Water-use efficiency (WUE) refers to the ratio of plant biomass to water lost by transpiration, can be defined either at the leaf, at the whole plant or a population/stand/field level:leaf level : photosynthetic water-use efficiency (also called instantaneous water-use efficiency WUEinst), which is defined as the ratio of the rate of net CO2 carbon assimilation (photosynthesis) to the rate of transpiration or stomatal conductance, then called intrinsic water-use efficiency (iWUE or Wi) plant level : water-use efficiency of productivity (also called integrated water-use efficiency or transpiration efficiency,TE), which is typically defined as the ratio of dry biomass produced to the rate of transpiration. field level : based on measurements of CO2 and water fluxes over a field of a crop or a forest, using the eddy covariance technique