Submitted to: Transactions of the ASAE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: March 6, 1996
Publication Date: N/A
Interpretive Summary: Agricultural irrigation water requirements may be changed by the effects of increased atmospheric carbon dioxide (CO2), which is expected to double sometime during the next century. Research has indicated that elevated CO2 levels cause significant growth and yield increases for many agricultural crops. It is not clear how a higher CO2 level will affect water consumption of the crops in the future. This paper describes the crop water use (evapotranspiration) for irrigated wheat grown in a natural, open-air field environment with CO2 enrichment (enriched 50% above present-day levels). Studies spanning two seasons indicated that the seasonal water use requirement for future wheat crops may decrease perhaps 4-5%, if changes in climate warming are slight. The studies also imply that high wheat yields (by present-day standards) should be attainable even with less irrigation due to better crop water use efficiency depending on the "yield enhancement effects" of CO2 enrichment. This work should help irrigated-wheat growers develop future optimum water management strategies and should also provide insight for those who will plan and manage agricultural water supplies during the next century.
Technical Abstract: The atmospheric CO2 concentration is increasing and is predicted to double present-day levels during the next century. One concern for future irrigated agriculture is how higher CO2 levels will affect crop evapotranspiration (ET) and, future irrigation requirements and management. The effects of elevated atmospheric CO2 concentration and irrigation quantity on evapotranspiration and water use efficiency were evaluated for two spring wheat crops using a Free-Air CO2 Enrichment (FACE) facility during 1992-93 and 1993-94 in central Arizona. The experiments consisted of four replicated main plots that were enriched with CO2 to a concentration of 550 micro-mol/mol (FACE) and four matching plots at ambient CO2 (CONTROL; 370 micro-mol/mol). Main plots were split to provide two levels of irrigation (WET, 100% replacement of ET and, DRY, 50% of the WET irrigation quantity). Measurements of soil water content were used to estimate wheat ET with a soil water balance. Total seasonal ET was reduced by an average of 5% for the FACE treatment under the WET irrigation; and was increased 5 and 1% for the FACE treatment under DRY irrigation during the first and second year, respectively. However, water use efficiency (grain yield per unit seasonal ET) was significantly increased for FACE treatments; 15 and 24% under DRY and 13 and 18% under WET irrigation. The results suggest water requirements for fully-irrigated wheat crops may be slightly lower in the future.