Spatiotemporal patterns of evapotranspiration, gross primary productivity, and water use efficiency of cropland in agroecosystems and their relation to the water-saving project in the Shiyang River Basin of Northwestern China

Abstract Water use efficiency over crop area (WUEc), defined as the ratio of carbon gain (i.e., gross primary productivity over crop area; GPPc) to water consumption (i.e., evapotranspiration over crop area; ETc) at the agroecosystems scale, is a critical variable linking the carbon and water cycles. In this study, the spatiotemporal patterns and trend characteristics of ETc, GPPc, and WUEc in agroecosystems over the period of 2000–2014 across the Shiyang River Basin in northwestern China were assessed with Moderate Resolution Imaging Spectroradiometer (MODIS) global products, and the impact of a water-saving project on WUEc is discussed. Results indicate that the spatiotemporal variability of ETc, GPPc, and WUEc were very common and similarly related to climatic variables and water allocation laws. It suggests that variations in climatic factors and the water-saving project controlled photosynthesis and ETc simultaneously; they changed the hydrological cycle by adjusting ETc. Based on a 15-year assessment, 97.25% of the basin showed a positive trend for WUEc. After the water-saving project launched in 2006, ETc was reduced by 23 mm yr−1 (7.3%), while WUEc increased by 0.11 g C kg−1H2O (7.2%). The WUEc was significantly correlated with precipitation, and it was mainly determined by ETc rather than GPPc. The limitation of this research is the fact that the driving factors are not quantitatively characterized, especially at the basin scale. However, this research will not only aid in predictions about the influence of climate change and human activity on carbon and water fluxes but also provide scientific guidance to regulate water resources allocation, and this guidance can be applied to other arid regions around the world.