This paper analyses the influence of the extreme phases of the winter North Atlantic\ud Oscillation (NAO) on water resources in the Spanish region of the Tagus River basin. By analysing\ud a winter NAO index based on station sea level pressure, the years between 1957 and 2003 were\ud classified as normal, positive, and negative NAO years. A statistical test was then applied to\ud monthly data series of precipitation, river discharge, reservoir storage, and reservoir release to\ud analyse the variations in these variables. For all four variables, significant differences were found\ud between positive and negative NAO years, the former resulting in reduced water availability\ud (negative anomalies), and the latter in increased water availability (positive anomalies). The\ud influence of extreme NAO winters was found to act with different time lags on different variables:\ud the effect of extreme NAO winters on precipitation was found to be quite immediate (and\ud significant for December to March), but this effect was observed later in the year and lasted longer\ud for river discharge, reservoir storage, and water release. Positive and negative NAO years were also\ud found to have different effects on these variables, in that the effects of positive years were more\ud sustained and those of negative years were more rapid and less prolonged. In spite of the high\ud variability of the availability of water resources, the strategies for management of the reservoir\ud system of the basin were found in most cases to provide a regular supply that meets water demands.\ud However, our results also indicate that these water management practises are not adequate for the\ud expected scenarios of climate change and increasing water demand
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