J. Hydrol. Hydromech., 64, 2016, 4, 357–366 DOI: 10.1515/johh-2016-0037 Variability of seasonal floods in the Upper Danube River basin Katarína Jeneiová1, Silvia Kohnová1*, Julia Hall2, Juraj Parajka2 1 Slovak University of Technology in Bratislava, Faculty of Civil Engineering, Radlinského 11, 810 05 Bratislava, Slovakia. 2 Institute of Hydraulic Engineering and Water Resources Management, Vienna University of Technology, Karlsplatz 13/222, A-1040, Vienna, Austria. * Corresponding author. Tel.: +421 (2) 59 274 623. E-mail: [email protected] 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 tem- poral concentration of floods. The flood seasonality of winter events is dominant in the Alps; however, along the north- ern 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 sum- mer 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, particu- larly for the assessment of the effect of increasing air temperature on snowmelt runoff and associated floods. Keywords: Seasonality; Summer and winter floods; Upper Danube River basin; Comparative hydrology. INTRODUCTION ent flood types based on the timing of floods. Beurton and Thieken (2009) analysed monthly frequencies of annual floods The growth of population and assets in flood prone areas has in Germany and divided the country into three regions based on increased the impacts of floods, therefore flood management the identified flood regime. Parajka et al. (2009 and 2010) through mitigation, protection and damage control is becoming applied a seasonality index to identify the main climatic and increasingly important (e.g. Di Baldassarre et al., 2014; physiographic drivers behind the floods generating processes. Zeleňáková et al., 2011). One of the prerequisite of effective Parajka et al. (2009) evaluated the differences in precipitation, flood management is a sound understanding of the underlying long-term hydrological regime and annual floods along a tran- flood processes in the region of interest. Recent severe floods sect from Austria to Slovakia. Parajka et al. (2010) then extend- worldwide have sparked a renewed interest in the analysis of ed the analysis to the Alpine Carpathian range. The evaluation long hydrological data to investigate the main drivers and pat- of annual flood seasonality in both studies demonstrated an terns of flood regimes and their changes. important role of soil moisture, evaporation and snow processes The Upper Danube region has been affected several times by on flood generation, but none of these studies analysed the extreme floods, most of which occurred in the summer season summer and winter flood separately. of the year. For example, major floods occurred in August The aim of this study is to extend the analyses of previous 1897, September 1899, July 1954, August 2002, with the latest studies and to explore the seasonal floods in the Upper Danube flood in late May and early June 2013 (e.g. Blöschl et al., 2013; River basin. The main motivation is to evaluate and compare Mitková et al., 2005; Pekárová et al., 2008, 2014). the seasonal flood concentration for winter (December–May) As floods are generally not limited to a specific season, sea- and summer (June–November) half-year. The analysis focuses sonality analysis can provide insight and explanations on re- on the understanding of the factors that control the spatial gimes and drivers of the extreme events. This has been shown changes of winter half and summer half-year flood occurrences in various studies over both small and large river basins (e.g. in 88 basins with different physiographic conditions. Black and Werritty, 1997; Collins et al., 2014; Hlavčová et al., The paper is organised as follows. First, we describe the 2015; Koutroulis et al., 2010; Szolgayová et al., 2014) as well methods applied for flood seasonality assessment. Next, we as over diverse geographical regions (e.g. Glaser et al., 2010; introduce the study region and the data used in the analysis. The Parajka et al., 2009, 2010; Petersen et al., 2012). A summary of results section compares floods in the summer and winter half- flood regimes and associated changes in Europe can be bound year and evaluates the temporal concentration seasonal floods. in Hall et al. (2014). Finally, we discuss the factors that control the spatial patterns The seasonality of annual flood and precipitation maxima in of the observed variability of seasonal floods in the Upper the Alpine – Carpathian region has been analysed in several Danube basin. studies. For example, Merz et al. (1999) and Merz and Blöschl (2003) used flood seasonality as an indicator to describe differ- 357 Katarína Jeneiová, Silvia Kohnová, Julia Hall, Juraj Parajka METHODS AND DATA 10°C in the lowlands to less than –8°C in the Alps and –3°C in Flood seasonality assessment the Tatra Mountains. The mean annual precipitation also exhibits strong spatial variability. A general decrease of The analysis of flood seasonality is based on circular statistics precipitation along the West to East gradient is imposed by an (e.g. Bayliss and Jones, 1993; Burn, 1997; Magilligan and increase caused by orographic enhancement. The lowest Graber, 1996). The date of each flood event is plotted on a unit precipitation occurs in the lowlands (less than 400 mm/year), circle, where the position of the event Θi is defined as: while the largest precipitation totals occur on the highest windward slopes of the Alps (more than 3000 mm/year) and the Θi = D 2π/Y, (1) Carpathian Mountains (more than 1,500 mm/year). The flood seasonality is derived from daily runoff data where D = 1 is chosen for 1 January and D = 365 for 31 provided by the Global Runoff Data Centre (GRDC, 2014). The December (366 for leap years), Y is number of days in the year. dataset includes 88 stations (Figure 1), with less than 10% of The concentration of the date of occurrence around the mean missing data over the period 1961–2010. Part of the GRDC date is represented by the length of vector r, which is defined as dataset for Slovakia was complemented with data from the (Bayliss and Jones, 1993): Slovak Hydrometeorological Institute. The size of the river basins studied is up to 130,000 km2, with 60 basins being 2 rxy=+22 (2) smaller than 1,000 km . The mean basin elevation ranges between 217 to 1,598 m a.s.l. (Table 1). A table of all stations where the position of x and y coordinates represents the and with their basin characteristics (i.e. area, elevation, and location) is provided in Appendix 1 as Table A1. mean date obtained from the sample of n extreme events by The annual flood series at each station represent the largest following relationships: flood events (in terms of peak discharge). For the seasonal n analysis, the annual flood data at each station is split into half- =Θ1 years based on the expected timing of the flood generation x cos(i ) (3) n i=1 processes in the analysed region. The seasonal flood maxima are derived in two steps. First, the date and half-year of annual n maximum of daily runoff is determined for each year and =Θ1 y sin(i ) (4) station. In the second step, the runoff maximum in the other n i=1 half-year is computed. In order to assure temporal independence between the summer half-year and winter half- The length of vector r represents the concentration of the year peaks, the individual peaks identified for each season had flood occurrences (referred to as ‘concentration index’ to be at least 30 days apart. This step is of particular importance thereafter) and ranges from zero (high variability of the date of for basins with an area larger than 1000 km2, where summer- occurrence, low seasonality) to r = 1 (all flood events occur on year and winter half-year flood events tend to occur in narrow the same day, low variability of the date of occurrence, high time intervals. The seasonal flood series therefore represent the seasonality). largest flood events (in terms of peak discharge) in the winter In order to investigate the influence of the magnitude of half-year (December–May) or the summer half-year (June– extreme events on the temporal flood concentration, the r index November) periods, respectively.