Fractional Vegetation Cover In Qinghai Lake Watershed Extraction And Change Analysis Based On Time-series NDVI Data Of Landsat And MODIS

Qinghai Lake is a typical dryland lake.The Qinghai Lake Watershed is located in the northeastern part of the Qinghai-Tibet Plateau.It is in the middle of the monsoon humid zone and the inland arid zone.The ecological location is extremely important,and the ecological environment is fragile,which is greatly affected by the global climate.In this paper,the Qinghai Lake Watershed is used as the research area,and the NDVI map of Landsat 2001,2007,2014 and 2016 time series data are used to reconstruct the vegetation growth curve based on the quadratic polynomial,which corresponds to MODIS MOD09 GQ MVC-NDVI(Maximum Value Composite NDVI).For each pixel,the value of MVC-NDVI in 2001,2007,2014 and 2016 is solved under the date constraint.Then,the dimidiate pixel model was used to estimate the maximum fractional vegetation cover of four years.Based on the value of maximum vegetation coverage,as well as meteorological data and area elevation,this paper discusses the relationship between the spatial and temporal changes of fractional vegetation cover in Qinghai Lake Watershed and regional climate and topography.The main conclusions of this paper are as follows:(1)Landsat MVC-NDVI reconstruction method has more practical application value because of the introduction of MODIS MVC-NDVI corresponding date,but the fitting result is subject to underlying surface,thus,performance of the method in area of low fractional vegetation cover(such as bare rock land,sand area,etc.)and area covered by the cloud shadows are not good.(2)The fractional vegetation cover of Qinghai Lake Watershed shows a gradual increase trend from 2001 to 2014,but with a slight decline in 2016.During 2001-2016,the vegetation in Qinghai Lake Watershed changes heavily,t he average CV value of vegetation coverage in this area is 0.2441,which means it is an unstable area.By analyzing the type shift of fractional vegetation cover in the Qinghai Lake Watershed from 2001 to 2016,conclusions below can be obtained: The vegetation coverage evolution type is mainly from middle and low vegetation coverage to high vegetation coverage,and the positive transfer of all kinds of vegetation coverage are higher than negative transfer;the bare land and sand land slowly decreases slighty,and the area of high fractional vegetation cover increases by 10.15%,the quality of vegetation coverage in the Qinghai Lake Watershed has improved.(3)The influence of elevation factors on fractional vegetation cover is significant,and the fractional vegetation cover increases first and then decreases with the increase of elevation;the temperature and the precipitation also have great impact on vegetation coverage-areas with significant linear correlation between the two factors and fractional vegetation cover accounts for more than 60%,and the distribution of these areas is related to the altitude.It is speculated that the variation of fractional vegetation cover in Qinghai Lake Watershed is the result of the interaction of various influencing factors.