Water use efficiency is defined here as yield of plant product (tonnes of wheat grain, Y) per unit of crop water use (megalitres of water lost by evapotranspiration, ET), and is important in all areas of plant production. Water use efficiency (Y/ET) is the outcome of an entire suite of plant and environmental processes operating over the life of a crop to determine both Y and ET. Consequently, biomass production per unit ET, has been used extensively as an interim measure of water use efficiency. Link to WWW