Evaluating simple transpiration-based models of crop productivity

There is a renewed interest in evaluating crop productivity using simple transpiration-based models of biomass accumulation. Transpiration-use efficiency (w), defined as the ratio of biomass (B) produced per unit water transpired (T), has been widely used to evaluate crop performance under limited water supply. Simple approaches to assess w have been used including: (1) w = kDa/Da and (2) w = kETo/ETo; where kDa and kETo are crop-dependent parameters. The concept is that normalization by Da or ETo would account for the effects of climate variations on w, while k Da or kETo would be reasonably constant across diverse environments. However, the evaluation of the transferability of these parameters is not simple due to the scarcity of experimental values and the lack of consistency of the methodology used in the available experiments. For this reason we have developed and tested a canopy transpiration and photosynthesis model (CTP) to obtain simulated values of w, kDa and kETo in different locations with a consistent methodology. Model simulations were compared with evapotranspiration estimated with weighing lysimeters for non stressed wheat and maize. Results showed good agreement between observed and simulated transpiration values for both crops, with the simulated values tracking well the daily fluctuations of the observed values. Daily values of simulated transpiration-use efficiency (w) were compared with observed values from literature and showed that the average and standard deviation of the simulated values were within the range of the observed data. The model was then used to evaluate the transferability of kDa and kETo values for wheat and maize across eight world locations with contrasting climate. The results indicated that kDa and kETo (maize) are not constant parameters; suggesting that calibration in contrasting climates would be desirable. However, a consistent trend of change of the values of these parameters as a function of Da or ET o was found, which can be represented by mathematical functions, allowing the possibility of transferring kDa and kETo values across climatic conditions. Verification of these equations with field data was performed. The simulation-based equations to w and kDa of wheat and maize, and kETo of maize appeared to be robust estimators of observed values, while kETo of wheat was better represented by a single value across climatic conditions.