Effects Of Land-use Landscape Pattern Change On The Runoff And Sediment In Yanhe River Watershed

Since the implementation of the project of returning cropland to woodland or grassland in the Loess Plateau in 1999, the land-use landscape pattern, runoff and sediment has changed significantly. This study take Yanhe River Basin as a case study area. According to the long-term(1970-2010) hydrological data measured at Ganguyi hydrological station and four periods of land use data(1978, 1990, 2000, 2010), the change in land-use landscape pattern, and runoff and sediment were analyzed in Yanhe River Basin. The relationship between landscape index and runoff and sediment were also studied on basin scale, using the GIS and RS techniques, through the simulated rainfall experiments and combing the methods of landscape ecology, qualitative, quantitative and mathematical statistics analysis. The runoff and sediment characteristics and benefit of natural grassland, artificial caragana korshinskii land and artificial robinia pseudoacacia forestland with different restoration stages on slops were analyzed, which provided the theoretical foundation for explaining the influence of land-use landscape pattern on runoff and sediment on basin scale. The main conclusions are as follows:(1) The landscape pattern of Yanhe River Basin has changed significantly after years of ecological construction and comprehensive management of watershed, especially the implementation of the project of conversing cropland to woodland or grassland. At the patch level, the fragmentation of grassland and cultivated land were higher than other landscape types and their patch shapes were more complex. At the landscape level, the landscape pattern dominated by cultivated landchanged to the new landscape pattern dominated by forest land and grassland. The landscape pattern dominated by forest land and grassland was beneficial for the improvement of the ecological environment because of their role in erosion reduction.(2) In the past 40 years, the annual change of runoff and sediment of Yanhe River Basin showed a downward trend, and the change rate of sediment was higher than runoff. Mann-Kendall non parametric statistical tests showed that the turning point of runoff and sediment was the year 2005. The change of runoff and sediment experienced three obvious periods. The main cause is not precipitation, but related with the changes in land-use caused by vegetation restoration in Yanhe River Basin.(3) The relations between landscape pattern dominated by cultivated land, forest land and grassland and changes of runoff and sediment were very significant on watershed scale. At the patch level, the SPLIT index of cultivated land, the CONNECT index of forest land and the NP index of grassland played an important role in the process of runoff, However, the ED index of cultivated land, the PLAND index of forest land and the IJI index of grassland were closely related to the process of sediment. The regression equations indicated that the soil and water erosion was lighter when the area of forestland and grassland was greater and the connectivity was higher and the distribution of cultivated land was more dispersed. At the landscape level, the runoff and sediment yield of Yanhe River Basin showed a negative correlation with NP index, AI index and showed a positive correlation with AREA-MN index and SHID index. The regression equations showed that the runoff and sediment discharge was less when the landscape fragmentation was lighter and the landscape diversity was richer and the landscape distribution was more concentrated.(4) The restoration year was an important factor that influenced the runoff and sediment yield of natural grassland, artificial caragana korshinskii and artificial robinia pseudoacacia forestland on slop in Yanhe River Basin. Under the same vegetation restoration mode, the variation of runoff rate and sediment yield intensity under vegetation at different restoration stages is quite different. With the increase of restoration years, the runoff rate and sediment yield intensity of natural grassland and artificial caragana korshinskii land decreased, but the changes in runoff rate of artificial robinia pseudoacacia forestland was not significant, and the sediment yield intensity of artificial robinia pseudoacacia forestland increased because of the increase of canopy density and tree height. There were good quadratic functions between accumulated sediment, total runoff and rainfall time on different restoration stage. The relationship between accumulated sediment and the total runoff can response by power function.(5) The sediment reductions of different restoration stages were all higher than the runoff reductions, which answered the question why the decreasing rate of sediment was higher than the runoff on watershed scale. The effect of artificial robinia pseudoacacia forestland on runoff and sediment reduction was the most outstanding. The benefit of natural grassland and artificial caragana korshinskii woodland increased with the increase of restoration years. The ecological functions of natural grassland and artificial caragana korshinskii woodland were relatively small in the early recovery stage(0-8 years). If the surface vegetation of artificial robinia pseudoacacia forestland was destroyed in the later recovery stage(more than 16 years), it was still one of the main source of sediment yield when encountered a heavy rainfall. Therefore, we must complete fencing and prohibiting grazing measures in the process of vegetation restoration.