Analysis Of Spatial And Temporal Differentiation And Driving Factors Of Soil Erosion In Brahmaputra River And Its Two Tributaries Area Of Tibet

The Brahmaputra river and its two tributaries area in Tibet is a typical development zone in western China and also one of the typical ecologically fragile areas in China.The main scope includes the central valley area of the Yarlung Zangbo River and its two main tributaries,the Lhasa River and the Nianchu River.Due to the large topographic drop in the region,low vegetation coverage,and the obvious trend of warming and humidification on the Qinghai-Tibet Plateau as well as the impact of human activities,soil erosion is becoming more severe and complex.The ecological degradation caused by serious soil erosion has become one of the important factors restricting economic development in the region.However,due to special natural conditions such as high altitude and coldness,soil erosion monitoring,evaluation,prevention and control are difficult to carry out in this area.In the context of ecological civilization construction and rapid economic development on the Qinghai-Tibet Plateau,it is a prerequisite and basis for strengthening soil erosion control in this area to carry out regional soil erosion risk assessment and driving factor analysis.Therefore,this study calculates the soil erosion intensity of The Brahmaputra river and its two tributaries area from 1995 to 2020based on the Revised Universal Soil Loss Equation（RUSLE）,analyzes the temporal and spatial characteristics of climate,vegetation,land use,nighttime light index and other factors,clarifies the temporal and spatial differentiation rules of soil erosion intensity in different periods in The Brahmaputra river and its two tributaries area,clarifies its soil erosion risk;and uses mathematical statistics methods to comprehensively analyze the relationship between soil erosion factors and temporal and spatial changes of soil erosion.The main conclusions are as follows:（1）Clarifying the temporal and spatial characteristics of soil erosion influencing factors in The Brahmaputra river and its two tributaries area.From 1995 to 2020,annual precipitation,temperature and NDVI overall showed a slight increasing trend.Among them,annual precipitation and NDVI generally showed a trend of being higher in the east and lower in the west,while the spatial distribution of temperature showed vertical zonality,with areas with higher average annual temperature mainly distributed in the lower altitude valley areas.Based on the future trend analysis,it can be seen that the annual precipitation and NDVI in most areas of the Brahmaputra river and its two tributaries area showed a decreasing trend,and the temperature showed an increasing trend.From 1995 to 2020,the land use categories with the largest changes in rate were residential land,sparsely vegetated land,bare land,and farmland.The areas with high nighttime light index in the study area are mainly concentrated in the urban areas of Lhasa,Shigatse and Shannan in the river valley,and sporadically distributed in towns and cities in various counties.The nighttime light index of the three main cities all showed a trend of first decreasing in the outer area and increasing in the inner area,and then decreasing in the inner area and increasing in the outer area.（2）This study clarifies the spatial and temporal differentiation of soil erosion in the Brahmaputra river and its two tributaries area from 1995 to 2020 and assesses the risk of soil erosion in the region.The soil erosion in the Brahmaputra river and its two tributaries area has high spatial and temporal heterogeneity.The average soil erosion modulus in 1995,2005,2015and 2020 were 30.32,31.66,29.79 and 29.64 t·hm^-2·a^-1 respectively;mainly slight erosion and mild erosion.The proportion of soil and water loss area in the four periods was 78.20%,57.52%,58.83%and 57.09%,showing a gradual decreasing trend overall.The intensity of soil erosion presents a pattern of being higher in the east and lower in the west,higher in river valleys and lower on mountain slopes.The areas with higher soil erosion intensity are mainly distributed in the SamzhubzêDistrict and Namling County.From 1995 to 2020,the soil erosion modulus in the Brahmaputra river and its two tributaries area showed a trend of first increasing and then decreasing.Among them,the period from 1995 to 2005 was the main period of increase in soil erosion modulus,which gradually decreased after 2005.The areas with large decrease in erosion modulus were mainly Lhatse County and Bainang County.（3）Identifying the key factors affecting soil erosion in the Brahmaputra river and its two tributaries area.Based on machine learning algorithms such as Support Vector Machine method,Random Forest algorithm,BP Neural Network and Multiple Linear Regression,the importance of factors affecting the spatial and temporal changes of soil erosion such as climate,vegetation,soil,topography and human activities were ranked respectively for 1995,2005,2015 and 2020.The results show that the importance ranking of factors based on different algorithms has similarities but also differences.The importance ranking of soil erosion factors based on the support vector machine method shows that the main controlling factors affecting the change of soil erosion in the Brahmaputra river and its two tributaries area from 1995 to2020 are vegetation,rainfall,elevation and slope;the main controlling factors obtained by the random forest algorithm are rainfall,vegetation and nighttime light Index;the main controlling factors obtained by the BP neural network are vegetation,soil organic matter content and land use status;the main controlling factors obtained by multiple linear regression are vegetation,land use status and soil organic matter content.The results of the random forest method are more accurate,so it can be used as the basis for the analysis of the main controlling factors of soil erosion in this area.Finally,the main controlling factors affecting soil erosion in the Yarlung Zangbo River and its two tributaries area are precipitation,vegetation and human activities.The spatial and temporal differentiation of soil erosion in the study area is mainly affected by natural factors such as climate change,followed by human activities.