A hundred years of Caposele spring discharge measurements: trends and statistics for understanding water resource availability under climate change

The 100 year discharge series of the Caposele karst spring in southern Italy enables insight into variations in water resource availability under climate change, given the length of the series, the systematic quality of the records and the absence of human-made alteration of the natural conditions of the aquifer. With this scope, frequency and trend analyses of hydro-meteorological variables were conducted. Various three-parameter probability distribution functions were fitted to the data to calculate the Standardized Discharge Index, which describes spring discharge variations in a standardized manner. Common goodness-of-fit tests were applied to evaluate the performance of each distribution. In addition, a new criterion based on the frequency of events exceeding specific extreme Z values is proposed to quantify the deviation of the calculated index from the theoretical Standard Normal Distribution. The Weibull distribution was found to be best for calculating standardized series of spring discharge as it fits the extreme values of both tails well. The relationship between spring discharge and climatic variables was investigated by using least square linear regression, Mann–Kendall and Sen’s slope trend detection tests, applied to the entire series and to 30-year moving time series. A statistically significant decrease of spring discharge of  −0.0045 m3 s–1/year has occurred over the last 100 years, whereas no statistically significant trends were found in annual or seasonal precipitation series. However, temperature has constantly been above the mean during the past two decades. An analysis of trends in the moving time series of all variables suggests a connection between the observed spring discharge decay and temperature increase, which can be considered common to other areas of central-southern Italy.