| In this study,the upper reaches of the Beiluo River,where the return of farmland to forests(grass)in the loess hilly and gully regions was selected,was used as the research area.Based on hydrological,meteorological,DEM,and imagery data,hydrological statistics and soil erosion model calculation methods were used to analyze the hydrological and meteorological trends from 1963 to 2011,land use changes,vegetation restoration levels and quantitative analysis of climate change and human activities impacts and driving forces on water and sediment changes in river basins.Methods for the division of soil erosion spatial assessment units to simulate soil erosion in watersheds were explored.The results are as follows:1)The average vegetation coverage increased from 17.51%to 40.16%from 1987 to 2014.The vegetation condition was slightly improved and then improved rapidly,and the stability was maintained at the end.The vegetation was degraded in a small part of the region and mainly scattered on the slope.The percentage of the area whose vegetation coverage is less than or equal to 45%decreased from 96%to 45%,while the percentage of the area whose vegetation coverage is more than 45%increased from 4%to 55%.2)The area of woodland and grassland increased significantly from 1987 to 2014,and the cultivated land decreased slowly and then decreased.As a whole,the area of cultivated land decreased by 36.55%,and the area of woodland,grassland,water and residential land increased by 18.54%,16.91%?0.06%,1.04%respectively.Based on the analysis of landscape pattern index,it was found that the landscape patches developed towards strong connectivity,high aggregation and heterogeneous.3)Land use,soil types,micro basins and channel lines are finally identified as factors for the division of soil erosion space evaluation units.Based on spatial correlation?specific combinations and differences between different geographical elements,by using hierarchical control mode and semantic similarity model,the units are automatically divided.The soil erosion modulus showed a significant decreasing trend,and the pattern of soil erosion changed greatly4)According to the mechanism of landscape pattern and soil erosion process,the revised landscape spatial load contrast index(RLLI)was proposed.The RLLI values of each period were 6.53,5.71,2.85 and 1.92 respectively,indicating that the land cover structure changed dramatically,the source landscape of cultivated land with low vegetatioto n coverage above 25 degrees was reduced,and the vegetation landscape with high vegetation coverage increased significantly after the conversion of cropland to forest and grassland5)There is no significant change in rainfall and temperature,but a significant decrease in runoff and sediment transport.The annual average change rate was-0.32mm·a-1 and-193.64t·km-2·a-1.Severe changes occurred in 2002 and 1999,when the comprehensive treatment of soil and water loss and the return of farmland to forest(grass)resulted in a decrease of runoff in the flood period and the level period,and a increase of runoff in the low water period.The amount of sediment transport was greatly reduced,and its reduction was far greater than the change degree of the runoff.When the frequency was less than or equal to 1%,critical runoff distribution index decreased,indicating it difficult to effectively control extreme runoff caused by extreme precipitation events through ecological restoration.6)The contribution of human activity and climate factor to runoff change was evaluated by the method of climate elastic analysis in the theory of water heat balance.Based on the nonlinear curve of runoff and sediment transport relationship and fractal thought,the contribution of human activity and climate change to sediment transport change was evaluated.Ecological restoration makes the contribution ratio of human activity and climate change to the change of runoff from 4:6 to 7:3,and the proportion of contribution to sediment transport from 5:5 to 8:2. |
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