Some statistical characteristics of monthly and annual pluviometric irregularity for the Spanish Mediterranean coast

Seven series of monthly pluviometric amounts, sometimes exceeding recording periods of 100 years and compiled by the Instituto Nacional rie Meteorolog ia (Spain), are used to study the irregularity of the pluviometric regime a long the Spanish Mediterranean coast and nearby Atlantic coast. First of al l, three statistical functions (gamma, log-normal and a combination of Pois son and gamma distributions) and moment-ratio diagrams are used to model th e monthly and annual empirical distributions of precipitation amounts, each distribution being tested by means of the Kolmogorov-Smirnov test. It is n oteworthy that, whereas most of the monthly cases require the gamma distrib ution, the pluviometric behaviour of the summer months is well described by the Poisson-gamma distribution. Moreover, both the log normal and the gamm a distributions satisfactorily model empirical annual amounts. Consequently , rainfall amounts are not identically distributed along a year for each ga uge tested. Second, temporal trends deduced for annual and seasonal amounts are computed and their statistical significance evaluated. The most notabl e fact is that, although some linear trends an close to 1 mm/year, their si gnificance levels exceed the assumed threshold value and, excepting the win ter season for Barcelona, they are considered non-significant from a statis tical point of view. Finally, by again using monthly and annual amounts, th ree temporal irregularity indexes are computed for each pluviometric series , the temporal disparity of the rainfall patterns of the Mediterranean regi on being enhanced as a result. It addition to the temporal irregularity, a change with latitude is observed both in the parameters of the statistical distributions and the temporal irregularity indexes for the rain gauges ana lysed. The two most southerly rain gauges constitute a special case in comp arison with the remaining stations, because they also receive the Atlantic influences due to their proximity to this ocean.