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Despite the importance of garlic (Allium sativum L.) among vegetable crops, very little information is currently available on its crop water requirements which depend mainly on crop evapotranspiration (ET). The objective of this work was to measure ET of garlic crops and to develop a simple model for its calculation based on the crop coefficient approach. Measurements of ET were performed in two commercial crops of garlic grown in Cordoba (Spain) in 2000 and Ecija (Spain) during 2002. The crop coefficient reached a maximum between 1.2 and 1.3 from canopy closure to maximum leaf area index (LAI), and then declined to values around 0.6 at the end of the growing season. These changes were parallel to changes in surface conductance which showed a maximum around 35 mm/s and decreased to values below 10 mm/s at the end of the growing season, partly due to leaf aging and leaf senescence. The decrease in surface conductance was also related to changes in vapor pressure deficit (VPD), which suggests that the maximum K-c of garlic crops should decrease when VPD increases. Seasonal water-use efficiency was 2.8 g dry matter per kg ET which corresponded to around 5 g fresh product per kg ET. Sensitivity of garlic ET to changes in both aerodynamic and surface conductance were rather low which may allow the direct application of the Penman-Monteith equation to calculate crop ET. (C) 2003 Elsevier B.V. All rights reserved.