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item Kimball, Bruce
item Hunsaker, Douglas - Doug
item Pinter Jr, Paul
item GARCIA, R.
item Wall, Gerard - Gary
item La Morte, Robert

Submitted to: Agricultural and Forest Meteorology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/15/1998
Publication Date: N/A
Citation: 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 especially include the 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 that 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 DEMETER was developed, which is capable of predicting the growth of a wheat crop and its 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 concentrations o 550 ppm and present-day ambient of about 370 ppm. The results showed that the model could track the temperature, evapotranspiration, and water use efficiency of the crop hour-by-hour reasonably well, and consistent with the data, future wheat water requirements may decrease slightly, perhaps 4%, if climate warming is minimal. This work should help future growers develop optimum management strategies, and, of course, should ultimately benefit all future food consumers.

Technical Abstract: Seasonal evapotranspiration (ET) and the diurnal courses of canopy temperature and latent heat flux of a spring wheat crop were simulated for atmospheric CO2 concentrations of 370 ppm and 550 ppm. The hourly weather data, soil parameters and the irrigation and fertilizer treatments of the Free-Air Carbon Dioxide Enrichment wheat experiment in Arizona (1992/93) were used to drive the model. The simulation results were tested against field measurements with special emphasis on the period between anthesis and maturity. A model integrating leaf photosynthesis and stomatal conductance was scaled to a canopy level in order to be used in the wheat growth model. The simulated intercellular CO2 concentration (Ci) is determined from the ratio of Ci to the CO2 concentration at the leaf surface (Cs), the leaf to air specific humidity deficit and a possibly unfulfilled transpiration demand. After anthesis, the measured assimilation rates of the flag leaves sdecreased more rapidly than their stomatal conductances, leading to a rise in the Ci/Cs ratio. In order to describe this observation, an empirical model approach was developed, which takes into account the leaf nitrogen content for the calculation of the Ci/Cs ratio. Simulation results obtained with the new model version were in good agreement with the measurements. If changes in the Ci/Cs ratio accorded to the decrease in leaf nitrogen content during leaf senescence were not considered in the model, the simulation revealed an underestimation in the daily ET of up to 20%; canopy temperatures were overestimated by up to two Kelvins; and the simulated seasonal ET decreased by 10%. The measured reduction in seasonal ET owing to CO2 enrichment, in comparison, was only 5%.