1Transpiration-use efficiency (w), defined as the ratio of biomass produced per unit water transpired, has been used to evaluate crop performance under limited water supply. However, the lack of consistency of w values through different environmental conditions has not allowed, using it as a transferable parameter. Thus, simple approaches have been developed, including: 1) w = kDa Da -1 and; 2) w = kEto ET0 -1 where kDa and kETo are crop-dependent parameters, with the underlying concept that normalization by Da or ET0 would accounts for the effects of climate variations on w, while these parameters would be reasonably constant across diverse environments. The objective of this study was to assess the transferability of kDa and kETo for wheat (Triticum aestivum L.) and maize (Zea mays L.). The scarcity of experimental information and discrepancy of the methodology used, justified the use of a canopy transpiration and photosynthesis model which was developed, tested, and fitted with weather data from eight environmentally different locations to simulate values of w, kDa and kETo. The results indicated that kDa and kETo were more variable than expected; suggesting that calibration would be desirable. A consistent trend of change of the parameter values as function of Da or ET0 was found, which can be represented by mathematical functions, allowing transferring w, kDa and kETo (maize). In contrast, the kETo for wheat correlated weakly with Da and ET0, but a low overall coefficient of variation (10%) allowed using an average value as a reasonable predictor of w.Key words: Transpiration-use efficiency, models of biomass production. griculture is challenged by the scarcity of water resources in many regions of the world, problem that is compounded by climate variability and expected to worsen in the future. There is a raising need for tools to evaluate crop productivity as a function of water to better guide development policies and field management practices aimed at producing "more crop per drop". Mechanistic simulation models of canopy photosynthesis and transpiration appear as suitable tools to evaluate the effect of interacting factors on water-use efficiency and productivity of crops. However, demanding parameterization and computing requirements of these models limit their applicability for long-term analysis that includes multiple species across the globe. As a result, there is a renewed interest in simple, transpiration-based models of crop productivity that can be readily applied to a large number of crop species across the range of climatic conditions where these crops are grown. Although these models were introduced as early as the beginning of the previous century, the experimental determination of the parameters (typically just one parameter) used in the models has been relatively scarce, probably due to the need of measuring crop transpiration for their determination. As a result of scarce experimental information, it is not easy to assess the variability and transferability among locations of the parame...
