Ladislav Gaál

Increasing river floods: fiction or reality?

There has been a surprisingly large number of major floods in the last years around the world, which suggests that floods may have increased and will continue to increase in the next decades. However, the realism of such changes is still hotly discussed in the literature. This overview article examines whether floods have changed in the past and explores the driving processes of such changes in the atmosphere, the catchments and the river system based on examples from Europe. Methods are reviewed for assessing whether floods may increase in the future. Accounting for feedbacks within the human‐water system is important when assessing flood changes over lead times of decades or centuries. It is argued that an integrated flood risk management approach is needed for dealing with future flood risk with a focus on reducing the vulnerability of the societal system. WIREs Water 2015, 2:329–344. doi: 10.1002/wat2.1079 For further resources related to this article, please visit the WIREs website.

Dependence between flood peaks and volumes: a case study on climate and hydrological controls

Abstract The aim of this paper is to understand the causal factors controlling the relationship between flood peaks and volumes in a regional context. A case study is performed based on 330 catchments in Austria ranging from 6 to 500 km2 in size. Maximum annual flood discharges are compared with the associated flood volumes, and the consistency of the peak–volume relationship is quantified by the Spearman rank correlation coefficient. The results indicate that climate-related factors are more important than catchment-related factors in controlling the consistency. Spearman rank correlation coefficients typically range from about 0.2 in the high alpine catchments to about 0.8 in the lowlands. The weak dependence in the high alpine catchments is due to the mix of flood types, including long-duration snowmelt, synoptic floods and flash floods. In the lowlands, the flood durations vary less in a given catchment which is related to the filtering of the distribution of all storms by the catchment response time to produce the distribution of flood producing storms. Editor Z.W. Kundzewicz

Inclusion of historical information in flood frequency analysis using a Bayesian MCMC technique: a case study for the power dam Orlík, Czech Republic

Inclusion of historical information in flood frequency analysis using a Bayesian MCMC technique: a case study for the power dam Orlík, Czech Republic The paper deals with at-site flood frequency estimation in the case when also information on hydrological events from the past with extraordinary magnitude are available. For the joint frequency analysis of systematic observations and historical data, respectively, the Bayesian framework is chosen, which, through adequately defined likelihood functions, allows for incorporation of different sources of hydrological information, e.g., maximum annual flood peaks, historical events as well as measurement errors. The distribution of the parameters of the fitted distribution function and the confidence intervals of the flood quantiles are derived by means of the Markov chain Monte Carlo simulation (MCMC) technique. The paper presents a sensitivity analysis related to the choice of the most influential parameters of the statistical model, which are the length of the historical period h and the perception threshold X0. These are involved in the statistical model under the assumption that except for the events termed as ‘historical’ ones, none of the (unknown) peak discharges from the historical period h should have exceeded the threshold X0. Both higher values of h and lower values of X0 lead to narrower confidence intervals of the estimated flood quantiles; however, it is emphasized that one should be prudent of selecting those parameters, in order to avoid making inferences with wrong assumptions on the unknown hydrological events having occurred in the past. The Bayesian MCMC methodology is presented on the example of the maximum discharges observed during the warm half year at the station Vltava-Kamýk (Czech Republic) in the period 1877-2002. Although the 2002 flood peak, which is related to the vast flooding that affected a large part of Central Europe at that time, occurred in the near past, in the analysis it is treated virtually as a ‘historical’ event in order to illustrate some crucial aspects of including information on extreme historical floods into at-site flood frequency analyses.

