Technical Abstract: 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 (ETd) (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 (ETw). 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 (ETd), crop transpiration (Td), and the ratios of ETd and Td to evapotranspiration and transpiration when water is not limited (ETw and Tw). Both years, yield increased linearly with seasonal irrigation, but the relationship varied from year to year. Combining data from both years, ETd had the best correlation to grain yield (yield = 0.028ETd–5.04, R2 = 0.95), and the water variables could be ranked from higher to lower R2 when related to grain yield as: ETd(R2=0.95) > Td(R2=0.93) > ETd = ETw (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 ETd) linearly increased with ETd/ETw (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.