CARBON-DIOXIDE ENRICHMENT AND IRRIGATION EFFECTS ON WHEAT EVAPOTRANSPIRATION AND WATER-USE EFFICIENCY

Evapotranspiration (ET) and water use efficiency were evaluated for tw o spring wheat crops, grown in a drip-irrigated field under ambient (a bout 370 mu mol mol(-1)) and enriched (550 mu mol mol(-1)) carbon diox ide (CO2) concentrations during the 1992-1993, and 1993-1994, Free-Air CO Enrichment (FACE) experiments in central Arizona. CO2-enriched (FA CE) and ambient CO2 (CONTROL) treatments were replicated in four circu lar plots, 25 m in diameter, and well-watered (WET) and water-stressed (DRY) irrigation treatments were imposed on one-half of each plot. Wh eat ET, measured over discrete time periods of several days by a soil water balance, was significantly higher for WET than DRY irrigation tr eatments after the first week in March in both years. Differences in E T between CO2 treatments during the season were generally small, altho ugh there was a consistent trend towards decreased ET for the FACE ove r CONTROL under the well-watered irrigation regime. The two-year avera ge reduction in seasonal ET owing to the FACE treatment was about 5% u nder WET irrigation and was consistent with the results from two paral lel investigations that used an energy balance and sap flow measuremen ts. Under the DRY irrigation treatment, seasonal ET was 5 and 0.9% hig her for the FACE treatment in the first and second years, respectively . Water use efficiency (grain yield per unit seasonal ET) was signific antly higher for FACE treatments; 15 and 24% higher than CONTROL under DRY irrigation, and 13 and 18% higher than CONTROL under WET irrigati on. The results indicate that irrigation requirements for fully irriga ted wheat may be slightly lower in the future high-CO2 environment.