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SU MMARYThe main objective of the current study was to analyse how water application through a sprinkler irrigation system influences yield of onion (Allium cepa L.), taking into account water application heterogeneity and the effects on theoretical crop evapotranspiration (ETc). Field experiments were conducted on commercial onion plots, irrigated with a permanent sprinkler irrigation system, located in Albacete, Spain, over two irrigation seasons. Two experimental plots were selected each study year : plot A (P A ), in which water was applied heterogeneously by using sprinklers with different nozzle combinations, and plot B (P B , used as the reference plot) in which the four sprinklers were maintained with the same nozzle combinations. Both experimental plots were divided into 25 subplots with the aim of studying the water distribution (measured as Christiansen uniformity coefficient (CU)), the impact on the actual evapotranspiration (ETa) and the yield obtained. Irrigation was scheduled using a daily simplified water balance method within the root area following the approach of the Food and Agriculture Organization. In the present study, sprinkler irrigation in P A resulted in lower CU (65-82 % lower in 2002 and 59-79 % lower in 2005) compared with P B (78-92 % lower in 2002 and 79-93 % lower in 2005). Between 30 May and 18 August 2002, the estimated crop water requirements in P A in the absence of water deficit was 22 mm over the accumulated value of ETc (491 v. 469 mm), while estimated crop water requirements under water deficit were 187 mm below ETc (282 v. 469 mm). In 2005, between 29 May and 25August, ETa without water deficit was more similar to ETc (458 v. 444 mm) but Eta under water deficit was 242 mm. The greater uniformity of water distribution in P B was translated into a greater uniformity of yield distribution. A smaller range in yield was observed in P B when compared with P A . No statistically significant differences were observed between P A and P B in the crop quality parameters bulb moisture content, total soluble solids, pH and total acidity.

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