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This paper describes the calibration (year 2005) and validation (year 2003) processes for the simulation of an onion crop (Himalaya cultivar) in a two-year field test under deficit irrigation conditions in Castilla-La Mancha (Spain) using the MOPECO model. Results show that MOPECO is suitable for simulating the yield versus total water use under the climatic and soil conditions in this study (RMSE = 7462 kg ha(-1), and relative error = 10.1%, between observed and simulated yields). Growing-degree-days (GDD) for the whole growth cycle is around 2283.4 degrees C, while the crop coefficient (K-c: 0.65, 1.20, and 0.75) and calibrated crop yield response (K-y: 0.45, 0.8, and 0.2) values for the growth stages proposed by FAO are similar to those presented in the literature. The electrical conductivity of the irrigation water in the area (0.85 dS m(-1)) can be neglected when simulating the yield response of onion to water (over-estimations up to 7% without considering the effect of salinity). Under the current harvest sale price scenarios (0.12 (sic) kg(-1) if onions are sold after harvest and up to 0.20 (sic) kg(-1) if onions are stored to sell at a higher price later), this crop is one of the most profitable in the area, reaching a gross margin between 1750 and 2620 (sic) ha(-1), respectively, for an average yield (70.000 kg ha(-1)). Even though optimized regulated deficit irrigation (ORDI) may slightly increase yield by 3-7%, the gross margin may significantly raise up to 30% compared with an irrigation strategy where the stress levels remain constant during the whole growth cycle. Hence, for medium and high water deficit conditions it is of interest to guarantee nascence and to favor the bulb formation stage as much as possible. For low and medium-low water deficit scenarios, nascence and establishment are guaranteed, and the model assigns more water to vegetative development than to ripening. (c) 2012 Elsevier B.V. All rights reserved.