effects On Runoff Features In The Watershed On Loess Plateau

To slow down the soil erosion, to improve ecosystem environment and to properly use the limited water resource are becoming critical issues in Loess Plateau. As a result, it is meaningful to study on the hydrologic effect on the different landuse types there. In this study, the data of rainfall, runoff and meteorology measured from Year2004to2010in the Caijiachuan Watershed in Loess Plateau region of Shanxi Province, northern China, have been statistically anyalyzed. Firstly, we examined the change regulation of the watershed features and stream features with different area scales. Secondly, we studied on the elements which influence the rainfall and runoff in three subwatersheds under different vegetation cover rates. Lastly, with the aid of SWAT model, we managed to seek the optimized scenario of vegetation space disposal mode in Caijiachuan Watershed.The results showed that:with watershed area increasing, the length of main stream, overall length of main stream and branch, meander coefficient and the coefficient of basin shape all increased; the basin asymmetry factor and the average elevation both decreased; the drainage density, stream gradient and average slope reduced at the beginning and then stayed stable. With weak ability to prevent from flooding, the study area must be protected urgently for the severe soil erosion. Concretely, the stream gradient should be controlled under6.1%, the drainage density below0.58km/km2, and the watershed under10km" must be the key to deal with the water and soil conservation in Caijiachuan Watershed. The area with slope above25°occupied52%of the overall area, so flooding is apt to take place there and the conservation of soil and water should be paid more attention to.During the study period, the average runoff depth for the lasting six years in the less-forest watershed is3.43times of it in the more-forest watershed, and4.95times of it in the no-forest watershed. That is to say, the heavier the vegetation cover is, the more the watershed’s runoff is. However, raito of less-forest watershed’s runoff in dry season is4.4%than it of more-forest watershed’s runoff, showing that the vegetation could dramatically adjust the runoff in the dry season. On the contrary, vegetation in no-forest watershed own scarcely adjusting ability itself, with the highly ratio33.54%. the early loss of rainfall in more-forest watershed is5.84mm more than it in less-forest watershed and7.167mm more than no-forest watershed. In a word, only ample rainfall could engender runoff in more-forest watershed. In addition, the major type of rainfall in the experimental watershed is the type of storm events with short duration and high intensity (A-type), which occupied64.48%of the total number of times, and47.25%of the total rainfall in the research period. Under different rainy type, the elements which have highly effect on the runoff are evidently different. As a result, it is necessary to take a consideration on the rainy type while a rainfall-runoff model is setting up in Loess Plateau.With the use of the Soil and Water Assesment Tool (SWAT), the runoff depth was simulated in the Caijiachuan watershed. In calibration period, Nash-Suttcliffe coefficient is0.836, relative coefficient0.861; in validation period, Nash-Suttcliffe coefficient is0.802, relative coefficient0.829. SWAT model is applicable to the Cijiachuan Watershed.Adopting the SWAT model, seven extreme landuse scenarios are simulated, and the result show that effectiveness of arbor, shrub and garden plot is more significant. Considering the terrain and the need of local people, an optimized scenario of vegetation space disposal is devised, whose average yearly runoff decreases by24.08%compared to the measured value.