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 growt h and yield of many important agricultural crops. One consequence of l arger and more vigorous plants may be increased crop evapotranspiratio n (ET) and irrigation water requirements. The objective of this work w as 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 irrig ation managements. Studies were conducted in 1990 and 1991 within a la rge, drip-irrigated cotton field in central Arizona. Cotton ET was mea sured during the growing seasons using a soil water balance, based on neutron gauge soil water measurements. ET, for periods from 7 to 14 da ys, was not significantly different between ambient and enriched CO2 t reatments at the 0.05 probability level, and the total seasonal ET for the CO2 treatments varied by 2% or less in either year. However, wate r-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 Jul y in 1990 and in early July in 1991 when the plants were about 75-90 d ays old. The result that CO2 enrichment to 550 mumol mol-1 did not sig nificantly change the ET of cotton was consistent with the results of co-investigators who measured ET in the same experiments using stem fl ow gauges and an energy balance. This result implies that irrigation w ater use would not have to be increased to produce cotton in a future high-CO2 world. However, if a concomitant change in climate occurs, su ch as global warming, cotton evapotranspiration may change in response to the changed weather condition.