A field experiment was conducted for three growing seasons to study the effects of seasonal water use and applied N fertilizer on yield attributes and water productivity indices of wheat in an arid region of Iran. The results revealed that yield attributes were significantly affected by irrigation and nitrogen treatments and growing season, and their interactions. Crop height, maximum leaf area index and biological yields were increasingly affected by the available water and N fertilizer. The findings indicated that the grain yield response to N was associated with water application levels. The water productivity indices were influenced by irrigation strategies and deficit irrigation effectively boosted productivity of irrigation water (W I ). The highest W I was obtained at a seasonal irrigation water of 156 mm for different levels of applied nitrogen . For levels of applied N 1 (application 70% of the required nitrogen), N 2 (required nitrogen), and N 3 (application 120% of the required nitrogen), W I ranged between 0.93 and 2.28, 1.30 and 2.75, and 0.98 and 2.47 kg m -3 , respectively. The data generated here suggest that under deficit irrigation, maximum water productivity (W ET ) would be achieved when 98 kg N ha −1 is combined with a 156 mm of supplemental irrigation. In this seasonal water use, W ET value may be increased to 30% with N appropriate practice (practice N 2 ). Consequently, when limited irrigation water is combined with N fertilizer appropriate management, wheat water productivity can be substantially and consistently increased in the region.
The present study evaluated the applicability of the CropSyst model under variable climatic, irrigation, and fertilizer-nitrogen regimes. The objective was to analyze wheat productivity responses to water and N-application for optimizing water productivity in an arid irrigated environment. Evaluation analysis showed that the model provided very satisfactory estimates for the emergence, flowering and physiological maturity dates. The performance of the model was reasonable as demonstrated by the close correspondence between simulated grain yield, biomass accumulation, seasonal ET, and irrigation water productivity (WP I ) with measured data. The normalized root mean square error ranged between 5 and 10% for most of the parameters. Overall, the Willmott index of agreement between simulated and observed values of grain yield, biomass and seasonal ET were 0.99, 0.98 and 0.97, respectively. The validated model was employed to assess interactive effects of irrigation and fertilizer N on grain yield and water productivity indices. Scenario analyses indicated that WP I and WP ET (ET water productivity) ranged from 0.16 to 2.07 kg m -3 , and from 0.07 to 1.49 kg m -3 , respectively. For predicting the best N and water application practices for maximization of water productivity, the best option found by the model was application of water and nitrogenous fertilizer in 70% and 90% of the required values, respectively, for WP I , and equal to the required values (100%) for WP ET . The simulations demonstrated that the current wheat productivity of 5.0 Mg ha -1 obtained by the local farmers can be achieved at 140 kg ha -1 fertilizer N and 30% deficit irrigation regime with a WP I of 1.73 kg m -3 . The CropSyst model can be applied to derive best management options in terms of N and irrigation application of wheat under arid conditions.
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