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There are concerns that recent climate change is altering the frequency and magnitude of river floods in an unprecedented way. Historical studies have identified flood-rich periods in the past half millennium in various regions of Europe. However, because of the low temporal resolution of existing datasets and the relatively low number of series, it has remained unclear whether Europe is currently in a flood-rich period from a long-term perspective. Here we analyse how recent decades compare with the flood history of Europe, using a new database composed of more than 100 high-resolution (sub-annual) historical flood series based on documentary evidence covering all major regions of Europe. We show that the past three decades were among the most flood-rich periods in Europe in the past 500 years, and that this period differs from other flood-rich periods in terms of its extent, air temperatures and flood seasonality. We identified nine flood-rich periods and associated regions. Among the periods richest in floods are 1560–1580 (western and central Europe), 1760–1800 (most of Europe), 1840–1870 (western and southern Europe) and 1990–2016 (western and central Europe). In most parts of Europe, previous flood-rich periods occurred during cooler-than-usual phases, but the current flood-rich period has been much warmer. Flood seasonality is also more pronounced in the recent period. For example, during previous flood and interflood periods, 41 per cent and 42 per cent of central European floods occurred in summer, respectively, compared with 55 per cent of floods in the recent period. The exceptional nature of the present-day flood-rich period calls for process-based tools for flood-risk assessment that capture the physical mechanisms involved, and management strategies that can incorporate the recent changes in risk., This work was supported by the ERC Advanced Grant ‘FloodChange’ project (no. 291152), the Horizon 2020 ETN ‘System Risk’ project (no. 676027), the DFG project FOR 2416, the FWF projects I 3174 and W1219-N22, the Spanish Agency of Science and FEDER/UE projects CGL2016-75475/R, CGL2017-86839-C3-1-R, CGL2016-75996-R and CTM2017-83655-C2-2-R, the ICREA Academia programme, and project CZ.02.1.01/0.0/0.0/16_019/0000797, Ministry of Education, Youth and Sports of the Czech Republic.

There is a need to estimate design floods for areal planning and the design of important infrastructure. A major challenge is the mismatch between the length of the flood records and needed return periods. A majority of flood time series are shorter than 50 years, and the required return periods might be 200, 500, or 1,000 years. Consequently, the estimation uncertainty is large. In this paper, we investigated how the use of historical information might improve design flood estimation. We used annual maximum data from four selected Norwegian catchments, and historical flood information to provide an indication of water levels for the largest floods in the last two to three hundred years. We assessed the added value of using historical information and demonstrated that both reliability and stability improves, especially for short record lengths and long return periods. In this study, we used information on water levels, which showed the stability of river profiles to be a major challenge.

This study compiles a new dataset, consisting of the longest available flow series from across Europe, and uses it to study the spatial and temporal clustering of flood events across the continent. Hydrological series at 102 gauging stations were collected from 25 European countries. Five geographically distinct large-scale homogeneous regions are identified: (i) an Atlantic region, (ii) a Continental region, (iii) a Scandinavian region, (iv) an Alpine region, and (v) a Mediterranean region. The months with a higher likelihood of flooding were identified in each region. The analysis of the clustering of annual counts of floods revealed an over-dispersion in the Atlantic and Continental regions, forming flood-rich and flood-poor periods, as well as an under-dispersion in the Scandinavian region that points to a regular pattern of flood occurrences at the inter-annual scale. The detection of trends in flood series is attempted by basing it on the identified regions, interpreting the results at a regional scale and for various time periods: 1900-1999; 1920-1999; 1939-1998 and 1956-1995. The results indicate that a decreasing trend in the magnitude of floods was observed mainly in the Continental region in the period 1920-1999 with 22% of the catchments revealing such a trend, as well as a decreasing trend in the timing of floods in the Alpine region in the period 1900-1999 with 75% of the catchments revealing this trend. A mixed pattern of changes in the frequency of floods over a threshold and few significant changes in the timing of floods were detected. (C) 2015 Elsevier B.V. All rights reserved.

