Currently, the 40-year-long reference period 1961–2000 is used in Slovakia for systematic assessment of the hydrological regime. The World Meteorological Organization recommends unifying the climatic normal 1991–2020 with a reference period for hydrological characteristics for the purpose of climate change research. We have analyzed changes in the long-term mean annual discharges at 113 water-gauging stations over different time periods. Considering the history of the reference periods used in Slovakia, we have focused on the analysis of the 10, 20, 30, 40, and 50-year-long moving averages of the long-term mean annual discharges in the period 1961–2020 in comparison with the 1961–2000 reference period and the whole selected and analyzed period. The deviations of the annual flow rates for the area of Slovakia and river sub-basins have been analyzed. The results indicate that the new reference period to be used in Slovakia should include the time period after the year 2000. To determine its precise length, a more detailed analysis is needed.

Reassessing the hydrological characteristics with regard to drought is very important in the context of a changing climate. In this paper, we evaluate the drought in terms of changes in hydrological characteristics for the 20-year period 2001-2020. The evaluation of changes is based on a comparison of data for this period with the currently valid reference period 1961-2000 in 13 selected water-gauging stations. In the analysis of the occurrence of subnormal mean annual discharges (Qr <90% Qa,1961-2000, in most of the evaluated stations, the percentage incidence of such years is higher in the period 2001-2020 than in the reference period. The distribution of runoff throughout the year in the period 2001-2020 in comparison with the reference period in most stations confirms the changes identified in the previous evaluation of the period 2001-2015, i.e. the transfer of part of the usual increased spring runoff to previous, winter months. An exception was water-gauging stations on streams in the mountainous areas of northern Slovakia, where changes are reflected to a lesser extent.

Hydrological data series that are measured on the Danube River are temporally limited. Instrumental flow data can be prolonged by documentary data from historical sources in archives. This paper deals with knowledge gained by studies of historical materials regarding the 1895 catastrophic floods on the Danube River and its tributaries as reflected in the contemporary local press and also in studies of other historical records (flood marks, chronicles, books and photos). Records from the newspapers (Wiener Zeitung, Preßburger Zeitung, The New York Times and Komáromi Lapok) and other analysed sources show the relatively large territorial impact of the floods in March and April 1895, which affected not only the Danube and its tributaries, but also some neighboring basins. Catastrophic consequences of the flood were especially reported from the lower parts of the Danube River (from its confluence with the Drava River up to the mouth of the Black Sea) and the tributaries of the Tisza and Sava rivers. In 1895, the second highest flood after the 2006 flood on the Lower Danube was observed since 1841.

This research is focused on the assessment of drought on surface watercourses in Slovakia. Low-flow characteristics and their changes in the 2001–2015 period in comparison with the 1961–2000 reference period were evaluated at selected representative water-gauging stations. Two different methods were used to calculate the flow duration curves (FDCs): the standard method, based on mean daily discharge data series for the whole evaluated period in descending order, and the alternative method, based on first calculating the values of FDCs for each year of the assessed period and then averaging the corresponding percentile values. The changes were evaluated for selected percentiles of the FDCs (330-, 355-, and 364-day discharge). The number of days with the mean daily flow below the set limits and the seasonality of their occurrence were assessed. The results show significant changes in cases of both methods in the compared time periods, while differences in individual regions of Slovakia were also found. The weakness of the standard method is in allowing the values of the smallest quantiles to be influenced by a small number of long-lasting drought episodes. The alternative method eliminates the aforementioned shortcoming and could be used to determine the ecological flows in Slovakia.

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