A measure of the amount of water used by plants per unit of plant material produced. The term can be applied at the leaf, whole-plant, and ecosystem levels. At the leaf level, it is more precisely referred to as the instantaneous transpiration efficiency, the CO2 assimilation rate (photosynthesis) divided by the transpiration rate (the moles of CO2 taken up divided by the moles of water lost through transpiration in a unit of time per unit leaf area). At the whole-plant level, it is more precisely referred to as the growth water-use efficiency, the units of dry matter synthesized divided by the units of water lost. At the ecosystem level, it is more precisely referred to as the crop water-use efficiency, the grams of dry weight gained by plants during the growing season per unit land area divided by the millimeters of water lost (including evaporation directly from the soil).