Irrigation water supplies are decreasing in many areas of the US Great Plains, which is requiring many farmers to consider deficit-irrigating corn ( Zea mays L.) or growing crops like winter wheat ( Triticum aestivum L.) that require less water, but that are less profitable. The objectives of this study were to: (1) quantify the yield response of corn to deficit irrigation, and (2) determine which of several seasonal water variables correlated best to corn yield in a semiarid climate. Eight (T1-T8) and nine (T1-T9) deficit-irrigated treatments (including dryland), were compared in 2003 and 2004 in North Platte, Nebraska. The actual seasonal crop evapotranspiration (ET d) (calculated with procedures in FAO-56) for the different treatments was 37-79% in 2003 and 63-91% in 2004 compared with the seasonal crop evapotranspiration when water is not limited (ET w). Quantitative relationships between grain yield and several seasonal water variables were developed. Water variables included, irrigation ( I), total water ( Wall), rain+irrigation ( WR+I ), evaporation ( E), crop evapotranspiration (ET d), crop transpiration ( Td), and the ratios of ET d and Td to evapotranspiration and transpiration when water is not limited (ET w and Tw). Both years, yield increased linearly with seasonal irrigation, but the relationship varied from year to year. Combining data from both years, ET d had the best correlation to grain yield (yield=0.028ET d-5.04, R2=0.95), and the water variables could be ranked from higher to lower R2 when related to grain yield as: ET d ( R2=0.95) > Td ( R2=0.93) > ET d/ET w ( R2=0.90) = Td/ Tw ( R2=0.90) > Wall ( R2=0.89) > E ( R2=0.75) > WR+I ( R2=0.65) > I ( R2=0.06). Crop water productivity (CWP) (yield per unit ET d) linearly increased with ET d/ET w ( R2=0.75), which suggests that trying to increase CWP by deficit-irrigating corn is not a good strategy under the conditions of this study.
