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In the old-world viticulture, there is a common but most often not scientifically proven consideration that supplemental irrigation should detrimentally affect berry and wine composition. In the semi-arid and warm climate of in-land Valencia, we tested the hypothesis that deficit irrigation might, not only improve yield, but also fruit composition. The experiment was performed with 'Cabernet Sauvignon' vines at the Celler del Roure SL vineyard, located in the D.O. Valencia. Rainfed vines were compared with three different post-veraison irrigation regimes with water application at either 10, 20, or 30% of reference evapotranspiration, resulting in water application of 26, 34 and 57, mm respectively. The experimental design was a randomised block with three replicates per treatment and 308 experimental vines per experimental plot. The experiment was conducted in a very dry and warm 2009 season, with substantial no rainfall from August up to harvest and average temperature during ripening of 24 degrees C. Rainfed vines experienced quite severe plant water stress with an average midday stem water potential of -1.45 MPa. Supplemental irrigation improved plant water status and increased yield in proportion to the amount of water applied mostly because irrigation avoided berry and whole clusters dehydration that occurred in the rainfed vines during ripening. The most important effect of irrigation was to avoid the excessive increase in berry sugar content that, at the right phenolic ripening time, reached in the rainfed treatment up to 16.5 degrees of probable alcohol. Irrigation did not affect must acidity and improved berry quality determined by a berry tasting panel. In addition the supplemental irrigation did not decrease total berry phenolic and anthocyanin potential. On the other hand irrigation slightly decreased the extractable values. This suggests that different maceration procedures should be applied depending on grape origin. Under very dry and warm seasons, irrigation can be used to mitigate the negative effect of low plant water status on berry dehydration and unbalanced ripening.