Rainfall Trends in Southern Portugal at Different Time Scales

Abstract

The evidence about the climate change is now considered to be unequivocal. But if visible changes are occurring at an increased rate over shorter periods, then the time series of the hydrologic variables more directly related to the climate, as rainfall or temperature, should denote progressively more pronounced signs of such changes, under the form of trends or non-homogeneities. Though some studies were previously developed for mainland Portugal, the expectable increasing signs of the changes in the values and behavior of some hydrological variables makes it essential: (i) to ensure the soundness of those studies by using the most suitable models applied to time series as long as possible; (ii) to update such studies by continuously incorporating the more recent data. In the scope briefly mentioned, the paper presents an analysis on long-term rainfall trends at different time scales in southern Portugal. For this purpose, 62 rain gages with 108 years of monthly rainfall data were analyzed based on the nonparametric Mann--Kendall (MK) test coupled with the Sen's slope estimator method. Further an analysis about sequential changes in trends of the rainfall series was conducted using the sequential MK (SQMK) method. The study shows that the rainfall is markedly decreasing. It also provides new insights about the within-the-year relative contribution of the wettest months of the rainy period.