Abstract:The objective of this study is to analyse the spatial variability of seasonal flood occurrences in the Upper Danube region for the period 1961-2010. The analysis focuses on the understanding of the factors that control the spatial variability of winter and summer floods in 88 basins with different physiographic conditions. The evaluation is based on circular statistics, which compare the changes in the mean date and in the seasonal flood concentration index within a year or predefined season.The results indicate that summer half-year and winter half-year floods are dominant in the Alps and northern Danube tributaries, respectively. A comparison of the relative magnitude of flood events indicates that summer half-year floods are on average more than 50% larger than floods in winter. The evaluation of flood occurrence showed that the values of seasonal flood concentration index (median 0.75) in comparison to the annual floods (median 0.58) shows higher temporal concentration of floods. The flood seasonality of winter events is dominant in the Alps; however, along the northern fringe (i.e. the Isar, Iller and Inn River) the timing of winter half-year floods is diverse. The seasonal concentration of summer floods tends to increase with increasing mean elevation of the basins. The occurrence of the three largest summer floods is more stable, i.e. they tend to occur around the same time for the majority of analysed basins. The results show that fixing the summer and winter seasons to specific months does not always allow a clear distinction of the main flood generation processes. Therefore, criteria to define flood typologies that are more robust are needed for regions such as the Upper Danube, with large climate and topographical variability between the lowland and high elevations, particularly for the assessment of the effect of increasing air temperature on snowmelt runoff and associated floods.

A number of floods have been observed in the Slovak Republic in recent years, thereby raising awareness of and concern about flood risks. The paper focuses on the trend detection in the annual maximum discharge series in the Vah River basin located in Slovak Republic. Analysis was performed on data obtained from 59 gauging stations with minimum lengths of the observations from 40 years to 109 years. Homogeneity of the time series was tested by Alexandersson test for single shift at 5% level of significance. The Mann-Kendall trend test and its correction for autocorrelated data by Hamed and Rao (1998) were used to analyse the significance of detected changes in discharges. The series were analysed at different lengths of 40, 50, 60 years and whole observation period. Statistically significant rising and decreasing trends in the annual maximum discharge series were found in different regions of the Vah River catchments

The study is focused on the evaluation of changes in the hydrological balance of the Krupinica River basin to the Plášťovce station for the entire 90-year period as well as for the three 30-year subperiods 1931-1960, 1961-1990 and 1991-2020. In the first part of the study, the hydrological balance is processed in an annual step on the basis of measured series of average monthly flows from the Krupinica: Plášťovce; monthly precipitation totals on the Krupinica catchment area and average monthly temperatures in the catchment area. The hydrological balance in the monthly step was processed in the second part. Changes in water resources in the river basin over the three mentioned time subperiods were analyzed. The long-term annual precipitation total in the Krupinica River basin for the whole period was 660 mm, the annual runoff depth was 182 mm and the balance evaporation was 478 mm. A comparison of 30-year periods shows a significant decrease in the runoff of Krupinicafrom 231 mm to 144 mm. This was even more pronounced in the runoff coefficient it fell from 0.32 over 0.27 to 0.21 in the last period 1991-2020. In the third part, a simple regression relationship between runoff, precipitation and air temperature was derived to estimate the future development of the annual runoff from the basin. The relationship shows that a 100 mm decrease in precipitation in the Krupinica River basin will cause an average decrease of 52 mm in runoff. And an increase in the average annual temperature by 1°C in the Krupinica River basin results in a decrease in runoff of about 33.5 mm.

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.

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