Hybrid Approach to Delineation of Homogeneous Regions for Regional Precipitation Frequency Analysis

Hybrid Approach to Delineation of Homogeneous Regions for Regional Precipitation Frequency Analysis Regional frequency analysis of heavy precipitation amounts based on the estimation of the parameters of a regional distribution function using L-moments is adopted for the specific geographical-climatological settings of Slovakia. The paper focuses on the first step of the regional L-moment algorithm (Hosking, Wallis, 1997), which is the delineation of homogeneous regions. Objective and process-based logical pooling techniques are used to form homogeneous pooling groups of rainfall gauging stations for regional frequency analysis of k-day precipitation amounts (k = 1 to 5 days). Even though the delineation of homo-geneous regions by means of objective methods is generally accepted and recommended in the literature, it is concluded here that such a pooling of similar sites should not be carried out automatically in precipitation analysis. Instead, a combination of physical/geomorphological considerations and objective methods should be preferred. Hybridný prístup k vyčleneniu homogénnych regiónov pre regionálnu frekvenčnú analýzu úhrnov zrážok Článok sa zaoberá regionálnou frekvenčnou analýzou mimoriadnych úhrnov zrážok, ktorá je založená na odhade parametrov regionálneho rozdelenia pravdepodobnosti pomocou L-momentov a ktorá sa aplikuje v špecifických geograficko-klimatických podmienkach Slovenska. Článok je užšie zameraný na prvý krok tzv. regionálneho L-momentového algoritmu (Hosking, Wallis, 1997), ktorým je vyčlenenie homogénnych regiónov pre k-denné úhrny zrážok (k = 1 až 5). Na formovanie homogénnych zoskupení klimatologických a zrážkomerných staníc sa použila objektívna aj subjektívna (logická) metodika. Napriek tomu, že odborná literatúra všeobecne uznáva a odporúča použiť objektívne postupy na vyčlenenie homogénnych regiónov, v štúdii sme usúdili, že by sa vo frekvenčnej analýze úhrnov zrážok navzájom podobné stanice nemali vyčleńovať automaticky. Namiesto toho odporúčame, aby sa k tomuto účelu použila kombinácia objektívnych postupov, resp. úvah založených na fyzicko-geografických charakteristikách krajiny.

A regional comparative analysis of empirical and theoretical flood peak-volume relationships

Abstract This paper analyses the bivariate relationship between flood peaks and corresponding flood event volumes modelled by empirical and theoretical copulas in a regional context, with a focus on flood generation processes in general, the regional differentiation of these and the effect of the sample size on reliable discrimination among models. A total of 72 catchments in North-West of Austria are analysed for the period 1976–2007. From the hourly runoff data set, 25 697 flood events were isolated and assigned to one of three flood process types: synoptic floods (including long- and short-rain floods), flash floods or snowmelt floods (both rain-on-snow and snowmelt floods). The first step of the analysis examines whether the empirical peak-volume copulas of different flood process types are regionally statistically distinguishable, separately for each catchment and the role of the sample size on the strength of the statements. The results indicate that the empirical copulas of flash floods tend to be different from those of the synoptic and snowmelt floods. The second step examines how similar are the empirical flood peak-volume copulas between catchments for a given flood type across the region. Empirical copulas of synoptic floods are the least similar between the catchments, however with the decrease of the sample size the difference between the performances of the process types becomes small. The third step examines the goodness-of-fit of different commonly used copula types to the data samples that represent the annual maxima of flood peaks and the respective volumes both regardless of flood generating processes (the traditional engineering approach) and also considering the three process-based classes. Extreme value copulas (Galambos, Gumbel and Hüsler-Reiss) show the best performance both for synoptic and flash floods, while the Frank copula shows the best performance for snowmelt floods. It is concluded that there is merit in treating flood types separately when analysing and estimating flood peak-volume dependence copulas; however, even the enlarged dataset gained by the process-based analysis in this study does not give sufficient information for a reliable model choice for multivariate statistical analysis of flood peaks and volumes.

