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

Abstract

Elevated concentrations of atmospheric CO 2 ( 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 (ψ 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 ψ 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 CO 2 enrichment experiment during 1998 and 1999. Model results, corroborated by field measurements, showed that elevated C a raised ψ C and lowered latent heat fluxes under high irrigation and delayed water stress under low irrigation. Changes in ψ C modeled under ambient vs. elevated C a varied diurnally, with lower ψ 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.