Analysis of river drying in Slovenia

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Intermittent watercourses lack water during some periods of the year, which especially applies to karst sinking streams. Droughts also occur in dry periods with no rainfall over a longer period of time. The drying up of Slovenian rivers was analysed using the data from the state hydrological monitoring of the Slovenian Environment Agency. By analysing the periodic statistics of minimum low discharges (nQnp), 18 gauging stations were extracted where the watercourses dried up during the measurement period. For these stations, sets of mean daily discharges were analysed, and dry days where the flow was equal to zero were listed. The number of dry days and the number of consecutive dry days were analysed by month and year, and the longest dry periods were determined. The results are presented graphically. At most of the gauging stations considered, the number of dry days was the highest in the years characterized by extreme hydrological droughts, namely 2003, 2012, and 1993. In some watercourses, drying up is a constant phenomenon that occurs almost every year, while others only dry up in years characterized by extreme droughts. Exceptions to this are the karst rivers, which sink and are therefore dry for part of the year. The Pivka and Branica Rivers regularly dry up. Watercourses in Pomurje and Velunja, Kožbanjšček, and Dragonja also dry up very often. Analysis results demonstrated that river drying has become more frequent in the past two decades. The driest month is August, followed by July and September.