Climate Change Scenarios of Precipitation Extremes in the Carpathian Region Based on an Ensemble of Regional Climate Models

The study examines projected changes in precipitation extremes, aggregated on several time scales (1 hour, 1 day, and 5 days), in simulations of 12 regional climate models (RCMs) with high spatial resolution (~25 km). The study area is the Carpathian Basin (Central and Southeastern Europe) which has a complex topography and encompasses the whole territory of Slovakia and Hungary as well as major parts of Romania and western Ukraine. We focus on changes in mean seasonal maxima and high quantiles (50-year return values) projected for the late 21st century (time slice 2070–2099) in comparison to the control period (time slice 1961–1990), for summer and winter. The 50-year return values are estimated by means of a regional frequency analysis based on the region-of-influence method, which reduces random variability and leads to more reliable estimates of high quantiles. In winter, all examined characteristics of precipitation (seasonal totals, mean seasonal maxima, and 50-year return values for both short-term and multi-day aggregations) show similar patterns of projected increases for the late 21st century. In summer, by contrast, drying is projected for seasonal totals in all RCMs while increases clearly prevail for the 50-year return values. The projected increases are larger for short-term (hourly) extremes that are more directly related to convective activity than multiday extremes. This suggests that the probability of occurrence of flash floods may increase more than that of large-scale floods in a warmer climate. The within-ensemble variability (and associated uncertainty) is, nevertheless, much larger in summer than in winter.

PROJECTED CHANGES IN FLOOD-GENERATING PRECIPITATION EXTREMES OVER THE CZECH REPUBLIC IN HIGH-RESOLUTION REGIONAL CLIMATE MODELS

Projected Changes in Flood-Generating Precipitation Extremes Over the Czech Republic in High-Resolution Regional Climate Models Climate change scenarios of high quantiles of 5-day precipitation amounts (proxies for flood-generating events) over the Czech Republic are evaluated in an ensemble of high-resolution Regional Climate Model (RCM) simulations from the ENSEMBLES project. The region-of-influence method of the regional frequency analysis is applied as a pooling scheme. This means that for any single gridbox, a homogeneous region (set of gridboxes) is identified and data from that region are used when fitting the Generalized Extreme Value distribution. The climate change scenarios for the late 21st century (2070-2099) show widespread increases in high quantiles of 5-day precipitation amounts in winter, consistent with projected changes in mean winter precipitation. In summer, increases in precipitation extremes occur despite an overall drying (prevailing declines in mean summer precipitation), which may have important hydrological implications. The results for summer suggest a possible substantial change in characteristics of warm-season precipitation over Central Europe, with more severe dry as well as wet extremes. The spatial pattern of projected changes in summer precipitation extremes, with larger increases in the western part of the area and smaller changes towards east, may also point to a declining role of Mediterranean cyclones in producing precipitation extremes in Central Europe in a future climate. However, uncertainties of the climate change scenarios remain large, which is partly due to biases in reproducing precipitation characteristics in climate models, partly due to large differences among the RCMs, and partly due to factors that are poorly or not at all represented in the examined ensemble. The latter are related also to uncertainties in future emission scenarios and socio-economic development in general. Predpokladané Zmeny v Zrážkových Extrémoch Nebezpečných Pre Tvorbu Povodní na Území Českej Republiky v Regionálnych Klimatických Modeloch s Vysokým Priestorovým Rozlíšením Práca analyzuje scenáre klimatickej zmeny pre vysoké kvantily 5-denných úhrnov zrážok (ktoré predstavujú možné riziko z pohľadu tvorby povodňových udalostí) na území Českej republiky, a to na základe širšej množiny simulácií z regionálnych klimatických modelov (RCM) s vysokým priestorovým rozlíšením, dostupných z projektu ENSEMBLES. Kvantily zrážkových extrémov sa odhadujú na základe metódy vplyvného regiónu, ktorá je jedným z variantov regionálnej frekvenčnej analýzy. To znamená, že pre každý gridový bod sa identifikuje jedinečný homogénny región (t.j. množina ďalších gridových bodov) a zrážkové údaje dostupné zo všetkých gridových bodov v rámci daného regiónu sa zužitkujú v procese odhadovania kvantilov využitím zovšeobecneného extremálneho rozdelenia. Scenáre klimatickej zmeny pre obdobie posledných troch dekád 21. storočia (2070-2099) naznačujú rozsiahly nárast vysokých kvantilov 5-denných úhrnov zrážok počas zimy, čo je v súlade s predpokladanými zmenami v priemerných úhrnoch zrážok za zimu. V lete sa tiež očakáva zvýšenie extrémnych úhrnov zrážok, čo môže v súvislosti s predpokladaným všeobecným úbytkom zrážok v tomto období (t.j. napriek prevažujúcemu poklesu priemerných úhrnov zrážok v lete) viesť k vážnym hydrologickým následkom. Výsledky pre leto naznačujú zásadnú zmenu v režime úhrnov zrážok v strednej Európe v teplom období roka, spojenú s častejším výskytom nepriaznivých suchých aj vlhkých extrémov. Priestorové rozdelenie predpokladaných zmien v extrémnych úhrnoch zrážok za leto - s vyšším nárastom v západných častiach skúmanej oblasti a postupne menej výrazným nárastom smerom na východ - zrejme poukazuje na slabnúcu úlohu stredomorských cyklón pri tvorbe zrážkových extrémov v strednej Európe v nastávajúcich klimatických podmienkach. Treba však podotknúť, že neurčitosť scenárov klimatickej zmeny je stále veľká, a to jednak v dôsledku nepresností v reprodukcii charakteristík úhrnov zrážok v klimatických modeloch, ďalej kvôli významným rozdielom medzi jednotlivými RCM, a nakoniec aj v dôsledku klimatických faktorov, ktoré sú slabo reprezentované, prípadne nie sú vôbec zahrnuté v analyzovanej množine výstupov klimatických modelov. Spomínané klimatické faktory takisto závisia od emisných scenárov skleníkových plynov, resp. od socio-ekonomického vývoja ľudstva vo všeobecnosti.

