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Attribution of satellite-observed vegetation trends in a hyper-arid region of the Heihe River basin, Western China

Yanfang Wang,M. Roderick,M. Roderick,Yanjun Shen,Fubao Sun,Fubao Sun

Published 2014 in Hydrology and Earth System Sciences

ABSTRACT

Terrestrial vegetation dynamics are closely influ- enced by both climate and by both climate and by land use and/or land cover change (LULCC) caused by human activ- ities. Both can change over time in a monotonic way and it can be difficult to separate the effects of climate change from LULCC on vegetation. Here we attempt to attribute trends in the fractional green vegetation cover to climate variabil- ity and to human activity in Ejina Region, a hyper-arid land- locked region in northwest China. This region is dominated by extensive deserts with relatively small areas of irrigation located along the major water courses as is typical through- out much of Central Asia. Variations of fractional vegeta- tion cover from 2000 to 2012 were determined using Mod- erate Resolution Imaging Spectroradiometer (MODIS) vege- tation index data with 250 m spatial resolution over 16-day intervals. We found that the fractional vegetation cover in this hyper-arid region is very low but that the mean grow- ing season vegetation cover has increased from 3.4 % in 2000 to 4.5 % in 2012. The largest contribution to the over- all greening was due to changes in green vegetation cover of the extensive desert areas with a smaller contribution due to changes in the area of irrigated land. Comprehensive anal- ysis with different precipitation data sources found that the greening of the desert was associated with increases in re- gional precipitation. We further report that the area of land irrigated each year can be predicted using the runoff gauged 1 year earlier. Taken together, water availability both from precipitation in the desert and runoff inflow for the irriga- tion agricultural lands can explain at least 52 % of the total variance in regional vegetation cover from 2000 to 2010. The results demonstrate that it is possible to separate the satellite- observed changes in green vegetation cover into components due to climate and human modifications. Such results in- form management on the implications for water allocation between oases in the middle and lower reaches and for water management in the Ejina oasis.

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