Catchment-scale Simulation Study On Soil Erosion In The Loess Hilly Region

The soil and water loss in the Loess hilly region is serious.The continuous soil erosion causes the eco-environmental problems,and the damage of the ecological environment further aggravates the soil erosion in turn.The vicious circle greatly restricts the development of the social economic environment in this region.Luoyugou watershed,located in the third sub-region of Loess Hilly Region,is a typical small watershed with representative soil erosion characteristic which has remarkable particularity in climate,terrain and human influence.One of the first order tributaries of Luoyugou watershed-Qiaozigou sub-watershed,including Qaozidonggou and Qiaozixigou sub-watersheds,was listed as contrast testing sub-watersheds by YRCC of MWR in 1985.Carrying out simulation study on soil erosion in the sub-watersheds can indicate representative results.Based on Standards for classification and gradation of soil erosion(SL190-2007)issued by MWR,this thesis accurately identified and judged the conditions and threats of surface erosion,gully erosion and hydraulic erosion in the Luoyugou through soil erosion monitoring.On this basis,the annual average runoff and sediment yield in watersheds with different spatial scales were simulated quantitatively by GeoWEPP physical model.In order to reveal the change law of spatial-temporal pattern of soil erosion and the main driving force factors in study areas,a comparative study on soil erosion control effect was carried out in 2 sub-watersheds,Qiaozidonggou and Qiaozixigou,which implementing different conservation measures.The research conclusions would provide important scientific basis for soil erosion monitoring,control and watershed management and bring forward important guiding and reference value for the control of soil erosion in hilly area of Loess Plateau.The main conclusions of this research were listed as following:(1)The soil erosion status of the surface erosion,gully erosion and hydraulic erosion in the Luoyugou was classified according to Standards for classification and gradation of soil erosion(SL190-2007)of MWR through soil erosion monitoring.The results showed that the mean annual soil erosion modulus of 16a was 4557.7 t/(km2·a),which went up to 355.77%over the soil loss tolerance,and indicated the level of hydraulic erosion in Luoyugou reached to middle degree.The areas of middle degree and above erosion intensity accounted for 89.67%of study area which was the main source of surface erosion.The gully erosion intensity was 4.244km/km2and reached the strong level in Luoyugou.We can conclude that the situation of all kinds of soil erosion in Luoyugou was severe and some reasonable soil erosion measures need to be taken according to concrete conditions.(2)A variety of statistical methods(Mann-Whitney test,K-S test,Wilcoxon test,et al.)were used to evaluate the applicability of the CLIGEN model in the Luoyugou and Qiaozigou respectively,to verify effectiveness for the input of the meteorological data of the WEPP model.Then,the*.par meteorological data statistics files were generated for future meteorological simulation.(3)Spatial-temporal patterns of soil erosion were simulated in multi-scale coupling study areas(watershed,sub-watershed)using GeoWEPP model and the results were tested in Relative Error-Re.The results showed that the GeoWEPP model was suitable in Qiaozidonggou(1.23 km~2)and Qiaozixigou(1.03 km~2),but not in Luoyugou(72.79 km~2),indicating that the model is more suitable for soil erosion simulation in smaller watershed.(4)Based on the simulation results,the temporal and spatial pattern of soil erosion and dynamic changes in Qiaozidonggou and Qiaozixigou were analyzed in detail,and the measures and effects of soil and water conservation were evaluated by comparative study.The results showed that the situation of soil loss in the Qiaozidonggou and Qiaozixigou of Phase 1(1988-1990),Phase 2(1991-1995),and Phase 3(1998-2000)all have been improved,and improvement efficiency in Phase 1-Phase 2 is higher than Phase 2-Phase 3.There are two reasons:on the one hand,precipitation on different phase was different and led to different influence on soil erosion,which is the natural factor of the decline of soil erosion in the study areas.On the other hand,the rectification of large area land use in the sub-watersheds during Phase 1-Phase 2,which is the artificial factor of the decline of soil erosion in the study areas,and the dominant factor is human influence.The result showed that Qiaozidonggou,as a control sub-watershed,suffered less damage than Qiaozixigou without conservation measures which means biological and engineering conservation control measures had played an important role in soil and water conservation.In general,the land use rectification measures on the"surface"are the dominant factors,and the soil erosion engineering/biological control measures on"point"are effective supplement.(5)The evolution of the soil erosion pattern in the Qiaozigou is closely related to the evolution of the land use pattern in the watershed during the same period.Land use patterns and relevant transformation are the main driving forces for the formation of the soil erosion pattern in the study area and have a long-term impact on the changes in soil erosion.