On the use of the simple scaling of heavy rainfall in a regional estimation of IDF curves in Slovakia

On the use of the simple scaling of heavy rainfall in a regional estimation of IDF curves in Slovakia The aim of this study was to test the applicability of a simple scaling methodology for a regional estimation of intensity-duration-frequency (IDF) curves in Slovakia. The analysis is based on the regionalization process of Gaál (2006), which focused on the delineation of homogeneous regions for a regional frequency analysis of precipitation maxima. In order to examine the regionally estimated IDF curves, a region covering the western parts of Slovakia was chosen. The selected region, which encompasses 19 raingauging stations, may be characterized by the dominant influence of Atlantic circulation patterns. Three of the 19 stations belonging to the target region were set aside and flagged as verification stations. The regional dimensionless growth curve of 1-day precipitation maxima in the warm season was derived for the region, and the local T-year quantiles were estimated by the index value method for the stations. At the same time, a regionally averaged scaling exponent was derived using all the stations except for the three verification ones. The local IDF curves at the verification stations were estimated by downscaling the T-year quantiles of the 1-day precipitation maxima using the regionally averaged scaling exponent. Finally, the IDF curves for these stations were compared with those defined by Šamaj, Valovič (1973). This study is the first step in assessing the applicability of a simple scaling theory for the regional estimation of IDF curves in Slovakia. Využitie metódy jednoduchého škálovania extrémnych zrážok pri regionálnom odhade návrhových dažďov na Slovensku Cieľom tejto práce bolo otestovať možnosť použitia metódy jednoduchého škálovania zrážok pri regionálnom odhade návrhových hodnôt zrážkových intenzít na Slovensku. Pre analýzu bolo vybraných 19 zrážkomerných staníc vo vopred vyčlenenom regióne na území západného Slovenska, pričom tri stanice boli separované ako verifikačné stanice. Pre vybraný región sme odvodili bezrozmernú regionálnu čiaru prekročenia jednodenných maximálnych úhrnov zrážok v teplom polroku a následne sme pre verifikačné stanice metódou indexovej hodnoty odhadli lokálne návrhové hodnoty pre rôzne významné doby opakovania T (tzv. T-ročné kvantily). Vo vyčlenenom regióne sme určili priemerný regionálny škálovací koeficient. Návrhové zrážkové intenzity pre tri verifikačné stanice sa stanovili zoškálovaním T-ročných kvantilov jednodenných maximálnych zrážok pomocou regionálneho škálovacieho koeficientu. Určené návrhové hodnoty sme porovnali s výsledkami Šamaja, Valoviča (1973). Štúdia potvrdila možnosť využitia metódy jednoduchého škálovania na regionálny odhad návrhových hodnôt zrážkových intenzít na Slovensku.

