Temporal and spatial variability of water use efficiency of vegetation and its response to precipitation and temperature in Heihe River Basin

Water use efficiency（WUE） is an important index to measure the coupling relationship between water and carbon in vegetation ecosystem. Research on the WUE spatial and temporal variability is of great significance to rational utilization and allocation of regional water resources. In this study, the net primary productivity（NPP） was estimated by improved Carnegie-Ames-Stanford Approach（CASA） model. Combined with the evapotranspiration data（ET） estimated by the ETWatch model, we studied the spatial pattern and temporal variations of the NPP, ET and WUE in Heihe River Basin from 2000 to 2013, and discussed the response of WUE to precipitation and air temperature. The results showed that: 1) the annual average value of NPP was 81.05 gC m-2 a-1 in Heihe River Basin in 2000-2013. Meanwhile, the annual ET averaged 133.38 mm and WUE was 0.448 gC mm-1 m-2. The spatial pattern of the NPP, ET and WUE were gradually decreasing from upstream and middle to downstream. 2) The average annual NPP and WUE showed an obvious upward trend from 2000 to 2013, while the ET showed a slightly upward trend. There is a corresponding relationship between slope of WUE of vegetation and its average value in spatial distribution. The high-value area of WUE is also the main area for its growth in spatial distribution. By contrast, the inter-annual variation of WUE tends to be stable in the area with lower average WUE value. 3) There were significant differences in WUE among different vegetation types. The main reason for the difference in WUE was the physiological and ecological parameters of vegetation which formed by the influence of the environment. The annual average WUE was listed in order of shrub > grassland > forest > farmland > swamp > desert. The average WUE of cultivated vegetation in the oasis area in the middle reaches is only 0.90 gC mm-1 m-2. We should to pay more attention to improving its utilization efficiency of water resources. 4) The inter-annual change of WUE is mainly affected by precipitation, but there showed a negative correlation between WUE change and precipitation in the middle reaches of oasis area, which indicating that human activity will weaken the influence of climatic factors on WUE of vegetation.