Rainfall uncertainty in the Mediterranean: dryness distribution

Dryness analysis is a major topic in the study of rainfall regime, mainly in regions with water shortage. The present study suggests a new approach for analyzing dry spells and their annual distribution and introduces a new term: dry days since last rain (DDSLR hereafter). The main goals of this study are to analyze and present the different annual courses of the DDSLR across the Mediterranean region. Daily rainfall totals for the period of 1931–2006 at 41 stations served as the main database. Two annual courses of the DDSLR, the median (50 percentile) and the 90 percentile, are presented for each Julian day in each station. Correlation matrices between stations according to their annual courses were calculated. A cluster analysis was performed on these correlations matrices according to which the stations were grouped. The main conclusions of the present study can be summarized as follows: Four regions were found in the Mediterranean basin according to their annual course of the DDSLR: Region Ia spread over the southern Mediterranean and has the most severe dryness conditions reaching maximum dryness in August and September. Region Ib is located mainly in northeastern Iberian Peninsula, south France, and northwestern Italy. Dryness is less severe than in Region Ia. The maximum dryness conditions are in July and August. Region IIa is located in the northeastern study area, with a bi-modal moderate dryness conditions. Maximum dryness is in August–October and a secondary dry period in January–March. Region IIb is located in the northwestern study area with a bi-modal moderate dryness conditions. Maximum dryness is in January–March and a secondary dry period in October. Both regions Ia and Ib can be combined into a single major region I. This region represents the classical Mediterranean rainfall regime with a very distinguishable dry period during summer. Similarly, regions IIa and IIb can be combined into a single major region II. This region benefits from rainfall all year round and therefore is not considered as having a classical Mediterranean climate. Uncertainty regarding the DDSLR distribution (from year to year) is much crucial in region II, since ecosystems are not adapted to long dry periods and their prolongation may have very severe environmental consequences.