Analysis On Dynamic Changes Of Sandy Desertification Land In The Middle And Upper Reaches Of Yarlung Zangbo River In Tibet

The research analyzed in this paper is centered on the desertification land in the upper and middle reaches of the Yarlung Zangbo River basin.With the four sets of Landsat TM remote sensing images,respectively taken in 1988,1995,2005 and 2015,the desertification land information extracted by 3S technology,and the socio-economic data,field survey data and meteorological data in the research area,great analyses were made on the distribution of desertification land in 2015,the dynamic changes of desertification land from 1988 to 2015,and the reasons behind.The purpose of the research is to provide theoretical data guidance for the protection and restoration of local ecological environment and the rational utilization and development of agricultural and animal husbandry resources in the Yarlung Zangbo River basin.The results show as following:(1)The total research area is 209694.43 km~2,among which 2.07%were desertification land in 2015,namely 4394.21 km~2.The gravel desertification land and the sandy desertification land cover 1079.93 km~2 and 3262.28 km~2respectively,accounting for 24.83%and 75.17%of the total desertification land area in the basin.As for the distribution,the desertification land area in the upper basin was the largest,accounting for 47.25%of the total,followed by that in the western section of the middle reaches(27.42%),the middle section of the middle reaches(20.11%)and the eastern section of the middle reaches(5.22%).It can be seen that the area of desertification land in the upper and middle reaches of the Yarlung Zangbo River basin decreases from the upstream to the middle reaches.The desertification land is generally distributed along the valley of the Yarlung Zangbo River basin,and the sandy desertification lands stand as the focus to deal with land desertification in the upper and middle reaches of the Yarlung Zangbo River basin.(2)In 2015,the desertification lands in the upper and middle reaches of the Yarlung Zangbo River basin were irregularly distributed,appearing as discontinuous strip or flake areas along the southern Tibetan valley.There were also obvious differences in the distribution of desertification lands in various counties(districts)like Zhongba County in the upstream basin,for instance,the desertification land area accounts for 47.02%of the total in the research area.Influenced by high-altitude westward airflow,topography and the rive valley microclimate,desertification lands were located more in the northern bank of the Yarlung Zangbo River basin than in the southern bank.(3)Back in 1988,the desertification land in the upper and middle reaches of the Yarlung Zangbo River basin covered 3975.21 km~2,while in 2015 the number changed to 4394.21 km~2.From 1998 to 2015,the area of desertification land expanded in all sections of the upper and middle reaches of the Yarlung Zangbo River basin and still enjoys a slow growth trend.Over the past 30 years,the total area of desertification land increased by 9.41%,namely 374.00 km~2,with an average annual growth of 13.85 km~2/a and a year-on-year increase rate of 0.33%.(4)The annual average growth rate varies in different periods in the past 30years.The highest annual growth rate,0.54%,of desertification land in the research area lies in the period from 1995 to 2005.It is seconded by that from2005 to 2015 which is 0.22%,while the average annual growth rate of desertification land from 1988 to 1995 is the lowest,that is,0.21%.It can be seen that the desertification land in the research area has experienced from the period of obvious growth to that of rapid increase,and finally to the one with slow growth,which means that there was a great return in the annual average growth rate of desertification land.It also implies that the annual average rate of desertification in the upper and middle reaches of the Yarlung Zangbo River basin has been under effective control.(5)With the help of field survey data collected respectively from the middle and upper reaches of the Yarlung Zangbo River basin in 2016,2017 and2018,a conclusion can be drawn from the analyses on the natural conditions of the research area,the socio-economic data,meteorological data and related existing documents:the expansion of desertification land in the upper and middle reaches of the Yarlung Zangbo River basin reflects the process of land desertification caused mainly by human activities,apart from natural factors.