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A gronomy J our n al • Volu me 101, I s sue 3 • 2 0 0 9 469 ABSTRACT Predicting yield is increasingly important to optimize irrigation under limited available water for enhanced sustainability and profi table production. Food and Agriculture Organization (FAO) of the United Nations addresses this need by providing a yield response to water simulation model (AquaCrop) with limited sophistication. In this study, AquaCrop was parameterized and tested for cotton (Gossypium hirsutum L.) under full (100%) and defi cit (40, 60, and 80% of full) irrigation regimes in the hot, dry, and windy Mediterranean environment of northern Syria. Model parameterization used the 2006 data and was straightforward within the designed user-interface, owing to the limited number of key parameters. Accurate simulation of canopy cover was central to sound prediction of evapotranspiration and biomass accumulation. Key user-input parameters for this purpose were identifi ed as the coeffi cients defi ning canopy development and the threshold soil water depletion levels for the water stress indices. Th e parameterized model was tested using data from the 2004 and 2005 seasons, resulting in accurate prediction of evapotranspiration (<13% error). Th e predicted yield values were within 10% of measurements, except in the 60 and 80% irrigation regimes in 2004, with errors up to 32%. Th e model closely predicted the trend in total soil water, but deviation existed for individual soil layers. Th is study provides fi rst estimate values for cotton parameters useful for future model testing and use.Model parameterization is site-specifi c, and thus the applicability of key calibrated parameters must to be tested under diff erent climate, soil, variety, irrigation methods, and fi eld management.

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Soil and water conservation is essential to the sustainability of Great Plains dryland agriculture. We hypothesized that cropping intensification improves the efficient use of precipitation. We evaluated long‐term observations of soil water at three locations in eastern Colorado for a range of pan evaporations (1050–1900 mm), soils, and cropping systems. Soils at various locations were mostly of the Argiustoll subgroup except for one Ustochrept and one Haplargid, both at the higher evaporation location. Normal precipitation at the three locations ranges from 400 to 425 mm yr‐1. Systems included a 2‐yr winter wheat (Triticum aestivum L.)‐fallow (WF) and more intense 3‐yr winter wheat‐corn (Zea mays L.)‐fallow and winter wheat‐sorghum [Sorghum bicolor (L.) Moench]‐fallow and 4‐yr rotations. To quantify the effectiveness of the intensified systems at utilizing precipitation, we introduce the System‐Precipitation‐Storage Index (SPSI) and System‐Precipitation‐Use Index (SPUI). Mean SPSI values were 0.19 and 0.28 for 2‐ and 3‐yr systems, respectively, meaning that the fallow periods in the 3‐yr rotation were collectively 47% more efficient at storing precipitation than fallow in WF. Inclusion of a summer crop, such as corn or sorghum, increased the fraction of precipitation allocated to growing‐season crop production (i.e., SPUI) from 0.43 in WF to 0.56 (i.e., an increase of 30%) in 3‐yr systems. The gains in efficient use of precipitation with intensification resulted from (i) reducing the frequency of the inefficient fallow preceding wheat, and (ii) using water for transpiration that would otherwise be lost during fallow through soil evaporation, runoff, and deep percolation.

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