MODELING ELEVATED CARBON DIOXIDE EFFECTS ON WATER RELATIONS, WATER USE, AND GROWTH OF IRRIGATED SORGHUM

Authors: GRANT, ROBERT - UNIV OF ALBERTA CANADA; Kimball, Bruce; Wall, Gerard - Gary; TRIGGS, JONATHAN - ARIZONA STATE UNIV; BROOKS, TALBOT - ARIZONA STATE UNIV; Pinter Jr, Paul; Conley, Matthew; OTTMAN, MICHAEL - UNIV OF ARIZONA; LAMORTE, ROBERT - USDA-ARS-USWCL PHOENIX; LEAVITT, STEVE - UNIV OF ARIZONA; THOMPSON, THOMAS - UNIV OF ARIZONA; MATTHIAS, ALLAN - UNIV OF ARIZONA

Submitted to: Agronomy Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/1/2004
Publication Date: 12/1/2004
Citation: Grant, R.F., Kimball, B.A., Wall, G.W., Triggs, J.M., Brooks, T.J., Pinter Jr, P.J., Conley, M.M., Ottman, M.J., Lamorte, R.L., Leavitt, S.W., Thompson, T.L., Matthias, A.D. 2004. Modeling elevated carbon dioxide effects on water relations, water use, and growth of irrigated sorghum. Agronomy Journal 96:1693-1705.

Interpretive Summary: The CO2 concentration in the atmosphere is increasing and is expected to double near the end of this century. The elevated levels of CO2 affect plant photosynthesis, and they also cause a partial closure of the stomata in plant leaves through which the plants exchange CO2 and water vapor with the atmosphere. The magnitudes of both affect the extent to which they change growth, yield, and water requirements of crops. In order to predict what effects the elevated CO2 will have on future crop production and to aid in developing improved management strategies, crop growth simulation models are being developed. This paper reports a successful test of one such model called ecosys, comparing model predictions against the results of an experiment where open-field-grown sorghum was exposed to elevated levels of CO2 using free-air CO2-enrichment (FACE) technology at ample and limited levels of soil water. Most model results were acceptably close to observed values, predicting for example, that CO2 concentrations of about 550 ppm, such as expected near the middle of the this century, should cause sorghum grain yields to change little at ample irrigation, but the amount of water required will decrease about 17%. On the other hand, when water supply is limited, sorghum yields could increase about 13%. This research benefits both future growers and consumers of agricultural crops.

Technical Abstract: Elevated concentrations of atmospheric CO2 (Ca) are believed to raise sorghum productivity primarily by improving sorghum water status. Model predictions of sorghum water requirements under hypothesized changes in climate must therefore be based on a robust and accurate simulation of sorghum water relations as affected by Ca. In ecosys, water relations are simuated by solving for the canopy water potential at which water uptake from a hydraulic model of soil-root- canopy water transfer equilibrates with transpiration from a first order closure of the canopy energy balance. These simulated water relations were tested with canopy water potential, water uptake and energy exchange measured under ambient (363 umol/mol) and elevated (566 umol/mol) Ca and high vs. low irrigation in a Free Air CO2 Enrichment (FACE) experiment during 1998 and 1999. Model results, corroborated by field measurements, showed that elevated Ca raised canopy water potential, and lowered water uptake and latent heat fluxes under high irrigation, and delayed the onset of water stress under low irrigation. Changes in sorghum water status under ambient vs. elevated Ca were shown to vary diurnally, with lower canopy water potential causing midafternoon stomatal closure to start earlier under ambient Ca. Ecosys was used to generated an irrigation response function for sorghum yield which indicated that yields would rise by around 13% for a range of irrigation rates if air temperatures were to rise by 3 degrees C and Ca by 50%. Current high sorghum yields could be achieved with approximately 120 mm or around 20% less irrigation water if these rises in temperature and Ca were to occur.