Similarity of empirical copulas of flood peak-volume relationships: a regional case study of North-West Austria

Abstract This paper analyses the bivariate relationship between flood peaks and corresponding flood event volumes modelled by empirical copulas in a regional context in the North-West of Austria. Flood data of a total of 69 catchments in the region are analysed for the period 1976–2007. In order to increase the sample size and the homogeneity of the samples for the statistical analysis, 24872 hydrologically independent flood events were isolated and assigned to one of three flood process types: synoptic floods, flash floods or snowmelt floods in contrary to the more traditional engineering approach of selecting annual maxima of flood peaks and corresponding flood volumes. The first major part of the paper examines whether the empirical peak-volume copulas of different flood process types are statistically distinguishable, separately for each catchment. The results indicate that the empirical copulas of flash floods tend to be different from those of the synoptic and snowmelt floods in the target region. The second part examines how similar are the empirical flood peak-volume copulas between catchments for a given flood type. For the majority of catchment pairs, the empirical copulas of all flood types are indeed statistically similar. The flash floods show the largest degree of spatial heterogeneity. It is concluded that there is merit in treating flood types separately and in pooling events of the same type in a region when analysing and estimating flood peak-volume dependence copulas; however, the sample size of the analysed events is a limiting factor in spite of the introduced event selection procedure.

Process-based selection of copula types for flood peak-volume relationships in Northwest Austria: a case study

Abstract The case study aims at selecting optimal bivariate copula models of the relationships between flood peaks and flood volumes from a regional perspective with a particular focus on flood generation processes. Besides the traditional approach that deals with the annual maxima of flood events, the current analysis also includes all independent flood events. The target region is located in the northwest of Austria; it consists of 69 small and mid-sized catchments. On the basis of the hourly runoff data from the period 1976- 2007, independent flood events were identified and assigned to one of the following three types of flood categories: synoptic floods, flash floods and snowmelt floods. Flood events in the given catchment are considered independent when they originate from different synoptic situations. Nine commonly-used copula types were fitted to the flood peak - flood volume pairs at each site. In this step, two databases were used: i) a process-based selection of all the independent flood events (three data samples at each catchment) and ii) the annual maxima of the flood peaks and the respective flood volumes regardless of the flood processes (one data sample per catchment). The goodness-of-fit of the nine copula types was examined on a regional basis throughout all the catchments. It was concluded that (1) the copula models for the flood processes are discernible locally; (2) the Clayton copula provides an unacceptable performance for all three processes as well as in the case of the annual maxima; (3) the rejection of the other copula types depends on the flood type and the sample size; (4) there are differences in the copulas with the best fits: for synoptic and flash floods, the best performance is associated with the extreme value copulas; for snowmelt floods, the Frank copula fits the best; while in the case of the annual maxima, no firm conclusion could be made due to the number of copulas with similarly acceptable overall performances. The general conclusion from this case study is that treating flood processes separately is beneficial; however, the usually available sample size in such real life studies is not sufficient to give generally valid recommendations for engineering design tasks.