Submitted to: Agricultural and Forest Meteorology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: January 25, 1999
Publication Date: N/A
Interpretive Summary: In order to predict the consequences of present and future global environmental changes on the security of world food production and on future irrigation requirements, efforts are underway to develop the capability to predict the growth, yield, and water use of major food crops. These global changes entail an increasing concentration of atmospheric carbon dioxide (CO2) which is expected to double sometime during the next century. Climate modelers have predicted that the elevated CO2 will cause the earth to warm and the precipitation patterns will change. Elevated CO2 is also known to alter the growth of plants and may affect their water requirements. Accordingly, a computer model called ecosys was developed, which is capable of predicting the growth of a wheat crop hour-by-hour, including the hourly consumption of water. This paper describes a specific validation test of the model comparing its predictions with actual data from a free-air-CO2 enrichment experiment on wheat at CO2 concentration's of 550 ppm and present-day ambient of about 370 ppm. The results showed that the model could track physiological responses of the crop to CO2 such as gas exchange (CO2, H20), and internal water content reasonably well and, consistent with the data, that future wheat water requirements may decrease slightly, perhaps 7%. This work should help future growers develop optimum management strategies and, of course, should ultimately benefit all future food consumers.
Increases in crop growth under elevated atmospheric CO2 concentration (CA) have frequently been observed to be greater under water-limited vs. non-limited conditions. Crop simulation models used in climate change studies should be capable of reproducing such changes in growth response to CA with changes in environmental conditions. We propose that changes with soil water status in crop growth response to CA can be simulated if stomatal resistance is considered to vary directly with air-leaf gradient, inversely with leaf carboxylation rate, and exponentially with leaf turgor. Resistance simulated in this way increases with CA relatively less, and CO2 fixation increases with CA relatively more, under water-limited vs. non-limited conditions. As part of the ecosystem model "ecosys," this simulation technique caused changes in leaf conductance and CO2 fixation, and in canopy water potential, temperature and energy balance in a modeling experiment, that were consistent with changes measured under 355 vs. 550 umol mol-1 CA and low vs. high irrigation in a Free Air CO2 Enrichment (FACE) experiment on wheat. Changes with CA in simulated crop water relations allowed the model to reproduce under 550 umol mol-1 CA and low vs. high irrigation, a measured increase of 20 percent vs. 10 percent in seasonal wheat biomass, and a measured decrease of 5 percent vs. 2 percent in seasonal evapotranspiration. The basic nature of the processes simulated in this model is intended to permit its use under a wide range of soil management and climate conditions.