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.

We simulate the water balance dynamics of 308 catchments in Austria using a lumped conceptual model involving 11 calibration parameters. We calibrate and verify the model for two non-overlapping 11-year periods of daily runoff data. A comparison of the calibrated parameter values of the two periods suggests that all parameters are associated with some uncertainty although the degree of uncertainty differs between the parameters. The regional patterns of the calibrated parameters can be interpreted based on hydrological process reasoning indicating that they are able to represent the regional or large-scale differences in the hydrological conditions. Catchment attributes explain some of the spatial parameter variability with coefficients of determination of up to R 2 ¼ 0:27; but usually the R 2 values are lower. Parameter uncertainty does not seem to cloud the relationship between calibrated parameters and catchment attributes to a significant extent as suggested by an optimised correlation analysis. The median Nash -Sutcliffe efficiencies of simulating streamflow decrease from 0.67 to 0.63 when moving from the calibration to the verification period. This is a small decrease, which suggests that problems with overparameterisation of the model are unlikely. We then compare regionalisation methods for estimating the model parameters in ungauged catchments, in terms of the model performance. The best regionalisation methods are the use of the average parameters of immediate upstream and downstream (nested) neighbours and regionalisation by kriging. For the calibration period, the average decrease in the Nash-Sutcliffe model efficiency, as a result of the regionalisation, is 0.10 which is about twice the decrease of moving from the calibration to the verification period. The methods based on multiple regressions with catchment attributes perform significantly poorer. Apparently, spatial proximity is a better surrogate of unknown controls on runoff dynamics than catchment attributes. q

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

[1] We propose a framework for identifying types of causative mechanisms of floods. The types are long-rain floods, short-rain floods, flash floods, rain-on-snow floods, and snowmelt floods. We adopt a catchment perspective, i.e., the focus is on the catchment state and the atmospheric inputs rather than on atmospheric circulation patterns. We use a combination of a number of process indicators, including the timing of the floods, storm duration, rainfall depths, snowmelt, catchment state, runoff response dynamics, and spatial coherence. On the basis of these indicators and diagnostic regional plots we identify the process types of 11,518 maximum annual flood peaks in 490 Austrian catchments. Forty-three percent of the flood peaks are long-rain floods, only 3% are snowmelt floods, and the relative contribution of the types changes with the flood magnitude. There are pronounced spatial patterns in the frequency of flood type occurrence. For example, rain-on-snow floods most commonly occur in northern Austria. Runoff coefficients tend to increase with rainfall depth for long-rain floods but are less dependent of rainfall depth and exhibit much larger scatter for flash floods. All types exhibit seasonal patterns, both in terms of flood magnitudes and catchment altitudes of flood occurrence. The coefficient of variation (CV) of the flood samples stratified by process type decreases with catchment area for most process types with the exception of flash floods for which CV increases with catchment area.