Supplementary irrigation in Citrus sinensis (L.) Osbeck 'Spring' navel and Citrus unshiu Marcow trees were studied according to fruit development stages on the northwestern region of Uruguay (32S, 58W) from 2000 to 2009. The effective rainfall (Pe) variability between consecutive years is bigger than the reference evapotranspiration (ETo) variability; despite the annual water balance seems to be adequate for excellent yields. Water deficit occurs at different intensities during fruit growth stages I, II and III. Supplementary irrigation was necessary in 'Spring' navel and satsuma 'Owari' during stage I and II, although 'Spring' navel requires occasionally irrigation during stage III. The major irrigation impact was to increase the fruit weight but not the number of fruits. The increment in the marketable fruit was related to the best size distribution of the fruits, also it was verified an attenuation on the alternate bearing. An inverse and significant relationship was found between the ETo/Pe ratio and the relative yield reduction in the stage I (R 2=0.73) and in stage II (R 2=0.91); and between the ETo/Pe and the fruit weight in satsuma 'Owari', (R 2=0.56) (R 2=0.85) respectively. The yield reduction between trees without irrigation and all year irrigated represents 20% in 'Spring' navel and 40% in satsuma 'Owari'. Maximum water application of 1000-1200 m 3 ha -1 was required for irrigated trees on stage I and between 1800-2000 m 3 ha -1 for the stages I+II. In a close up view, supplementary water is required to reach maximum marketable yield and alternate bearing reduction in citrus production in temperate conditions.
