THE EFFECT OF DOUBLED CO2 ON WATER-USE BY ALFALFA AND ORCHARD GRASS -SIMULATING EVAPOTRANSPIRATION USING CANOPY CONDUCTANCE MEASUREMENTS

Alfalfa and orchard grass crops were grown at ambient and twice ambien t carbon dioxide concentrations in field plots for several years in Be ltsville, MD, using semiopen chambers. Canopy conductances throughout many days were determined from water vapour exchange measurements, and indicated significant reductions in canopy conductance to water vapou r at elevated carbon dioxide in both species. However, recognizing tha t the artificial ventilation in the chambers made direct comparisons o f evapotranspiration rates questionable, we used a soil-vegetation-atm osphere model to determine what field-scale evapotranspiration rates w ould have been with natural ventilation. Unlike the 'omega' approach, the model used allowed feedbacks between the canopy and the atmosphere , such that, for example, canopy conductance responses affected profil es of temperature and water vapour. Simulations indicated that althoug h canopy conductances were lower at elevated carbon dioxide by as much as 20% in alfalfa and 60% in orchard grass, evapotranspiration rates never differed by more than 3% in alfalfa or 8% in orchard grass. Dail y totals of evapotranspiration were only 1-2% lower at elevated carbon dioxide in alfalfa, and 2-5% lower in orchard grass. The results are partly explained by the fact that aerodynamic conductances to water va pour were generally smaller than the stomatal conductance, and also by canopy-atmosphere feedback processes which largely compensated for th e lower conductance at elevated carbon dioxide by increasing the gradi ent for evaporation.