COTTON EVAPOTRANSPIRATION UNDER FIELD CONDITIONS WITH CO2 ENRICHMENT AND VARIABLE SOIL-MOISTURE REGIMES

The CO2 concentration of the atmosphere is predicted to double by the next century, and this is expected to increase significantly the growth and yield of many important agricultural crops. One consequence of larger and more vigorous plants may be increased crop evapotranspiration (ET) and irrigation water requirements. The objective of this work was to determine ET of cotton (Gossypium hirsutum L. cv. 'Deltapine 77') grown under ambient (about 370 mumol mol-1) and enriched (550 mumol mol-1) CO2 concentrations for both well-watered and water-stress irrigation managements. Studies were conducted in 1990 and 1991 within a large, drip-irrigated cotton field in central Arizona. Cotton ET was measured during the growing seasons using a soil water balance, based on neutron gauge soil water measurements. ET, for periods from 7 to 14 days, was not significantly different between ambient and enriched CO2 treatments at the 0.05 probability level, and the total seasonal ET for the CO2 treatments varied by 2% or less in either year. However, water-stress treatments, which were initiated on 3 July (day of year (DOY) 184) in 1990 and on 20 May (DOY 128) in 1991, had significantly lower (P < 0.05) ET than well-watered treatments starting at the end of July in 1990 and in early July in 1991 when the plants were about 75-90 days old. The result that CO2 enrichment to 550 mumol mol-1 did not significantly change the ET of cotton was consistent with the results of co-investigators who measured ET in the same experiments using stem flow gauges and an energy balance. This result implies that irrigation water use would not have to be increased to produce cotton in a future high-CO2 world. However, if a concomitant change in climate occurs, such as global warming, cotton evapotranspiration may change in response to the changed weather condition.