The exchange of water for carbon between the vegetation and the atmosphere constitutes the most important feedback mechanism underlying the regulation of water and carbon (C) cycles by the terrestrial biosphere. Water use efficiency (WUE) is the trait that serves to describe this intimate link between the C and water cycles and is a crucial parameter incorporated in many land-surface-models to predict vegetation-climatic feedbacks under future climate change scenario. WUE can be estimated using multiple approaches and methodologies concerning different temporal spatial and temporal scales.