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. With uncertainty in future irrigation water availability and regulations on nutrient application amounts, experimentally determined effects of â€œcontrollableâ€ management strategies such as nitrogen (N), water, and their combination on crop water productivity (CWP, also known as crop water use efficiency) and actual evapotranspiration (ET a ) are essential. The effects of various N application rates (0, 84, 140, 196, and 252 kg ha -1 ) under fully irrigated (FIT), limited irrigation (75% FIT), and rainfed conditions on maize (Zea mays L.) yield and various CWP indices were investigated in 2011 and 2012 growing seasons under linear-move sprinkler irrigation in south central Nebraska. CWP was presented as crop water use efficiency (CWUE), irrigation water use efficiency (IWUE), and evapotranspiration water use efficiency (ETWUE). The seasonal rainfall amounts in 2011 and 2012 were 371 mm and 296 mm, respectively, as compared with the long-term average of 469 mm. Two experimental seasons were contrasted with extreme warmer temperatures, greater solar radiation, and lower rainfall in 2012. Maximum grain yield of 12.68 metric tons ha -1 and 14.42 tons ha -1 was observed in 2011 and 2012, respectively, under the fully irrigated and 252 kg N ha -1 treatment. Grain yield was linearly related to ET a and curvilinearly related to N and irrigation application amounts. Lower N treatments were more susceptible to interannual effects on the grain yield response to irrigation water amount. CWUE ranged from 1.52 kg m -3 (FIT and 84 kg N ha -1 ) to 2.58 kg m -3 (rainfed and 196 kg N ha -1 ) with an average of 2.15 kg m -3 in 2011, and from 1.49 kg m -3 (FIT and 0 kg N ha -1 ) to 2.72 kg m -3 (rainfed and 252 kg N ha -1 ) with an average of 2.33 kg m -3 in 2012. CWUE had a positive quadratic relationship with N application amount and decreased with both the presence and amount of irrigation at a given N application amount. The maximum IWUE for 75% FIT and FIT in 2011 was 1.80 kg m -3 (252 kg N ha -1 ) and 1.51 kg m -3 (252 kg N ha -1 ), respectively, whereas in 2012 the maximum IWUE values were 1.40 kg m -3 (196 kg N ha -1 ) and 1.78 kg m -3 (252 kg N ha -1 ), respectively. A curvilinear relationship was observed between IWUE and N application amount. An optimal N application amount of 196 kg ha -1 was identified for the pooled data to maximize the increase in grain yield above rainfed conditions per unit of applied irrigation water under limited irrigation management practices. In 2011, ETWUE ranged from 0.22 kg m -3 (140 kg N ha -1 ) to 1.46 kg m -3 (196 kg N ha -1 ) and from -0.21 kg m -3 (84 kg N ha -1 ) to 3.74 kg m -3 (252 kg N ha -1 ) for 75% FIT and FIT, respectively, whereas in 2012 ETWUE ranged from -0.07 kg m -3 (0 kg N ha -1 ) to 1.87 kg m -3 (252 kg N ha -1 ) and from -0.14 kg m -3 (0 kg N ha -1 ) to 3.65 kg m -3 (196 kg N ha -1 ) for 75% FIT and FIT, respectively. The results support that there is an optimal N level for each irrigation regime and, in general, lower N application amounts are required to reach maximum productivity (e.g., CWUE) under limited and rainfed conditions as compared with the FIT. In other words, there is an optimal N application amount to maximize the effectiveness of irrigation water on increasing grain yield above rainfed yields. The optimal N level for maximum productivity varied not only between the irrigation levels, but also exhibited interannual variability for the same irrigation level, indicating that these variables are impacted by the climatic conditions.

Impact of Water and Nitrogen Management Strategies on Maize Yield and Water Productivity Indices under Linear-Move Sprinkler Irrigation