Predicting water use efficiency of crops

Water use efficiency (WUE) is a critical factor in the determination of adaptation and productivity of crops in water-limited areas, either under the present climate or future global change. Data on WUE are often highly variable and a unifying and quantitative approach is needed to analyze and predict crop WUE for different environments. This study tests the validity and applicability of a set of paradigm equations based on leaf gas exchange, proposed by Hsiao (1993) for this purpose. Cotton and other crops were grown in growth chamber under three levels of CO2 and their WUE for biomass production measured, along with leaf temperature and climatic factors. The equation predicted accurately the differences in WUE of cotton under different CO2 levels, and worked also well for other crops. The equations were also tested on crop in the open field, with WUE expressed as the ratio of CO2 assimilation to evapotranspiration. The prediction of the hour-to-hour changes in canopy WUE of cotton over daily cycles was quite good, provided that stomatal response to humidity was accounted for. The prediction for sweet corn was good for most parts of the day except early in the morning. Possible causes of the differences between predicted and measured WUE are discussed.