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Agronomic Modeling

Modeling Elevated Carbon Dioxide Effects on Water Relations, Water Use, and Growth of Irrigated Sorghum

^a Dep. of Renewable Resour., Univ. of Alberta, Edmonton, AB, Canada T6G 2H1
^b USDA-ARS, U.S. Water Conserv. Lab., 4331 E. Broadway, Phoenix, AZ 85040
^c Lab. of Tree-Ring Res., Tucson, AZ 85721
^d Dep. of Soil, Water, and Environ. Sci., Univ. of Arizona, Tucson, AZ 85721
^e Maricopa Agric. Cent., Univ. of Arizona, Maricopa, AZ 85239
^f Dep. of Geogr., Arizona State Univ., Tempe, AZ 85287

^* Corresponding author (robert.grant{at}ualberta.ca)

Received for publication November 28, 2004. Elevated concentrations of atmospheric CO₂ (C_a) are believed to raise sorghum [Sorghum bicolor (L.) Moench] productivity by improving water relations. In ecosys, water relations are simulated by solving for the canopy water potential (_C) at which water uptake from a model of soil–root–canopy water transfer equilibrates with transpiration from the canopy energy balance. Simulated water relations were tested with _C, water uptake, and energy exchange measured under ambient (363 µmol mol^–1) and elevated (566 µmol mol^–1) C_a and high vs. low irrigation in a free air CO₂ enrichment experiment during 1998 and 1999. Model results, corroborated by field measurements, showed that elevated C_a raised _C and lowered latent heat fluxes under high irrigation and delayed water stress under low irrigation. Changes in _C modeled under ambient vs. elevated C_a varied diurnally, with lower _C causing earlier midafternoon stomatal closure under ambient C_a. Modeled changes in sorghum water status caused elevated C_a to raise seasonal water efficiency under high and low irrigation by 20 and 26% (vs. 20 and 13% measured) in 1998 and by 9 and 27% (vs. 6 and 26% measured) in 1999. Ecosys was used to generate an irrigation response function for sorghum yield, which indicated that yields would rise by 13% for a range of irrigation rates if air temperatures were to rise by 3°C and C_a by 50%. Current high sorghum yields could be achieved with 120 mm or 20% less irrigation water if these rises in temperature and C_a were to occur.

Abbreviations: C_a, atmospheric CO₂ • C_i, intercellular CO₂ concentration • DOY, day of year • FACE, free air CO₂ enrichment (experiment) • g_C, stomatal conductance • GCM, general circulation model • g_L, leaf conductance • H, sensible heat • IRTs, infrared thermometers • LE, latent heat • Rn, net radiation • T_C, canopy temperature • VPD, vapor pressure deficit • WUE, water use efficiency • _C, canopy water potential • _S, soil water potential • _T, canopy turgor potential • , osmotic potential • _R, root–canopy hydraulic resistances • _S, soil–root hydraulic resistances

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