A canopy transpiration and photosynthesis model for evaluating simple crop productivity models

There has been a renewed interest in evaluating the performance of simple models based on water-use efficiency (w), defined as the ratio of biomass produced per unit water transpired, or radiation-use efficiency (e), defined as biomass produced per unit of solar radiation intercepted. Water-use efficiency is typically estimated as w = K/D _a, where K is a parameter fitted empirically and D _a is the daytime vapor pressure deficit. The expectation is that e and K are conservative parameters that can be used across diverse climatic conditions. Experimental determination of K has been constrained by the need to measure crop transpiration, while the lack of consistency of the methodologies used in reported field experiments has limited the assessment of the transferability of bothe and K values. A two-leaf hourly-time-step canopy transpiration and photosynthesis model (CTP) was formulated which after evaluation could be used to assess the transferability of these parameters across climatic conditions while eliminating experimental and biological variability. Model simulations of transpiration, tested on the basis of lysimetric data for wheat (Triticum aestivum L.) and maize (Zea mays L.), agreed well with the magnitude and time evolution of the data. Simulated values of w for wheat and maize in eight world locations with contrasting climatic conditions, plotted as a function of air vapor pressure deficit, were compared with experimental values from several other locations. Despite differences in cultivars, crop management, methods to estimate transpiration, sampling methods for biomass, and other sources of variability and experimental error in the available data, the agreement was adequate. These evaluations provide support to the use of the CTP model as a tool to assess the applicability of simple models of biomass production across climatic conditions.