Rainfall uncertainty in the Mediterranean: definitions of the daily rainfall threshold (DRT) and the rainy season length (RSL)

One of the major dilemmas in rainfall-regime research is regarding the representativity of rainfall amounts. The annual total (TOTAL) represents the summation of the contributions of all rainfall events. However, this parameter is obtained mainly by contributions of some major events, whereas, many others may contribute minute quantities of a negligible impact. A selected daily rainfall threshold (DRT) is used to filter these small events and to retain only the effective ones (for a wide range of activities), and enable focusing on them in order to better detect any important temporal changes in the rainfall regime. Two different procedures are applied to the data in order to filter out slight and/or sporadic rains: (a) definition of a DRT for each station which filters out the slight rains all year around regardless of their timing, and (b) definitions of the rainy season beginning date (RSBD) and the rainy season ending date (RSED), which filter out sporadic rain events at the beginning or the end of the rainy season, regardless of their amount. The current study presents the appropriate DRT and effective rainy season length (RSL) in 41 Mediterranean stations, and analyzes their influence on some other parameters. The main conclusions are: (1) the filtering process reduced considerably the number of rain-spells (NRS) and therefore the average intensities increased; (2) the filtering process reduced the average time that elapsed between the beginning of two consecutive rain-spells, and (3) the RSL within the study area has a clear spatial distribution, longer in the northern parts and shorter in the southern parts.