Water conservation in irrigated agriculture of the semi-arid Great Plains relies on accurate prediction of crop water use to gain the greatest benefit from declining irrigation water supplies. One method for estimating crop water use applies crop specific coefficients to adjust reference evapotranspiration (ET{sub}0). We compared daily measured evapotranspiration (ET{sub}m) of limited and fully irrigated grain sorghum to simulated ET (ET{sub}c) calculated using single and dual crop coefficients (K{sub}c) and a grass-referenced ET{sub}0. We also compared simulated and actual applied irrigation water requirements that were based on full replacement of ET. The dual K{sub}c procedure contained separate coefficients for crop transpiration, soil water evaporation, and water stress, as compared with one coefficient in the single K{sub}c procedure. Short-season grain sorghum was grown in weighing lysimeters containing monolithic soil cores of Pullman, Ulysses, or Amarillo soil located in a rain shelter facility. With the dual K{sub}c procedure, the difference during the season between cumulative ET{sub}c and ET{sub}m varied from 2 mm to around 70 mm, and by the end of the season the maximum difference in all treatments was about 60 mm, or 10. The single K{sub}c procedure underestimated final cumulative ET{sub}m in the fully irrigated treatments by as much as 120 mm. Simulated and actual applied irrigation amounts in the fully irrigated treatments were similar using the dual K{sub}c methodology, but the single K{sub}c methodology under-simulated irrigation needs by more than 100 mm in all treatments. The dual K{sub}c procedure improved water use predictions compared with the single K{sub}c procedure.
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