Historical drought occurrence on Slovak rivers during the period 1931–202

Abstract

In the last decade (2011-2020), Slovakia has experienced a significant decrease in flows in most river basins. The aim of this study is to statistically analyse the changes in the hydrological regime of selected Slovak streams based on observations over a 90-year period (1931-2020), using measured average daily discharges. Several regions in Europe, particularly in the Mediterranean area, including Spain, Italy, and Greece, have been affected by significantly dry years. The occurrence of dry years is not unusual, as extreme droughts have also occurred in the Danube River basin in the past, such as in 1863, 1921, 1947, 1992-93, and 2003. Our goal is to analyse changes in the low flow regime of rivers in Slovakia, which have long observation records. Understanding the low flow regime is one of the inputs for determining hydro-ecological limits, which is essential for maintaining and achieving good ecological status of surface waters. Several flow and non-flow characteristics were evaluated when assessing hydrological drought. Flow characteristics include: minimum average daily flows (in monthly or annual steps, over the entire period), M-day flows (the exceedance curve of average daily flows), minimum monthly and annual flows, and T-year minimum flows. Non-flow characteristics evaluated include changes in the timing of dry periods (date of occurrence), the number of low flow days, the longest drought episode, and the deficit volumes. Individual flow characteristics were calculated from the series of average daily flows. The trend analysis showed that in the given sub-catchments there is a decrease in the T-year minimum flows, as well as a decrease in the basic runoff. The comparison of the 100-year minimum specific runoff with the values of the specific base runoff shows that the 100-year minimum runoff can be up to ca. 4 times lower than the estimated specific base runoff in an extremely dry year.   Acknowledgement This work was supported by the project VEGA No. 2/0015/23 “Comprehensive analysis of the quantity and quality of water regime development in streams and their mutual dependence in selected Slovak basins”, and by the project APVV No. 20-0374 “Regional detection, attribution and projection of impacts of climate variability and climate change on runoff regimes in Slovakia”