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Carbon Dioxide Effects on Crop Energy Balance: Testing ecosys with a Free-Air CO₂ Enrichment (FACE) Experiment

Dep. of Soil Science, Univ. of Alberta, , Edmonton, AB T6G 2E3, Canada

Bruce A. Kimball, Paul J. Pinter, Jr., Gerard W. Wall, Richard L. Garcia, Robert L. La Morte and Douglas J. Hunsaker

U.S. Water Conservation Lab, USDA-ARS, Phoenix, AZ 85040

^* Corresponding author (Email: rgrant@gpu.srv.ualberta.ca).

Elevated CO₂ concentrations (C_e) have been observed to decrease short-term plant water use under controlled conditions by increasing stomatal resistance. The extent to which this decrease occurs over a growing season in the field is uncertain, however, because stomatal resistance is only one of many mechanisms that control water use. In this study, we tested the ecosystem simulation model ecosys, which reproduces an hourly energy balance through soil-vegetation systems under defined atmospheric boundary conditions, using energy exchange data measured as part of the Free-Air CO₂ Enrichment (FACE) experiment at C_e = 550 vs. 370 µmol mol^–1. The model reproduced reductions in measured upward latent heat fluxes that varied from –10 to +40 W m^–2, depending on atmospheric conditions. In the model, the primary effect of elevated C_e on latent heat fluxes was through canopy stomatal conductance. This effect was largely offset by secondary effects through canopy temperature that enabled the model to reproduce measured changes in sensible heat fluxes. The total effect simulated by ecosys of C_e = 550 vs. 370 µmol mol^–1 on evapotranspiration during the entire FACE experiment was a reduction of 7%. This reduction compares with one of 11% estimated from accumulated daily measurements of latent heat flux. In the model, the different effects of C_e on plant water use depend on atmosphere and soil boundary conditions, and are highly dynamic. Consequently the simulated C_e-water use relationship is likely to be sitespecific. The use of models such as ecosys allows site-specific boundary conditions to be considered in the study of C_e effects on plant growth and water use.

Received for publication June 6, 1994.

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