A Comparative Study Of The Relationship Between Vegetation And Variation Of Water And Sediment In Typical Basin In Loess Hilly And Gully Region

Vegetation is an important factor to prevent soil and water loss, and quantitative analysis on how vegetation help the soil to hold water is conducive to accurately estimate the amunt of soil-water consrvaton by vegetations, also, is of great significance to the regional ecological environment restoration. With the development of hydrological simuation technology, hydrological models has become an effective means of basin hydrology management. Different models have their own focuses, as a consequence, different models in the same basin possibly produce different results. Understanding the differenes among models and selecting right model in line with local conditions is important for accurate estimation of the impact of vegetations.. In this paper, Yanhe basin of Loess hilly region which suffered most from soil erosion was taken as the case study area. The variation characterstics of runoff and sediment from 1970 to 2010, and its relationship with rainfall and vegetation variation was analyzed. The soil ersion of the typical years at the different perionds was simulatied by using the Revised Universal Soil Loss Equation(RUSLE) model and Morgan – Morgan – Finery(MMF) model, besides, the veracity and applicability of the two models were evaluated. The main results of the thesis are as follow:(1) The runoff and sediment of Yanhe basin obviously changed with passage of time. The time mutation point of the variation tendency of annual runoff and sediment is 1996. The quantity of water and sand increased in volatility from 1970 to 1996, while discharged obviously from 1996 to 2010.(2) The vegetation coverage of Yanhe basin has changed significantly. The reslt shows that farmland transated into woodland and grassland. From 1978 to 2010, the area of farmland reduced from 5248.63 km2 to 1217.13 km2, but the area of forest and grassland increased from 1854.35 km2 to 6281.45km2. With the farmland and abandoned land changed into woodland and grassland, the area of woodland declining, the area of grassland will increased. The spatiotemporal pattern and variation of vegetation kept in step with the important events happened in history in the basin, such as the reform of forest rights system in the early 1990’ s, the Project of Returning farmland to forestland or grassland around 2000, etc.(3) The soil erosion of the Yanhe basin were simulated based on the RUSLE model. The results of the model showed that the average erosion modulus was 4434.7 t/(km2·a) in 1978, and 1026.7 t/(km2·a) in 2010,which has reduced by 76.8%. The intensity of soil erosion of the forest region in southeastern basin and the both sides of river was low, but high of the grassland region in northwest basin and the farmland region. Considering the dams in 1978 and 2010, the relative error between the meaured and simulated values were-3% and 36%, by reducing sediment and unoff, the dams in Yanhe basin play an important role in soil & water conservation.(4) The soil erosion of the Yanhe basin were also simulated based on the MMF model. The results of the model showed that the simulated values of the sediment discharge was 3691×104t, relative error was-29%, and 2586×104t, relative error was 49%,after revised, the corrected values of the sediment was 2506×104t in 1978, and 868×104t in 2010. The simulation results showed that MMF model for runoff performed obviously better than for sediment. The simulated runoff in 2010 was at 56.7% in 1978, while the simulated sediment was 16.5%.(5) The RUSLE and MMF models were compared and evaluated by using a series of methods. The results showed that the MMF model described the physical process of soil erosion better than the RUSLE model.Compared with the RUSLE model, the MMF model was more comlex in theory, and more difficult in the process of modeling.Taking the dams into consideration, the RUSLE model had higher simulated precision and better forecasting effect.