Abstract
Southern Portugal is characterized by an irregular distribution of precipitation which is highly variable from year to year, requiring a clear ascertaining if the pattern of the precipitation is changing due to climate variability or climate change. In this paper, the authors applied several statistical methods to annual and seasonal time series of precipitation during the period 1931–2006, in order to detect trends and evaluate the variability in the frequencies of occurrence of dry and wet periods and also to assess spatial distribution patterns. The statistical methods include homogeneity tests, Mann–Kendall test, simple moving averages and cluster analysis. In spring, the authors detected a lack of homogeneity in the precipitation time series and a significant decreasing trend of precipitation was identified, by using simple moving averages and the Mann–Kendall test. Cluster analyses also confirmed the results and identified significant trends in the interannual and spatial distributions of dry, normal and wet years. It is concluded that the annual precipitation regime in the North interior sector is becoming drier, as the winter and spring also become drier. The only season in which a precipitation increase is observed is at fall and near the sea, but even for that region springs are also becoming drier. Copyright © 2009 Royal Meteorological Society