This study investigates changes in seasonal runoff and low flows related to changes in snow and climate variables in mountainous catchments in Central Europe. The period 1966-2012 was used to assess trends in climate and streamflow characteristics using a modified Mann-Kendall test. Droughts were classified into nine classes according to key snow and climate drivers. The results showed an increase in air temperature, decrease in snowfall fraction and snow depth, and changes in precipitation. This resulted in increased winter runoff and decreased late spring runoff due to earlier snowmelt, especially at elevations from 1000 to 1500 m a.s.l. Most of the hydrological droughts were connected to either low air temperatures and precipitation during winter or high winter air temperatures which caused belowaverage snow storages. Our findings show that, besides precipitation and air temperature, snow plays an important role in summer streamflow and drought occurrence in selected mountainous catchments.

This paper presents an analysis of trends and causes of changes of selected hydroclimatic variables influencing the runoff regime in the upper Hron River basin (Slovakia). Different methods for identifying trends in data series are evaluated and include: simple mass curve analysis, linear regression, frequency analysis of flood events, use of the Indicators of Hydrological Alteration software, and the Mann-Kendall test. Analyses are performed for data from two periods (1931-2010 and 1961-2010). The changes in runoff are significant, especially in terms of lower Q Max and 75 percentile values. This fact is also confirmed by the lower frequency and extremity of flood events. The 1980s are considered a turning point in the development of all hydroclimatic variables. The Mann-Kendall test shows a significant decrease in runoff in the winter period. The main causes of runoff decline are: the considerable increase in air temperature, the decrease in snow cover depth and changes in seasonal distribution of precipitation amounts.

This paper deals with the occurrence of hydroclimatic extremes in the upper Hron River basin in Slovakia in 1951/61-2010. Select trends in hydroclimatic parameters in 1931/61-2010 are also studied. Emphasis is placed on the occurrence, frequency, and seasonality of dry episodes and flood events. The de Martonne index, the relative precipitation index, base flow index, low flow index, linear regression, the Mann-Kendall test and IHA software were used in the analyses. Mann-Kendall test pointed to significant changes in runoff of the upper Hron River basin. The annual runoff has decreased. Changes in minimums and maximums have had a fundamental influence on this decrease, particularly in the cold half year. Major changes occurred in Q 1 , Q Max , Q 75pct and 1-day, 3-day and 7-day maximum values. Approximately since the 1980s there has been a decrease in discharge events equal to or greater than one-year flood Q 1. Events where discharges were equal or greater than Q 5 occurred only in the first period 1951-1980 (in 1954 and 1974). The de Martonne index, the relative precipitation index and the low flow index show similar results, the longest aboveaverage period of annual values reached from 2007 to 2010 and the longest below-average period from 1990 to 1993.

The main focus of this research is concentrated on the flood analyses in the upper course of the Hron River in the period 1930–2010. The study includes an evaluation of the flood seasonality, frequency and extremity in two periods 1930–1991 and 1992–2009. The year 2010 has been added because of a very high amount of precipitation between May–September 2010 which caused extreme flooding. The most extreme flood in the 20th century occurred in October 1974. At the gauging station Banská Bystrica, discharge values reached 560 m3.s−1 which corresponds to the 100-year flood. In the last decade, extreme floods occurred in years 2002 and 2010. The main reason for the flooding was an intense rainfall and local storms with high amount of precipitation. The frequency analysis (in equally long periods 1950–1979 and 1980–2009) proved that there is a higher frequency of floods since 1980 (17 in the period 1950–1979 and 27 between 1980–2009). Higher water levels during floods were reached in the period 1950–1979. The summer floods dominate in both observed periods, but winter floods also occurred very often (7 floods in the period 1950–1979 and 12 in 1980–2009). This is due to the hollow relief of the upper course of the Hron River.