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This report has been produced to summarise the results of the floods and droughts research infrastructure scoping study for the interested community.

In this report we document work aimed at improving the quality of the representation of land and rivers in multi-year, coupled atmosphere-land (UM-JULES) simulations over the British Isles. The approach taken was to use standalone (uncoupled) simulations of JULES to investigate the potential to improve the coupled system. The use of alternative soil ancillary information, generated by using a data assimilation framework and observations of soil moisture from the COSMOS-UK network to optimise the constants in a pedotransfer function, was found to result in improved simulations by JULES of river flow in a diverse sample of British rivers (as measured by standard statistics). The revised soil parameters tended to increase the variability of the simulated river on short timescales, and reduce variability on the annual timescale. In catchments with a large influence of slow baseflow the revised parameters tended to give poorer simulations, with too much variability on short timescales. A new representation of groundwater processes was implemented in JULES and applied, for the first time, across Great Britain. This was shown to allow an influence of groundwater on nearsurface hydrology and fluxes over large parts of the country. Modelled river flows were more realistic in many cases, though much of the improvement was due to differences in the representation of runoff generation rather than the introduction of groundwater. A more physically-complete parameterisation of river physics, using the local inertial equation, was applied in a range of catchments, again for the first time. Simulated river flows were improved in most cases. The final section of this report offers a perspective on how terrestrial hydrology and its impacts could be considered in the next generation of UK Climate Projections. The work reported here has significantly improved our capability in several areas of land surface modelling, to the extent that new developments could be tested in nationwide sim

A ‘roadmap’ for the future of UK flood hydrology over the next 25 years has been published, based on a wide-ranging and inclusive co-creation process involving more than 270 individuals and 50 organisations from different sectors and disciplines. This paper highlights key features of the roadmap and its development as a community-owned initiative. The roadmap's relationship with hydrological research and practice is discussed, as is its context within the wider flood risk management innovation landscape, including funding. While the paper has a focus on UK flood hydrology, reflecting the scope of the roadmap, it is also considered in the context of advances in hydrology internationally.

Flooding is a recurring challenge across Malaysia, causing loss of life, extensive disruption and having a major impact on the economy. A new collaboration between Malaysia and UK, supported by the Newton-Ungku Omar Fund, aims to address a critical and neglected aspect of large-scale flood risk assessment: the representation of damage models, including exposure, vulnerability and inundation. In this paper we review flood risk and impact across Malaysia and present an approach to integrate multiple sources of information on the drivers of flood risk (hazard, exposure and vulnerability) at a range of scales (from household to national), with reference to past flood events. Recent infrastructure projects in Malaysia, such as Kuala Lumpur’s SMART Tunnel, aim to mitigate the effects of flooding both in the present and, ideally, for the foreseeable future. Our collaborative project aims to develop and assess a new multi-scale model of flood risk in Malaysia for current and projected future scenarios, and to address climate adaptation questions of policy relevance for flood stakeholders. This approach will enable us to identify the preferred adaptation pathways given multiple scenarios of climate and socio-economic change in Malaysia, and, beyond the life of our project, internationally

It is hypothesized that more accurate prediction and warning of natural hazards, such as of the impacts of severe weather mediated through various components of the environment, require a more integrated Earth System approach to forecasting. This hypothesis can be explored using regional coupled prediction systems, in which the known interactions and feedbacks between different physical and biogeochemical components of the environment across sky, sea and land can be simulated. Such systems are becoming increasingly common research tools. This paper describes the development of the UKC2 regional coupled research system, which has been delivered under the UK Environmental Prediction Prototype project. This provides the first implementation of an atmosphere–land–ocean–wave modelling system focussed on the United Kingdom and surrounding seas at km-scale resolution. The UKC2 coupled system incorporates models of the atmosphere (Met Office Unified Model), land surface with river routing (JULES), shelf-sea ocean (NEMO) and ocean waves (WAVEWATCH III). These components are coupled, via OASIS3-MCT libraries, at unprecedentedly high resolution across the UK within a north-western European regional domain. A research framework has been established to explore the representation of feedback processes in coupled and uncoupled modes, providing a new research tool for UK environmental science. This paper documents the technical design and implementation of UKC2, along with the associated evaluation framework. An analysis of new results comparing the output of the coupled UKC2 system with relevant forced control simulations for six contrasting case studies of 5-day duration is presented. Results demonstrate that performance can be achieved with the UKC2 system that is at least comparable to its component control simulations. For some cases, improvements in air temperature, sea surface temperature, wind speed, significant wave height and mean wave period highlight the potential benef

