Studies on water use efficiency (WUE) have primarily dealt with crops grown under water limited conditions and have usually not considered crop management factors other than irrigation. Improvements in crop management strategies, such as supplemental irrigation, proper fertilizer use, plant population and row-spacing, and new cultivars, attempt to minimize a crop exposure to soil water deficits. Under these circumstances, factors other than water supply can become limiting and production related to evaporation may not be a useful approach. Crop simulation models like CERES and SALUS have proven capability to provide reasonable estimates of each evaporation component as affected by management. Using simulated transpiration to estimate growth through transpiration efficiency, or simulated growth to estimate transpiration has major limitations when management influences yield and water supply is fixed or non-limiting. The microenvironment in sparse canopies strongly influences transpiration and soil evaporation due to large possible variations in sensible heat arising from variations in wetness of the soil surface. Crop growth is related to photosynthesis and plant development rates and is influenced primarily by intercepted photosynthetically active radiation whereas evaporation is influenced by a wider spectrum of energy. Factors related to management such as plant population and nutrient supply influences biomass production but has less influence on evaporation rates. Our analysis of published data and simulation studies indicates the use of simulated transpiration to estimate growth, or growth simulations to estimate transpiration is an unreasonable approach when management influences yield and water supply is fixed or not-limiting.

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