EVALUATION OF SIMULATION-BASED METHODS FOR ESTIMATING TRANSPIRATION-USE EFFICIENCY OF WHEAT AND MAIZE

In a previous research, (Kremer and Stockle, 2012), developed equations to estimate crop transpiration-use efficiency (w, g CO 2 kg �1 H 2 O), to determine parameters used in simple approaches to estimate (w, g CO 2 kg -1 H2O kPa; , g CO 2 m �2 ground area). These equations assess w, , and , as a function of climatic conditions represented by daytime air vapor pressure deficit (Da) or reference crop evapotranspiration (ETo). To develop the equations, simulations using a mechanistic canopy transpiration and photosynthesis model were performed using weather data from eight world locations with contrasting climate, but these simulations are a first approximation to overcome the spatial transferability of w, , and , however field validation will be required before adoption, for that reason, these equations expressed in terms of CO 2 assimilation per unit ground area were converted to aboveground biomass per unit ground area using a conversion factor (0.36 for wheat, and 0.33 for maize), and evaluated with available field data. Experimental w data in the literature are not only scarce, but they are highly variable due to differences in cultivars, crop management, methods to estimate crop transpiration and biomass, and other sources of variability. Despite these limitations, the simulation-based equations to estimate w and of wheat and maize appeared to be robust estimators of observed values, while , for maize has a good tendency however is needed more field data to be more