Flooding is a very costly natural hazard in the UK and is expected to increase further under future climate change scenarios. Flood defences are commonly deployed to protect communities and property from flooding, but in recent years flood management policy has looked towards solutions that seek to mitigate flood risk at flood-prone sites through targeted interventions throughout the catchment, sometimes using techniques which involve working with natural processes. This paper describes a project to provide a succinct summary of the natural science evidence base concerning the effectiveness of catchment-based ‘natural’ flood management in the UK. The evidence summary is designed to be read by an informed but not technically specialist audience. Each evidence statement is placed into one of four categories describing the nature of the underlying information. The evidence summary forms the appendix to this paper and an annotated bibliography is provided in the electronic supplementary material.

Understanding the rainfall-runoff behaviour of urban land surfaces is an important scientific and practical issue as stormwater management policies increasingly aimto manage flood risk at local scales within urban areas, whilst controlling the quality and quantity of runoff that reaches receiving water bodies. By reviewing field measurements reported within the literature on runoff, infiltration, evaporation and storage on common urban surfaces, this study describes a complex hydrological behaviour with greater rates of infiltration than often assumed,contradicting a commonly adopted, but simplified classification of the hydrological properties of urban surfaces. This shows that the term impervious surface, or impermeable surface, referring to all constructed surfaces (e.g. roads, roofs, footpaths, etc.) is inaccurate and potentially misleading. The hydrological character of urban surfaces is not stable through time, with both short seasonal) and long term (decadal) changes in hydrological behaviour, as surfaces respond to variations in seasonal characteristics and degradation in surface condition. At present these changing factors are not widely incorporated into hydrological modelling or urban surface water management planning, with static values describing runoff and assumptions of imperviousness often used. Developing a greater understanding of the linkages between urban surfaces and hydrological behaviour will improve the representation of diverse urban landscapes within hydrological models.

The aim of this section is to report on the current state of environmental observations related to natural hazards and identify the future requirements. Natural hazards can result in harm to human health and loss of life. They can also destroy livelihoods and cause significant environmental and economic damage, particularly to infrastructure. Hazards include floods, storm waves, coastal erosion, drought, heat waves, cold weather, windstorms, wildfires, pests and diseases, air quality, ground instability (e.g. landslides and subsidence), ash, gas and aerosols from overseas volcanic eruptions and space weather.

This report describes the results of a project to investigate the development of plausible high-end climate change scenarios. It covers the following climate hazards: heat waves, cold snaps, low and high rainfall, droughts, floods and windstorms. The scope of the project does not extend into defining the consequences of these hazards such as mortality, property damage or impacts on the natural environment. The scenarios created for this report are referred to as H++ scenarios, and are typically more extreme climate change scenarios on the margins or outside of the 10th to 90th percentile range presented in the 2009 UK climate change projections (also known as ‘UKCP09’).

Water-related issues are the most prominent amongst the commonly identified impacts of climate change. In particular, water-related extremes, storms, floods and droughts, are the likely most immediate and damaging manifestations of a changing climate. This chapter outlines how research into climate change impacts developed at CEH, introduces the key issues that have been encountered over the years and presents some examples of impact assessments. Moreover, the 11 global models were shown to reproduce relatively well the regional development of flood and drought episodes, which means they are useful tools to explore the impact of climate change on the hydrology at the regional to continental scale, not possible from catchment-scale models. The challenges of the future are many; climate change poses huge risks. Scientists have the responsibility to provide the best possible assessment of the science and associated risks to a, sometime, sceptical public and media.

