Nicholas S. Reynard

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 benefits of coupling between environmental model components. Results also illustrate that the coupling itself is not sufficient to address all known model issues. Priorities for future development of the UK Environmental Prediction framework and component systems are discussed.

Floods are one of the biggest natural hazards to society, and there is increasing concern about the potential impacts of climate change on flood occurrence and magnitude. Furthermore, flood risk is likely to increase in the future not just through increased flood occurrence, but also through socio-economic changes, such as increasing population. The extent to which adaptation measures can offset this increased risk will depend on the level of future climate change, but there exists an urgent need for information on the potential impacts of climate change on floods, so that these can be accounted for by flood management authorities and local planners aiming to reduce flood risk. Agencies across the UK have been pro-active in providing such guidance for many years and in refining it as the science of climate change and hydrological impacts has developed. The history of this guidance for fluvial flood risk in England is presented and discussed here, including the recent adoption of a regional risk-based approach. Such an approach could be developed and applied to flood risk management in other countries, and to other sectors affected by climate change.

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 storm water management policies increasingly aim to 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 potential impacts of climate change are an increasing focus of research, and ever‐larger climate projection ensembles are available, making standard impact assessments more onerous. An alternative way of estimating impacts involves response surfaces, which present the change in a given indicator for a large number of plausible climatic changes defined on a regular sensitivity domain. Sets of climate change projections can then be overlaid on the response surface and impacts estimated from the nearest corresponding points of the sensitivity domain, providing a powerful method for fast impact estimation for multiple projections and locations. However, the effect of assumptions necessary for initial response surface development must be assessed. This paper assesses the uncertainty introduced by use of a sensitivity framework for estimating changes in 20‐year return period flood peaks in Britain. This sensitivity domain involves mean annual and seasonal precipitation changes, and a number of simplifications were necessary for consistency and to reduce dimensionality. The effect of these is investigated for nine catchments across Britain, representing nine typical response surfaces (response types), using three sets of climate projections. The results show that catchments can have different causes of uncertainty and some catchments have an overall higher level of uncertainty than others. These differences are compatible with the underlying climatological and hydrological differences between the response types, giving confidence in generalization of the results. This enables the development of uncertainty allowances by response type, to be used alongside the response surfaces to provide more robust impact estimates. Copyright © 2013 John Wiley & Sons, Ltd.

Evaporation is an important component of the hydrological cycle. Potential evaporation (PE) from a vegetated surface is the amount of water that would be lost to the atmosphere were the supply unlimited; actual evaporation (AE) is a fraction of PE dependent on soil wetness. Many formulae exist for estimating PE from meteorological data. PE is usually a required input, with rainfall, for hydrological modelling, but PE accuracy is generally considered less important than rainfall accuracy for model performance. Few studies investigate historical evaporation trends in Britain, but generally indicate increases. Most studies presenting future PE projections for Britain indicate increased annual PE, but some suggest small decreases in some months. Limited consensus on the best formulae to derive PE projections from climate model data is further complicated by possible changes in plant behaviour (transpiration and growth) under higher carbon dioxide concentrations. Appropriate PE estimation could be particularly important in regions where precipitation and PE are in close balance, but PE uncertainty could be less important than climate model uncertainty for hydrological impacts. Further research is needed into which PE formulae are likely to be most reliable when applied with climate model data, and into climate change and plant feedbacks.