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UPDATED RAINFALL SERIES AND THEIR TRENDS FOR MAINLAND PORTUGAL (1913–2019)

This paper summarises the main findings from a comprehensive study on rainfall trends based on updated long-running rainfall series from more than 500 stations uniformly dispersed over mainland Portugal (89015 km2, i.e., almost 60 stations each 10,000 km2) considering a total period of 106 hydrological years, from October 1913 to September 2019. To understand the rainfall behaviour at different time scales, cumulative intra annual (monthly, quarterly and half-yearly), and annual rainfall series were analysed. To identify possible shifts in the long-term rainfall pattern, two additional contiguous periods were analysed along with the 106-year total period: the initial period, with 55 years and, followed by the final one, with the last 51 years. The detection of the rainfall trends at the different time scales and periods were addressed via the Mann–Kendall (MK) test together with the Sen’s slope estimator for measuring the magnitude of the trends. The majority of results achieved revealed that the generalised positive trends in the initial 55 years were followed by negative trends in the last 51 years of the total period clearly outweighing the trends observed in the initial years. This counterbalance resulted in widespread downward trends for the total period. The spatial representation of the magnitude of the trends clearly showed an increasing asymmetry between northern and southern conditions, the latter becoming progressively and remarkable more arid. Such behaviour is also consistent with the previously identified pattern of the “dry gets drier” paradigm for the south western region of the Iberian Peninsula. It was also found that, closely to the year 1968, the intra annual rainfall pattern changed. Since about that year, the rainfall in the last wet months of the hydrological year (January, February, and March) has showed a sustained decrease of its relative weight, thus diminishing its relative contribution to the groundwater refill and to the water needs during the forthcoming dry months.