Abstract:Water use efficiency (WUE) is a very important index for understanding the relationship between ecosystem carbon and water cycles. Studies on the spatiotemporal variations in WUE at the ecosystem level are essential for effectively predicting the impact of global climate change on ecosystem functions. At present, several algorithms are used for calculating ecosystem WUE. On the one hand, the different algorithms result in differences in the meanings for the different complexity of water processes involved, and on the other hand, they are closely associated with each other for the similar key processes involved. The traditional techniques for estimating ecosystem WUE are based on the measurements of vegetation biomass and soil hydrological parameters, but these approaches have limitation with estimating ecosystem WUE at finer timescales. Some new methods developed in the past years, especially the eddy covariance technique are the landmarks for studying ecosystem WUE at multiple timescales. Similar to the cases in the leaf level, the main factors affecting ecosystem WUE are vapour pressure deficit (VPD), soil moisture, air CO2 concentration, etc. Besides, the water balance characteristics in ecosystem also play an important role in controlling ecosystem WUE. There are distinct differences in the daily and seasonal dynamics of ecosystem WUE between forest and grassland ecosystems. Meanwhile, the forest and crop ecosystems generally have higher WUE than grassland, desert and tundra ecosystems. Overall, current studies on ecosystem WUE are just at the early stage, and it is fairly needed to make further investigations into the variations in WUE at different spatiotemporal scales and their underlying mechanisms.