Region-of-influence approach to a frequency analysis of heavy precipitation in Slovakia

International audienceThe L-moment-based regionalization approach developed by Hosking and Wallis (1997) is a frequently used tool in regional frequency modeling of heavy precipitation events. The method consists of the delineation of homogeneous pooling groups with a fixed structure, which may, however, lead to undesirable step-like changes in growth curves and design value estimates in the case of a transition from one pooling group to another. Unlike the standard methodology, the region-of-influence (ROI) approach does not make use of groups of sites (regions) with a fixed structure; instead, each site has its own "region", i.e. a group of sites that are sufficiently similar to the site of interest. The aim of the study is to develop a version of the ROI approach, which was originally proposed in order to overcome inconsistencies involved in flood frequency analysis, for the modeling of probabilities of heavy precipitation amounts. Various settings of the distance metric and pooled weighting factors are evaluated, and a comparison with the standard regional frequency analysis over the area of Slovakia is performed. The advantages of the ROI approach are assessed by means of simulation studies. It is demonstrated that almost any setting of parameters of the ROI method yields estimates of growth curves and design values at individual sites that are superior to the standard regional and at-site estimates

Selection of intense rainfall events based on intensity thresholds and lightning data in Switzerland

This paper presents a method to identify intense warm season storms with convective character based on intensity thresholds and the presence of lightning, and analyzes their statistical properties. Long records of precipitation and lightning data at 4 stations and 10 min resolution in different climatological regions in Switzerland are used. Our premise is that thunderstorms associated with lightning generate bursts of high rainfall intensity. We divided all recorded storms into those accompanied by lightning and those without lightning and found the threshold <i>I</i>^* that separates intense events based on peak 10 min intensity <i>I</i>_p &ge; <i>I</i>^* for a chosen misclassification rate α. The performance and robustness of the selection method was tested by investigating the inter-annual variability of <i>I</i>^* and its relation to the frequency of lightning strikes. The probability distributions of the main storm properties (rainfall depth <i>R</i>, event duration <i>D</i>, average storm intensity <i>I</i>_a and peak 10 min intensity <i>I</i>_p) for the intense storm subsets show that the event average and peak intensities are significantly different between the stations. Non-parametric correlations between the main storm properties were estimated for intense storms and all storms including stratiform rain. The differences in the correlations between storm subsets are greater than those between stations, which indicates that care must be exercised not to mix events of different origin when they are sampled for multivariate analysis, for example, copula fitting to rainfall data

Estimation of The Relationship Between The Travel Time of Flood Peaks and Peak Discharge on The Poprad River by Multilinear Flood Routing

The empirical relationship between travel-time of flood peaks and peak discharge was studied on a reach of the Poprad River in Slovakia. The data were fitted by regression and compared with the expected shapes as described in the literature. Further a chain of linear segments has been considered as the model of that relation. The number of segments parameters and the angles between theses in this piecewise linear model were fitted by optimisation of a conceptual multilinear flood routing model performance on a large flood wave with the help of a genetic algorithm. In the setup of the multilinear model the travel-time parameter of the model was allowed to vary with discharge according to the piecewise linear model of the travel time of flood peaks. The discrete state space representation of the Kalinin-Miljukov model was used as the basis for a multilinear discrete cascade flood routing model. The resulting relationship was compared with empirical data on travel times and used to model the variability of the time parameter in the discrete state space representation of the Kalinin and Miljukov model on three verification floods. The modelling results showed that the inclusion of empirical information on the variability of the travel-time with discharge even from one flood enables satisfactory accuracy for the prediction of the flood propagation process

A process-based analysis of the suitability of copula types for peak-volume flood relationships

The work aims at analyzing the bivariate relationship between flood peaks and flood volumes, with a particular focus on the type and seasonality of flood generation processes. Instead of the usual approach that deals with an analysis of the annual maxima of flood events, the current analysis includes all independent flood events in a catchment. Flood events are considered independent when they originate from distinguishably different synoptic/meteorological situations. The target region is located in the northern part of Austria, and consists of 72 small and mid-sized catchments. On the basis of the discharge measurements with a time resolution of 1 h from the period 1976–2007, independent flood events were identified and were assigned to one of the three following flood generation type categories: synoptic floods, flash floods and snowmelt floods. These were subsequently divided into two seasons, thereby separating predominantly rainfall-fed and snowmelt-fed floods. Nine frequently-used copula types were locally fitted to the samples of the flood type and seasonal data. Their goodness-of-fit was examined locally as well as analyzed in a regional scope. It was concluded that (i) treating flood processes separately is beneficial for the statistical analysis; (ii) suitability patterns of acceptable copula types are distinguishably different for the seasons/flood types considered, (iii) the Clayton and Joe copulas shows an unacceptable performance for all the seasons/flood types in the region; (iv) the rejection rate of the other copula types depends on the season/flood type and also on the sample size; (v) given that usually more than one statistically suitable dependence model exists, an uncertainty analysis of the design values in the engineering studies resulting from the choice of model seems unavoidable; (vi) reducing uncertainty in the choice of model could be attempted by a deeper hydrological analysis of the dependence structure between flood peaks and volumes in order to give hydrological support to the decision on model's suitability in specific regions and for typical flood generation mechanisms

