Julia Hall

Changing climate shifts timing of European floods

Flooding along the river Will a warming climate affect river floods? The prevailing sentiment is yes, but a consistent signal in flood magnitudes has not been found. Blöschl et al. analyzed the timing of river floods in Europe over the past 50 years and found clear patterns of changes in flood timing that can be ascribed to climate effects (see the Perspective by Slater and Wilby). These variations include earlier spring snowmelt floods in northeastern Europe, later winter floods around the North Sea and parts of the Mediterranean coast owing to delayed winter storms, and earlier winter floods in western Europe caused by earlier soil moisture maxima. Science, this issue p. 588 see also p. 552 Climate change is affecting the timing of river flooding across Europe. 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.

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.

Climate-driven trends in mean and high flows from a network of reference stations in Ireland

Abstract This paper introduces a reference hydrometric network for Ireland and examines the derived flow archive for evidence of climate-driven trends in mean and high river flows. The Mann-Kendall and Theil-Sen tests are applied to eight hydroclimatic indicators for fixed and variable (start and end date) records. Spatial coherence and similarities of trends with rainfall suggest they are climate driven; however, large temporal variability makes it difficult to discern widely-expected anthropogenic climate change signals at this point in time. Trends in summer mean flows and recent winter means are at odds with those expected for anthropogenic climate change. High-flow indicators show strong and persistent positive trends, are less affected by variability and may provide earlier climate change signals than mean flows. The results highlight the caution required in using fixed periods of record for trend analysis, recognizing the trade-off between record length, network density and geographic coverage. Editor Z.W. Kundzewicz; Associate editor H. Lins Citation Murphy, C., Harrigan, S., Hall, J., and Wilby, R.L., 2013. Climate-driven trends in mean and high flows from a network of reference stations in Ireland. Hydrological Sciences Journal, 58 (4), 755–772.

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

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.

Robust adaptation assessment – climate change and water supply

Purpose – This paper aims to develop a framework to assist the identification of robust adaptation options that account for uncertainty in future climate change impacts for the water sector.Design/methodology/approach – The water evaluation and planning (WEAP) tool, is to identify future water resource vulnerability in the Glore sub‐catchment within the Moy catchment in the West of Ireland. Where water stress is evident, a detailed hydrological modelling approach is developed to enable an assessment of the robustness to uncertainty of future adaptation decisions. WEAP is coupled with a rainfall runoff model (hydrological simulation model), and forced using climate scenarios, statistically downscaled from three global climate models to account for the key sources of uncertainty. While hydrological models are widely applied, they are subject to uncertainties derived from model structure and the parameterisation of the catchment. Here, random sampling of key parameters is employed to incorporate uncertainty ...

Variability of seasonal floods in the Upper Danube River basin

Abstract The objective of this study is to analyse the spatial variability of seasonal flood occurrences in the Upper Danube region for the period 1961-2010. The analysis focuses on the understanding of the factors that control the spatial variability of winter and summer floods in 88 basins with different physiographic conditions. The evaluation is based on circular statistics, which compare the changes in the mean date and in the seasonal flood concentration index within a year or predefined season. The results indicate that summer half-year and winter half-year floods are dominant in the Alps and northern Danube tributaries, respectively. A comparison of the relative magnitude of flood events indicates that summer half-year floods are on average more than 50% larger than floods in winter. The evaluation of flood occurrence showed that the values of seasonal flood concentration index (median 0.75) in comparison to the annual floods (median 0.58) shows higher temporal concentration of floods. The flood seasonality of winter events is dominant in the Alps; however, along the northern fringe (i.e. the Isar, Iller and Inn River) the timing of winter half-year floods is diverse. The seasonal concentration of summer floods tends to increase with increasing mean elevation of the basins. The occurrence of the three largest summer floods is more stable, i.e. they tend to occur around the same time for the majority of analysed basins. The results show that fixing the summer and winter seasons to specific months does not always allow a clear distinction of the main flood generation processes. Therefore, criteria to define flood typologies that are more robust are needed for regions such as the Upper Danube, with large climate and topographical variability between the lowland and high elevations, particularly for the assessment of the effect of increasing air temperature on snowmelt runoff and associated floods.

Against a ‘wait and see’ approach in adapting to climate change

Simulations of future climate change impacts are highly uncertain, particularly for catchment hydrology, where output from models of complex dynamic systems (global climate) are used as inputs to models of complex dynamic systems (hydrology models). This is problematic where decision-making for adaptation is underpinned by future climate predictions, and where policy-makers have opted to delay adaptation until either uncertainties are reduced, or climate change signals emerge from observations. This paper, using the Boyne catchment in the east of Ireland as a case study, discusses the uncertainties involved in climate change impact assessment for catchment hydrology and highlights why uncertainties are unlikely to be constrained or reduced in the time-scale required for adaptation. In addition, by calculating the time required for climate change signals to emerge from the observational record and the magnitude of change required for detection, it is highlighted that waiting for climate signals to be statistically detectable is not an option for effective adaptation. The paper concludes by considering how a paradigm shift in how we use the output from climate impact assessments can progress the adaptation agenda given the limits to prediction identified.

Detection of trends in magnitude and frequency of flood peaks across Europe

ABSTRACT This study analyses the differences in significant trends in magnitude and frequency of floods detected in annual maximum flood (AMF) and peak over threshold (POT) flood peak series, for the period 1965–2005. Flood peaks are identified from European daily discharge data using a baseflow-based algorithm and significant trends in the AMF series are compared with those in the POT series, derived for six different exceedence thresholds. The results show that more trends in flood magnitude are detected in the AMF than in the POT series and for the POT series more significant trends are detected in flood frequency than in flood magnitude. Spatially coherent patterns of significant trends are detected, which are further investigated by stratifying the results into five regions based on catchment and hydro-climatic characteristics. All data and tools used in this study are open-access and the results are fully reproducible.

Spatial Patterns and Characteristics of Flood Seasonality in Europe

In Europe, floods are typically analysed within national boundaries and it is not well understood how the characteristics of local floods fit into a continental perspective. To gain a better understanding at the continental-scale, this study analyses 10 seasonal flood characteristics across Europe for the period of 1960-2010. The timing within the year of annual maximum discharges or water levels of 4105 stations from a European flood database is analysed. A cluster analysis is performed to identify regions with different flood seasons. The clusters are determined using the monthly relative frequencies of the annual maxima, and are further analysed to determine the temporal flood characteristics of each region and the European-wide patterns of bimodal and unimodal flood seasonality distributions. 15 Below 60° latitude, the mean timing of floods of individual stations transitions from winter floods in the West to spring floods in the East. Summer floods occurring in mountainous areas interrupt this West to East transition. Above 60° latitude, spring floods are dominant, except for coastal areas in which autumn and winter floods are observed. The temporal concentration of flood occurrences around a specific time of the year is highest in North-Eastern Europe, with most of the floods being concentrated within 1-2 months. The cluster analysis suggests that six regions with geographically distinct flood seasonality 20 distributions exist. Most of the stations (~73%) with more than 30 years of data exhibit a unimodal flood seasonality distribution (one or more consecutive months with high flood occurrence). Few stations (~3%), mainly located on the foothills of mountainous areas, have a clear bimodal distribution. Overall, the geographical location of a station in Europe can give an indication of its flood characteristics throughout the year and is more relevant than catchment area and outlet elevation for the observed flood seasonality. 25 Hydrol. Earth Syst. Sci. Discuss., https://doi.org/10.5194/hess-2017-649 Manuscript under review for journal Hydrol. Earth Syst. Sci. Discussion started: 15 November 2017 c