A warming climate is expected to have an impact on the magnitude and timing of river floods; however, no consistent large-scale climate change signal in observed flood magnitudes has been identified so far. We analyzed the timing of river floods in Europe over the past five decades, using a pan-European database from 4262 observational hydrometric stations, and found clear patterns of change in flood timing. Warmer temperatures have led to earlier spring snowmelt floods throughout northeastern Europe; delayed winter storms associated with polar warming have led to later winter floods around the North Sea and some sectors of the Mediterranean coast; and earlier soil moisture maxima have led to earlier winter floods in western Europe. Our results highlight the existence of a clear climate signal in flood observations at the continental scale.

Climate change has led to concerns about increasing river floods resulting from the greater water-holding capacity of a warmer atmosphere 1 . These concerns are reinforced by evidence of increasing economic losses associated with flooding in many parts of the world, including Europe 2 . 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 3 , 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 farsuggest 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 4,5 , suggesting that climatedriven changes are already happening and supporting calls for the consideration of climate change in flood risk management.River floods are among the most costly natural hazards. Global annual average losses are estimated at US$104 billion 6 and are expected to increase with economic growth, urbanization and climatic change 2,7 . Physical arguments of increased heavy precipitation resulting from the enhanced water-holding capacity of a warmer atmosphere and

No abstract

Abstract. 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.

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 online 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 focused on the 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. ARTICLE HISTORY

No abstract

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.

[1] We analyze the controls on flood duration based on the concept of comparative hydrology. Rather than modeling a single catchment in detail, we compare catchments with contrasting characteristics in order to understand the controls in a holistic way. We analyze the hydrographs of 9223 maximum annual flood events in 396 Austrian catchments ranging from 5 to $10,000 km 2 as a function of climatic controls such as storm type (synoptic and convective storms, rain-on-snow, snowmelt), and catchment controls such as soils, soil moisture, geology, and land form. The ratio of the flood volume and the flood peak is used as a measure of the flood duration or flood timescale. The results indicate that, spatially, the median flood timescales range from 16 h in the hilly catchments, where convective storms prevail, to 104 h in the lowland catchments where substantial inundation into the floodplain occurs. The range is even larger for different flood types, from 7 h for flash floods in the hilly catchments to 200 h for snowmelt floods in an Alpine area with deeply weathered rocks and deep soils. The results also indicate that the catchment area is not the most important control on the flood timescales. For the range of catchments considered here, climate is very important through storm type and antecedent soil moisture, and geology is very important through soil characteristics. The concept of comparative hydrology is also used to interpret the interplay of the processes controlling the flood duration at timescales from hours to millennia. It is argued that the flood timescale is a rich fingerprint of the hydrological processes in a catchment because it integrates a range of climate and catchment characteristics by a time parameter.