Understanding Flood Regime Changes in Europe: a state-of-the-art assessment

There is growing concern that flooding is becoming more frequent and severe in Europe. A better understanding of flood regime changes and their drivers is therefore needed. The paper reviews the current knowledge on flood regime changes in European rivers that has traditionally been obtained through two alternative research approaches. The first approach is the data-based detection of changes in observed flood events. Current methods are reviewed together with their challenges and opportunities. For example, observation biases, the merging of different data sources and accounting for nonlinear drivers and responses. The second approach consists of modelled scenarios of future floods. Challenges and opportunities associated with flood change scenarios are discussed such as fully accounting for uncertainties in the modelling cascade and feedbacks. To make progress in flood change research, we suggest that a synthesis of these two approaches is needed. This can be achieved by focusing on long duration records and flood-rich and flood-poor periods rather than on short duration flood trends only, by formally attributing causes of observed flood changes, by validating scenarios against observed flood regime dynamics, and by developing low-dimensional models of flood changes and feedbacks. The paper finishes with a call for a joint European flood change research network

Changing climate both increases and decreases European river floods

Climate change has led to concerns about increasing river floods resulting from the greater water-holding capacity of a warmer atmosphere1. These concerns are reinforced by evidence of increasing economic losses associated with flooding in many parts of the world, including Europe2. Any changes in river floods would have lasting implications for the design of flood protection measures and flood risk zoning. However, existing studies have been unable to identify a consistent continental-scale climatic-change signal in flood discharge observations in Europe3, because of the limited spatial coverage and number of hydrometric stations. Here we demonstrate clear regional patterns of both increases and decreases in observed river flood discharges in the past five decades in Europe, which are manifestations of a changing climate. Our results\u2014arising from the most complete database of European flooding so far\u2014suggest that: increasing autumn and winter rainfall has resulted in increasing floods in northwestern Europe; decreasing precipitation and increasing evaporation have led to decreasing floods in medium and large catchments in southern Europe; and decreasing snow cover and snowmelt, resulting from warmer temperatures, have led to decreasing floods in eastern Europe. Regional flood discharge trends in Europe range from an increase of about 11 per cent per decade to a decrease of 23 per cent. Notwithstanding the spatial and temporal heterogeneity of the observational record, the flood changes identified here are broadly consistent with climate model projections for the next century4,5, suggesting that climate-driven changes are already happening and supporting calls for the consideration of climate change in flood risk management

Land use change impacts on floods at the catchment scale: Challenges and opportunities for future research

Research gaps in understanding flood changes at the catchment scale caused by changes in forest management, agricultural practices, artificial drainage and terracing are identified. Potential strategies in addressing these gaps are proposed, such as complex systems approaches to link processes across time scales, long-term experiments on physical-chemical-biological process interactions, and a focus on connectivity and patterns across spatial scales. It is suggested that these strategies will stimulate new research that coherently addresses the issues across hydrology, soil and agricultural sciences, forest engineering, forest ecology and geomorphology

Changing climate both increases and decreases European river floods

Climate change has led to concerns about increasing river floods resulting from the greater water-holding capacity of a warmer atmosphere. These concerns are reinforced by evidence of increasing economic losses associated with flooding in many parts of the world, including Europe. Any changes in river floods would have lasting implications for the design of flood protection measures and flood risk zoning. However, existing studies have been unable to identify a consistent continental-scale climatic-change signal in flood discharge observations in Europe, because of the limited spatial coverage and number of hydrometric stations. Here we demonstrate clear regional patterns of both increases and decreases in observed river flood discharges in the past five decades in Europe, which are manifestations of a changing climate. Our results—arising from the most complete database of European flooding so far—suggest that: increasing autumn and winter rainfall has resulted in increasing floods in northwestern Europe; decreasing precipitation and increasing evaporation have led to decreasing floods in medium and large catchments in southern Europe; and decreasing snow cover and snowmelt, resulting from warmer temperatures, have led to decreasing floods in eastern Europe. Regional flood discharge trends in Europe range from an increase of about 11 per cent per decade to a decrease of 23 per cent. Notwithstanding the spatial and temporal heterogeneity of the observational record, the flood changes identified here are broadly consistent with climate model projections for the next century, suggesting that climate-driven changes are already happening and supporting calls for the consideration of climate change in flood risk management

Twenty-three unsolved problems in hydrology (UPH) – a community perspective

This paper is the outcome of a community initiative to identify major unsolved scientific problems in hydrology motivated by a need for stronger harmonisation of research efforts. The procedure involved a public consultation through on-line media, followed by two workshops through which a large number of potential science questions were collated, prioritised, and synthesised. In spite of the diversity of the participants (230 scientists in total), the process revealed much about community priorities and the state of our science: a preference for continuity in research questions rather than radical departures or redirections from past and current work. Questions remain focussed on process-based understanding of hydrological variability and causality at all space and time scales. Increased attention to environmental change drives a new emphasis on understanding how change propagates across interfaces within the hydrological system and across disciplinary boundaries. In particular, the expansion of the human footprint raises a new set of questions related to human interactions with nature and water cycle feedbacks in the context of complex water management problems. We hope that this reflection and synthesis of the 23 unsolved problems in hydrology will help guide research efforts for some years to come