| In different ecosystems of the earth,the river basin is a very important ecological system.It has abundant water resources and provides a powerful impetus for human survival and development.Therefore,the change of water environment has become the focus of river basin.The middle route of South-to-North Water Diversion Project is a major water diversion project in China.The change of the ecological environment in the water source area is widely concerned.Among them,the human activities are the dominant factors of the ecological environment change in the water source area,and the problem of the non-point source pollution from the soil erosion has important influence on the ecological security of the water resources and the healthy development of the water environment.In this study,the water source area of the middle route of the South-to-North Water Diversion Project was selected as the study area.Landsat TM(2000)and HJ-1A / 1B(2010 and 2015)images were used to obtain the land cover based on the object-oriented decision tree classification method.Normalized difference vegetation index combined with the pixel dichotomy model,access to fractional vegetation coverage(FVC)data.The changes of land cover and fractinal vegetation cover were analyzed comprehensively,and the characteristics of vegetation coverage of forest,shrub and grassland were analyzed and discussed.Based on the land cover data,the key driving force of the ecological environment change---human activity intensity---in the water source area was estimated,and the erosion risk index(ERI),which was the key influencing factor of environmental degradation,was established.From the spatial pattern,the time change,flow-length distance and its four characteristic regions,a comprehensive analysis was conducted,the main conclusions are as follows:(1)The change of forest,cropland and settlements is the main land cover change in the water source area.From 2000 to 2015,the acreage of forest increased,acreage of cropland decreased,and the acreage of settlements increased significantly.The FVC of forest,shrub and grassland increased significantly.Among them,the FVC of forest showed an increasing trend,at an increase of 25.4%,and the FVC above 0.6 takes account of the largest proportion of forest area,reaching 94.4% by 2015.The implementation of ecological engineering projects is the main reason for the increase of forest and the decrease of cropland.In addition,immigration policy and economic development resulted in the significant expansion of settlements.But after the ecological policy achieving a certain effect,the focus shifted to the vegetation density and growth to strengthen.(2)HAILS index based on remote sensing technology can effectively reflect the spatial distribution of human activity intensity and its changes.The index is highly correlated with population density and GDP,and can describe the spatial distribution,production capacity and social and economic benefits of the population to a certain extent,which mainly reflects the situation and changes of human beings in agricultural activities.The overall distribution of human activity is characterized by a relatively high mountainous area along the valley,plain and southeastern waters,and relatively low in the northern and southern steep mountainous areas.From 2000 to 2015,the area of human activity decreased by 60230.25 km2,accounting for 63.7% of the total area of the water source area.However,in the Han River tributary of the valley and surrounding Danjiangkou reservoir,the intensity of human activities increased.(3)The overall distribution of ERI index is higher in the valley area along the main stream,in the eastern and western hills,but lower in the northern and southern regions,and in the northern part of the Qinling Mountains and the northern Micang Mountains.The ERI index showed a decreasing trend from 2000 to 2015,but the it increased with high value in the Hanjiang tributary valley and the surrounding area of the Danjiangkou Reservoir.From 2000 to2015,the erosion risk in the study area showed a significant reduction in low-risk level,a slight increase in the risk level,and a decrease in high-risk level.There is an increasing trend in risk-free level.In the case of risky level and high-risk level where there is a certain risk,the acreage is greatly reduced from 2000 to 2015,but the acreage increased significantly from2010 to 2015.For extreme-high-risk level,the acreage continues to decrease.(4)Based on the flow-length distance and its four special regions,we anaylized the influence of human activity intensity to the erosion risk,it can be seen that: 1.The intensity ofhuman activity in the riparian area is closely related to the risk of erosion.The main role of human activities on the risk of erosion in ecological construction and other policies and water area population,economic development.According to the study,it was found that HAILS and ERI were at a high level in the riparian area,and the trend was the same,which decreased with the increase of distance.2.In valley slope zone,the intensity of human activities reduced in 15 years,and the acreage with low risk of erosion is increasing,and the area with high risk is decreasing.3.The intensity of human activity in the back slope zone is also closely related to the erosion risk.The implementation of the ecological policy in back slope zone turn a large number of sloping land into natural vegetation,at the same time,ecological migration from poor villages to relocate,greatly reducing the cultivated land area and deforestation activities.From 2000 to 2015,the HAILS index continued to decline,and the ERI index showed an increasing trend in the risk-free and low-risk levels,with high-risk and extreme-high-risk levels.4.In the top of the mountain,HAILS index and ERI index showed no regular changes.The region is the farthest away from the water source,and human activities are related to the local tourism industry,and the development of the tourism industry increases the risk of soil erosion in the water source area. |
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