Water use efficiency of net primary production in global terrestrial ecosystems

The carbon and water cycles of terrestrial ecosystems, which are strongly coupled via water use efficiency (WUE), are influenced by global climate change. To explore the relationship between the carbon and water cycles and predict the effect of climate change on terrestrial ecosystems, it is necessary to study the WUE in global terrestrial ecosystems. In this study, the 13-year WUE (i.e., net primary production (NPP)/evapotranspiration (ET)) of global terrestrial ecosystems was calculated based on the Moderate Resolution Imaging Spectro-radiometer (MODIS) NPP (MOD17A3) and ET (MOD16A3) products from 2000 to 2012. The results indicate that the annual average WUE decreased but not significantly, and the 13-year mean value was 868.88 mg C m (-2) mm (-1). The variation trend of WUE value for each pixel differed greatly across the terrestrial ecosystems. A significant variation (P < 0.05) occurred in about 18.50% of the land surface. WUE was spatially distributed from 0 to 2541 mg C m (-2) mm (-1), and 58.78% of the WUE values were concentrated in the interval of 600-1200 mg C m (-2) mm (-1). The WUE increased from north to south in Africa and Oceania and from east to west in Europe and South America. Both latitudinal and longitudinal gradients existed in Asia and North America. The following trends in the WUE of different continents and Koppen-Geiger climates were observed: Europe (1129.71 mg C m (-2) mm (-1))> Oceania (1084.46 mg C m (-2) mm (-1))> Africa (893.51 mg C m (-2) mm (-1))> South America (893.07 mg C m (-2) mm (-1))> North America (870.79 mg C m (-2) mm (-1))> Asia (738.98 mg C m (-2) mm (-1)) and warm temperate climates (1094 mg C m (-2) mm (-1))> snowy climates (862 mg C m (-2) mm (-1))> arid climates (785 mg C m (-2) mm (-1))> equatorial climates (732 mg C m (-2) mm (-1))> polar climates (435 mg C m (-2) mm (-1)). Based on the WUE value and the present or future rainfall, the maximum carbon that fixed in one region may be theoretically calculated. Also, under the background of global climatic change, WUE may be regarded as an important reference for allotting CO (2) emissions offsets and carbon transactions.