Detecting Trends in the Annual Maximum Discharges in the Vah River Basin, Slovakia

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

Posúdenie obdobia 2001-2015 voči referenčnému obdobiu 1961-2000 z pohľadu zmien dlhodobých prietokových charakteristík vo vybraných vodomerných staniciach na Slovensku

Obdobie rokov 2000 až 2015, je obdobím, v ktorom sme pozorovali výrazné rozdiely a extrémy v hydrologickom režime slovenských tokov. Slovenský hydrometeorologický ústav v rámci svojich úloh spracúva komplexné posúdenie hydrologických charakteristík so zameraním na hodnotenie sucha. Tento článok sa venuje jednej z častí celkového hodnotenia, a to hodnoteniu zmien dlhodobých prietokových charakteristík obdobia 2001-2015 voči referenčnému obdobiu 1961-2000 z pohľadu zmeny v hodnotách dlhodobých priemerných prietokov za obdobie a priemerných mesačných prietokov za uvedené obdobie. Snahou je prehodnotiť obdobie nadväzujúce na aktuálne platné referenčné obdobie a jeho prípadné ovplyvnenie hydrologických charakteristík za obdobie 1961-2015. Celkovo bolo vybraných 216 staníc, ktoré boli roztriedené a hodnotené na základe dĺžky časového radu a miery ovplyvnenia. Výsledky analýz preukázali, že v období 2001-2015 na Slovensku došlo k poklesu prietokov, napriek tomuto faktu, trendové analýzy týchto hydrologických charakteristík vo väčšine prípadov nepotvrdzujú významne klesajúce trendy. Identifikované zmeny v hydrologickom režime slovenských tokov za ostatných 15 rokov nie sú takého charakteru, aby došlo k zmene referenčného obdobia, ale sú určité náznaky zmien v rozdelení odtoku v roku.313

Effects of the Time Period Length on the Determination of Long-Term Mean Annual Discharge

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

Variability of seasonal floods in the Upper Danube River basin

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

Changes in Selected Low-Flow Characteristics in the 2001–2015 Period Compared to the 1961–2000 Reference Period in Slovakia

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