Submitted to: Decennial National Irrigation Symposium
Publication Type: Proceedings
Publication Acceptance Date: November 14, 2000
Publication Date: N/A
Interpretive Summary: The amount of water available to agricultural producers that can be used for irrigating crops is declining. One way to conserve the water is to insure that the amount of irrigation water applied does not exceed the water needs of the crop. We evaluated two methods for predicting the water needs of a grain sorghum crop by comparing the predicted amounts with measured amounts for both fully irrigated and limited irrigated crops grow in three different soils. Both methods used a predicted reference water use calculated from daily weather variables such as solar radiation and temperature. The reference water use was then adjusted specifically for water use by grain sorghum based on its stage of growth. One method, however, used further adjustments based on the soil's water holding and water evaporation characteristics which varied from soil to soil. The difference between measured and predicted crop water use averaged 9 percent tfor the simpler method and 4 percent for the more complicated method. If the information is available to use the more complicated method, that method would improve crop water use prediction and help conserve our declining water supplies.
Technical Abstract: Water conservation in irrigated agriculture of the semi-arid Great Plains relies on accurate prediction of crop water use to obtain the greatest benefit from declining irrigation water supplies. One method for estimating crop water use applies crop specific coefficients to adjust reference evapotranspiration (ETo). We compared daily measured evapotranspiration (ETm) of limited and fully irrigated grain sorghum to modeled ET (ETc) calculated using single and dual crop coefficients (Kc) and a grass-based ETo. The dual Kc procedure contained separate coefficients for crop transpiration, soil water evaporation, and crop water stress, as compared with one coefficient in the single Kc procedure. Short season grain sorghum was grown in lysimeters on deck scales containing monolithic soil cores of either Pullman, Ulysses, or Amarillo soil located in a rain shelter facility. With the dual Kc procedure, the difference during the season between cumulative ETc and ETm varied from 2 to around 7 mm, and by the end of the season the maximum difference in all treatments was about 70 mm, or 15%, with an average of 4%. The single Kc procedure underestimated final cumulative ETm in the fully irrigated treatments by an average of 9%. The dual Kc procedure was sensitive to errors in required initial specifications for soil water balance and crop response to water stress in the limited irrigation treatments, but overall it improved water use predictions compared with the single Kc procedure.