Use of a Coupled Land Surface General Circulation Model to Examine the Impacts of Doubled Stomatal Resistance on the Water Resources of the American Southwest

Tiny openings on the surfaces of leaves, stomata, control the flux of CO2, water vapor, and other gases between the atmosphere and the earth’s vegetated surface. An increase in atmospheric CO2 could have an effect on stomatal openings, causing indirect changes in many surface hydroclimatogical variables that could be comparable in magnitude to the direct radiative effects. Increased atmospheric CO2 is expected to increase water use efficiency in many plant types because of the closure of the stomatal openings on the leaf surface. The present study assesses this stomatal effect by doubling the stomatal resistance in two land surface schemes, the Biosphere– Atmosphere Transfer Scheme and the Land Surface Model, which are coupled to the National Center for Atmospheric Research’s Community Climate Model version 3 atmospheric general circulation model, and by evaluating the resulting hydrometeorological responses, particularly for the western United States. Because the simulated reduction of stomatal openings restricts evapotranspiration, latent heat fluxes are reduced, causing global average annual and seasonal decreases in precipitation as well as increases in sensible heat flux, surface temperatures, runoff, and root-zone soil water. Global seasonal decreases in latent heat flux of up to 7% occur, corresponding to surface temperature increases of up to 0.58C and precipitation decreases of up to 3%. Regional responses vary. A focus of this study was to examine how these changes affect runoff and stream flow in the southwestern United States. Contrary to a previous empirical study of this effect, which showed an 87% mean increase in Arizona basin stream flow, this coupled land surface–atmospheric model shows no significant changes in any of the variables examined for this region.