Abstract
Abstract. Terrestrial vegetation dynamics are closely influenced by both climate and by both climate and by land use and/or land cover change (LULCC) caused by human activities. 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 variability and to human activity in Ejina Region, a hyper-arid landlocked 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 throughout much of Central Asia. Variations of fractional vegetation cover from 2000 to 2012 were determined using Moderate Resolution Imaging Spectroradiometer (MODIS) vegetation 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 growing season vegetation cover has increased from 3.4% in 2000 to 4.5% in 2012. The largest contribution to the overall 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 analysis with different precipitation data sources found that the greening of the desert was associated with increases in regional 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 irrigation 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 inform management on the implications for water allocation between oases in the middle and lower reaches and for water management in the Ejina oasis.
Highlights
Terrestrial vegetation plays a key role in energy, water, and biogeochemical cycles, and changes in vegetation can significantly influence atmospheric processes (Pielke et al, 1998; Gerten et al, 2004)
We investigate satellite-observed (MODIS) vegetation trends (NDVI) in a hyper-arid region of the Heihe River basin located in northwest China
Over the whole period we found that the contributions to regional vegetation cover change due to changes in irrigated vegetation cover and the area fraction (A∗I ) were 7.8 and 20.5 %, respectively, while changes in the non-irrigated vegetation cover and area fraction (A∗D) accounted for 75.8 %, and −4.1 %, respectively
Summary
Terrestrial vegetation plays a key role in energy, water, and biogeochemical cycles, and changes in vegetation can significantly influence atmospheric processes (Pielke et al, 1998; Gerten et al, 2004). Y. Wang et al.: Attribution of satellite-observed vegetation trends vegetation dynamics from regional to global scales (Tucker, 1979; Box et al, 1989; Fensholt et al, 2013). Greening trends have been detected on global (Myneni et al, 1997; Nemani et al, 2003; Donohue et al, 2013; Fensholt et al, 2013) and regional (Fang et al, 2004; Herrmann et al, 2005; Donohue et al, 2009) scales, but attribution of those trends in terms of the underlying biophysical and socioeconomic causes remains a difficult task. One approach to handle this complexity is to use regional knowledge to constrain the problem
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