Based on downscaled daily precipitation values from the global climate model of the Max Planck Institute in Hamburg, time series of 20 years have been generated to describe the current climate of 1980–1999 (control data) and the future climate of 2030–2049 (scenario data) for Norway. These time series serve as training data for the Randomised Bartlett-Lewis Rectangular Pulse Model (RBLRPM), a precipitation simulation model, and time series of 1000 years length have been generated to assess possible changes in the extreme precipitation regime due to climate change. The analysis of changes in extreme value patterns for annual and seasonal values in the scenario and control data sets shows tendencies towards increased extreme values and seasonal shifts for the scenario period. A general increase in mean and standard deviation of the extreme value sample and for values of 10 and 100 years return period is found, although the regional variability is significant. For some regions the increase is in the order of 10 to 50 % for both annual and seasonal values.

In glacier-dominated catchments, glaciers have an important effect on the water balance. It is important to understand the glacier control on inflow in order to assess historic trends and future scenarios. The mass balance of a 100 km2 glacier sub-basin of Svartisen ice cap, northern Norway, was reconstructed for 1917 to 1995. The reconstruction was carried out using three different methods, the hydrological method, the correlation between mass balance and meteorological observations and a precipitation-degree-day model. Calibration data were derived from field observations, map comparisons and correlation of observations at different glaciers. The reconstructed series showed a total loss of ∼3 × 109 m3 of water, most of which occurred between 1920 and 1950. After 1950, the net balance increased gradually and is currently near equilibrium with the present climate. Suggested climate scenarios for this region gave a loss between zero and 5 ×109 m3 of water until the year 2050.

Climate change impacts on hydrological processes in Norway have been estimated through combination of results from the IPCC SRES A2 and B2 emission scenarios, global climate models from the Hadley Centre and the Max-Planck Institute, and dynamical downscaling using the RegClim HIRHAM regional climate model. Temperature and precipitation simulations from the regional climate model were transferred to meteorological station sites using two different approaches, the delta change or perturbation method and an empirical adjustment procedure that reproduces observed monthly means and standard deviations for the control period. These climate scenarios were used for driving a spatially distributed version of the HBV hydrological model, yielding a set of simulations for the baseline period 1961– 1990 and projections of climate change impacts on hydrological processes for the period 2071–2100. A comparison between the two methods used for transferring regional climate model results to meteorological station sites is provided by comparing the results from the hydrological model for basins located in different parts of Norway. Projected changes in runoff are linked to changes in the snow regime. Snow cover will be more unstable and the snowmelt flood will occur earlier in the year. Increased rainfall leads to higher runoff in the autumn and winter.

A distributed version of the HBV-model using 1 km2 grid cells and daily time step was used to simulate runoff from the entire land surface of Norway for the period 1961-1990. The model was sensitive to changes in small scale properties of the land surface and the climatic input data, through explicit representation of differences between model elements, and by implicit consideration of sub-grid variations in moisture status. A geographically transferable set of model parameters was determined by a multi-criteria calibration strategy, which simultaneously minimised the residuals between model simulated and observed runoff from 141 Norwegian catchments located in areas with different runoff regimes and landscape characteristics. Model discretisation units with identical landscape classification were assigned similar parameter values. Model performance was evaluated by simulating discharge from 43 independent catchments. Finally, a river routing procedure using a kinematic wave approximation to open channel flow was introduced in the model, and discharges from three additional catchments were calculated and compared with observations. The model was used to produce a map of average annual runoff for Norway for the period 1961-1990. Keywords: distributed model, multi-criteria calibration, global parameters, ungauged catchments.

The paper focuses on floods in Europe derived from documentary evidence during the past millennium. The beginnings of early-instrumental and systematic-instrumental hydrological observations in Europe are discussed. Basic sources of documentary evidence related to floods and the problems that may be encountered in the use of historical source materials are described. An overview of documentary-based flood research in the form of long-term frequency and severity series, as well as a summary of the most disastrous floods in the Mediterranean, Western, Central and Northern Europe, is presented. The role and value of floods described in the documentary record to better understanding current flood risks in Europe are discussed.