Water related issues are prominent amongst the commonly identified impacts of climate change. The issue of uncertainty remains highly pertinent, particularly for projections of changes in climate and the subsequent impact on hydrological extremes. An alternative to the traditional top-down impact studies are the bottom-up, vulnerability-based, approaches, which focus on sensitivity testing and adaptation options rather than climate change projections. A case study of such an approach to inform climate change policy development for flood management in Great Britain is described in this chapter. To assess the impact of climate change on hydrological droughts a multi-model experiment was undertaken, including seven global impact models (GIMs) driven by climate data from five global climate models (GCMs). This study demonstrates that different representations of terrestrial water cycle processes in GIMs can be responsible for considerable uncertainty in the response of hydrological drought to climate change.

During the winter of 2013/14, much of the UK experienced repeated intense rainfall events and flooding. This had a considerable impact on property and transport infrastructure. A key question is whether the burning of fossil fuels is changing the frequency of extremes, and if so to what extent. We assess the scale of the winter flooding before reviewing a broad range of Earth system drivers affecting UK rainfall. Some drivers can be potentially disregarded for these specific storms whereas others are likely to have increased their risk of occurrence. We discuss the requirements of hydrological models to transform rainfall into river flows and flooding. To determine any general changing flood risk, we argue that accurate modelling needs to capture evolving understanding of UK rainfall interactions with a broad set of factors. This includes changes to multiscale atmospheric, oceanic, solar and sea-ice features, and land-use and demographics. Ensembles of such model simulations may be needed to build probability distributions of extremes for both pre-industrial and contemporary concentration levels of atmospheric greenhouse gases.

Nick Reynard, of the UK’s Centre for Ecology and Hydrology, underlines the need to foster research as regards natural hazard risks, in order to ensure policy makers are informed.

The Environment Agency’s Water Resources Strategy (March 2009) states: “Climate change will affect the amount of rain that supports river flows and replenishes groundwater, and when it does so it will also influence the demand for water and its quality, as well as the way land is used – all of which will put pressure on water resources.” This statement summarises the often conflicting pressures on the management of the water resources system and the additional stress that climate change could place on this. However, the UK water resources sector is undoubtedly one of the more advanced sectors in terms of addressing the potential consequences of future climate change. Indeed, it is virtually unique world-wide in explicitly incorporating climate change into the water resource management process. Both the water industry and regulators have a sound understanding of the potential risks and have provided quantitative evidence of the potential climate-change risks to their business. This understanding has fed into methodologies for incorporating climate change within water resources planning, for both water supply and demand, for the last three Periodic Reviews. These include guidelines from regulators on appropriate methods and data sources and the results of climate change modelling within water company plans. While the majority of the quantitative work has focused on water supply, the water resources sector also needs to understand the potential risks to water demand and water quality, and some quantification of the potential risk to these from climate change have been considered within water resources planning.

The study presents a novel framework for undertaking climate change impact studies, which can be used for testing the robustness of precautionary climate change allowances. It explores the relationship between climate change impacts on peak flows and catchment characteristics in a scenario-neutral framework. A national climate change sensitivity domain, defined to encapsulate magnitude and seasonality of changes in precipitation and temperature in Great Britain as currently projected by the Global Climate Models developed for the Fourth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC-AR4), serves as a framework for an extensive impact study on 154 catchments. The response of the catchments to these changes are analysed for a selected flood indicator, the 20-year return period flood (RP20), and mapped into the climate change domain, thus defining a vulnerability pattern for each catchment. When compared with the likelihood of future climate change scenarios (the climate ‘hazard’), an estimation of the climate change risk can be made easily. The probabilistic risk of exceeding a specific design allowance, such as an allowance of a 20% increase in flood peak, can be assessed for individual catchments or by regions. Using projections of 16 IPCC-AR4 GCMs for the 2080s based on the A1B emission scenarios, the study showed that 10.9% of the projections suggest RP20 will increase by at least 20% for the Enrick at Mill Tore, and across GB, 15% of the projections generate a RP20 increase of 20% or greater for at half of the 154 study catchments. Such conclusion can help evaluate the robustness of the tested allowance.

