The effects of soil water depletion and irrigation cutoff during ripening on yield, quality and evapotranspiration (ET) of table grapes were studied to refine irrigation strategies needed during hot, dry summers. Six irrigation treatments were applied to two cultivars, viz. Sunred Seedless and Muscat Supreme, from the 1998/99 until 2000101 seasons in a field trial in the Hex River valley of South Africa. Grapevines of all treatments were irrigated with micro-sprinklers at 40% plant available water (PAW) depletion before veraison and after harvest. Two treatments were irrigated at 20% and 40% PAW depletion from veraison to harvest. Four more treatments were applied by cutting off irrigation at these two depletion levels when total soluble solids reached 12°B and 15°B, respectively. According to earlier results, best overall performance for Sunred Seedless was obtained where irrigation at 40% PAW depletion was cut off at 15°B compared to the worst, i.e. where irrigation at 20% depletion' was cut off at 15°B. For Muscat Supreme, irrigation at 20% PAW depletion, either continued until harvest, or cut off at 15°B, resulted in optimum overall performance. Although yield and quality responded differently, this study showed that the ET of the two cultivars responded in a similar way to the irrigation treatments under the given conditions. The ET of grapevines irrigated at 40% PAW increased from 1.6 mm day-1 in September to a maximum of 5.4 mm day-1 in January. Irrigation at 20% PAW depletion increased the maximum ET to 7.3 mm day-1. For both soil water depletion levels, irrigation cutoff at 12°B and 15°B resulted in, respectively, ca. 75% and 65% PAW depletion at harvest. Mean daily ET during January decreased to 4.1 mm day-1 and 5.0 mm day-1 after the irrigation had been cut off at 12°B and 15°B, respectively. The crop coefficients, which decreased accordingly, can be used to reduce irrigation to obtain optimum table grape yield and quality, particularly for Sunred Seedless.
