Rainfall regime uncertainty (RRU) in an Eastern Mediterranean region - A methodological approach

A methodological investigation of the temporal rainfall regime uncertainty (RRU) in a typical Mediterranean station is presented, based on daily rainfall totals from the University of Haifa station for the period 1976/77-2001/02 and two additional stations from northern Israel for a shorter period. By defining the most expectable regime (MER), we calculated the deviations of its various components along the study period, and then analyzed the RRU. The components we used to define the MER were: NRS_(d)-number of rainspells (in days); RSY_(d -rainspell yield (in mm). By multiplying NRS_(d) by RSY_(d) we obtained the total rainfall accumulated from all rainspells of a given duration d, which was denoted as Total_(d. In addition, we checked the RSL-rainy season length (in days, from the date when 10% of the annual rainfall amount was accumulated to the date of 90%) and the DAP_(i)-the median date of an accumulated percentage of the annual rainfall. Finally, an uncertainty index (UI) was defined. The main conclusions of the study are: The aforementioned parameters reflect the complexity of such a regime better than the commonly used averages and standard deviations. The RSL can vary from about two and half months to about five months. The RSL is the longest for a median rainy season and considerably shorter for either early or late seasons. The DAP appear in a variety of dates, spreading over two months. The RSL demonstrates a slight decreasing tendency. The RRU is characterized by an increasing tendency.