Spatial And Temporal Changes Of Soil Erosion And Its Driving Factors Before And After The "Grain For Green" Project In The Loess Plateau

The Loess Plateau?LP?is vulnerable to eco-environment,has become one of regions with the most serious soil and water loss in China and even the world.The severe soil erosion in the Loess Plateau has seriously affected the local sustainable development,and the sediment deposition has brought serious threats to the lower reaches of Yellow River.In order to control serious soil erosion,a series of soil and water conservation projects and forestry ecological engineering have been carried out in this area,especially“Grain for Green?GFG?”project began in 1999 has been achieving remarkable effect in controlling soil and water erosion and improving eco-environment,such as significant increment in vegetation coverage and sharp decline in runoff and sediment loads due to the“GFG”project.Soil erosion is a complex spatiotemporal variation process under the combined influence of climate change and human activities on the underlying surface.Clarifying spatial and temporal changes of soil erosion and its driving factors before and after the“GFG”project in the Loess Plateau and scientifically defining the contribution rate of the“GFG”project on the impact of soil erosion,we can scientifically understand the driving effect of the“GFG”project on temporal and spatial changes of soil erosion on the Loess Plateau,can promote the development of regional soil erosion theory,and can provide scientific basis for soil and water conservation and ecological construction and health management of the Yellow River.Based on soil erosion changes before and after the“GFG”project in the Loess Plateau,the upper reaches of the Beiluo River basin with large-scale vegetation restoration in the loess hilly and gully region was selected as research area in this study.Using Multi-source data?such as multi-temporal remote sensing image,digital elevation model?DEM?,hydrometeorological data,etc.,?and comprehensively using different methods?such as GIS,RS,hydrological statistics,RUSLE model simulation,etc.,?,rainfall factors,land use/land cover change?LUCC?,variation characteristics of sediment load and spatial and temporal changes of soil erosion in the basin before and after the“GFG”project were systematically analyzed.The changes and driving factors of soil erosion in the basin before and after the“GFG”project were clarified,and the contribution rates of rainfall change and the“GFG”project on soil erosion change in the basin were defined.The main conclusion were as follows:?1?From 1981 to 2010,the average annual rainfall,erosive rainfall and rainfall erosivity in the upper reaches of the Beiluo River were 408.8 mm,232.8 mm and 1252.4MJ·mm/?hm²·h?,respectively.The overall distribution pattern of them were decreasing from southeast to northwest.The annual rainfall in the basin during 30 years showed no significant decrease,and the linear tendency rate was-0.4 mm/a.The annual erosive rainfall and rainfall erosivity in the basin showed a rising trend in volatility with linear tendency rates of 0.45 mm/a and 7.3 MJ·mm/?hm²·h·a?,respectively.This trend indicates that the climate background of the basin from 1981 to 2010 has potential conditions that exacerbate soil erosion.?2?In 1990,the upper reaches of the Beiluo River basin were dominated by cropland and grassland,accounting for 42.62%and 54.35%of the total area respectively,showing the characteristics of land use structure in which agriculture and animal husbandry account for the absolute majority.In 2000,the basin still maintained the land use structure characteristics of agriculture and animal husbandry,and the crop land and grassland area accounted for42.69%and 54.20%of the total area,respectively.In 2010,the area of crop land,grassland and forest land in the basin accounted for 14.59%,63.43%and 20.95%of the total area respectively,showing the characteristics of land use structure based on animal husbandry,forestry and animal husbandry.Under the influence of“GFG”project,from 2000 to 2010,the land use conversion in the basin was mainly transformed from crop land to forest land and grassland,and converted from grassland to forest land.Since 2000,the change of cropland,grassland and forest land with topography has been concentrated in the range of elevation 1350¹650 m and the slope was more than 15°.From the perspective of vegetation coverage,the average coverage of the basin in 1990,2000 and 2010 was 16.81%,30.78%and 47.57%,respectively,showing an increasing trend.Since 2000,vegetation restoration has been particularly obvious,and the proportion of excellent vegetation area has increased significantly.?3?From 1981 to 2010,the average sediment transport volume in the basin was 27.611million tons,which showed a significant reduction trend,with an average annual variability of-572.28 million t/a.The sediment transport in the basin from 1981 to 2010 could be divided into two periods from 1981 to 2000 and from 2001 to 2010.Compared with1981-2000,the annual average erosive rainfall and rainfall erosivity increased by 6.3%and6.8%respectively in the basin from 2001 to 2010.Rainfall factors had potential conditions for increasing sediment transport,while sediment transport during the same period has been significantly reduced.The land use/cover change caused by“GFG”project was the decisive factor for the reduction of sediment transport.?4?The soil erosion modulus of the basin in the 3 different periods(I₁₉₉₀??₂₀₀₀??₂₀₁₀)before and after the implementation of the“GFG”project,were 8612.62,8078.91 and3060.71 t/?km²·a?,respectively.During the period I₁₉₉₀-II₂₀₀₀,the area of soil erosion intensity in the basin was not significantly changed,the area where the intensity of soil erosin was reduced and increased was equivalent.During the period II₂₀₀₀-III₂₀₁₀,the area of erosion intensity reduction accounted for 69.84%,while the area with increased erosion intensity accounted for only 6.52%.Under the influence of“GFG”project,during the period II₂₀₀₀-III₂₀₁₀,the proportion of erosion on steep slopes above 15°decreased significantly,and the severe erosion decreased most prominently,with an average reduction of 84.5%.The average soil erosion modulus under different land use types showed the characteristics of cropland>grassland>forest,and the average soil erosion modulus decreased with the increase of vegetation coverage.During the period I₁₉₉₀-II₂₀₀₀,under the background of increasing rainfall erosivity,the average soil erosion modulus was basically increasing under the same vegetation coverage level,while the average erosion modulus of grassland and forest decreased.During the period II₂₀₀₀-III₂₀₁₀,the average erosion modulus under different land use and land cover conditions showed a decreasing trend,and the higher the vegetation coverage level,the greater the reduction in soil erosion modulus.?5?After the implementation of“GFG”project,the area with 81.87%of total basin area had the characteristics of small decrease of rainfall erosivity and large increase of vegetation coverage.The combination of the two drove the soil erosion in this area to decrease significantly.The average decline was 65.31%before the implementation of the project.The“GFG”project based on vegetation restoration contributed 85%to the reduction of soil erosion in the basin.In Wuqi County,due to the higher degree of vegetation restoration than Dingbian County,the contribution of the project to the reduction of soil erosion in Wuqi County was slightly higher than that in Dingbian County.The contribution of reduced rainfall to reducing soil erosion in the basin was only 15%.