The possible effects of changing climate on a southern and a north-eastern English river (the Thames and the Yorkshire Ouse, respectively) were examined in relation to water and ecological quality throughout the food web. The CLASSIC hydrological model, driven by output from the Hadley Centre climate model (HadCM3), based on IPCC low and high CO2 emission scenarios for 2080 were used as the basis for the analysis. Compared to current conditions, the CLASSIC model predicted lower flows for both rivers, in all seasons except winter. Such an outcome would lead to longer residence times (by up to a month in the Thames), with nutrient, organic and biological contaminant concentrations elevated by 70-100% pro-rata, assuming sewage treatment effectiveness remains unchanged. Greater opportunities for phytoplankton growth will arise, and this may be significant in the Thames. Warmer winters and milder springs will favour riverine birds and increase the recruitment of many coarse fish species. However, warm, slow-flowing, shallower water would increase the incidence of fish diseases. These changing conditions would make southern UK rivers in general a less favourable habitat for some species of fish, such as the Atlantic salmon (Salmo salar). Accidental or deliberate, introductions of alien macrophytes and fish may change the range of species in the rivers. In some areas, it is possible that a concurrence of different pressures may give rise to the temporary loss of ecosystem services, such as providing acceptable quality water for humans and industry. An increasing demand for water in southern England due to an expanding population, a possibly reduced flow due to climate change, together with the Water Framework Directive obligation to maintain water quality, will put extreme pressure on river ecosystems, such as the Thames.

Global warming is predicted to cause significant changes to the world’s climate, but uncertainties remain about the precise nature of these changes. This is particularly true with regard to possible changes at a regional or local level and to changes in the climate extremes that produce catchment flooding. Such changes might include more frequent short-duration, high-intensity rainfall or more frequent periods of long-duration, sustained rainfall of the type responsible for the Autumn 2000 floods. To address the uncertainty surrounding this issue, DEFRA guidance on flood defence scheme appraisal currently suggests sensitivity allowances for climate change, for example a 20% increase in peak flows over the next 50 years. The need to further develop this policy and guidance on climate change impacts is being informed by improved modelling capabilities and climate change scenarios. For example, statistical rainfall models and fine resolution regional climate model data are now available to drive the hydrological models used to investigate the potential impacts of climate change on flood flows. CEH-Wallingford has been commissioned by DEFRA and the Environment Agency to use these data sets and models to assess the impacts and this paper describes the final project results for a range of catchments across the UK.

The manner in which regulators apply environmental risk assessment to their decisions on managing risk is changing. Expectations of risk assessment work are becoming clearer, the social issues agenda is having an impact on risk assessment practice, and there is a trend toward harmonizing approaches to the treatment of environmental risk. For risk analysts, the multiplicity of environmental problems is providing opportunities for the transfer of expertise between the different contexts of applying environmental risk assessment. With the latter as a focus, we summarize recent policy developments in the United Kingdom and illustrate how Government guidance on environmental risk assessment and management is being implemented. We emphasize the need for proportionality in risk analysis, the targeting of regulatory effort to risk, and the explicit treatment of uncertainty. These developments are contributing toward better "risk-informed" environmental decisions in which risk analysis plays an important part alongside other considerations. The forward agenda is likely to see further practical integration between technical risk issues and economic and social concerns, and the positioning of environmental risk assessment within a broader landscape of decision-making tools.