The primary objective of FD2020 ‘Regionalising the impacts of climate change on flood flows’ was to assess the suitability of current FCDPAG3 guidance given the advances in climate change science since its publication. PAG3 requires an allowance of 20% to be added to peak flows for any period between 2025 and 2115 for any location across Britain. This guidance was considered a precautionary value and its derivation reflected the evidence available at that time. FD2020 has been designed to increase this evidence base, and it is anticipated that the research will lead to the development of regional, rather than national, guidelines for changes to peak flows due to climate change.\ud \ud A scenario-neutral approach based on a broad sensitivity analysis to determine catchment response to changes in climate as chosen for FD2020. The method separates the climate change that a catchment may be exposed to (the hazard) from the catchment response (change in peak flows) to changes in the climate (the vulnerability). By combining current understanding of climate change likelihood (the ‘hazard’) with the vulnerability of a given catchment, it is possible to evaluate the risk of flood flow changes. \ud \ud The vulnerability of a catchment is characterised in two steps: first, the response of a set of catchments to a range of climatic changes is modelled, then analysed for similarity, and second the main responses are characterised according to catchment properties. This is achieved by defining a sensitivity framework of changes to the mean and seasonality of precipitation and temperature and modelling the response of each catchment within this fixed framework. \ud \ud This milestone report describes the first step of the vulnerability assessment. The changes in flood peaks for 154 catchments across Britain are modelled in a comprehensive ‘scenario-neutral’ sensitivity study based on 4,200 patterns of changes in rainfall, temperature and potential evaporation (PE). These changes were defined using a single harmonic function peaking in January for rainfall, and either in January or in August for temperature, with the range in mean annual change and seasonal change guided from latest IPCC-AR4 and PRUDENCE scenarios for the UK. The sensitivity analysis is composed of 525 rainfall patterns of change, combined with eight temperature and PE patterns of change.\ud \ud The harmonic functions lead to ‘smoothed’ monthly climate change factors, producing daily climate input series using the change factor method. These time series were input to the hydrological model to generate river flow time series. These ‘changed’ river flow time series were compared with those simulated from the observed climate series. Changes in the magnitude of flood peaks of 2, 10, 20 and 50-year return period were selected as the indicators of flood change. Because the sensitivity study scans systematically through a domain of possible changes in rainfall, temperature and PE, the results obtained are ‘scenario-neutral’. The percentage changes in the flood indicators are representative of the response of the catchment to a variety of different climates, and hence are not directly linked to specific climate change scenarios. \ud \ud For each catchment, these changes are organised according to the mean annual and seasonal change of rainfall for all eight temperature and PE scenarios: they form eight flood response patterns per flood indicator, i.e. the response of a catchment to a range of climatic changes.\ud \ud The analysis of all the individual flood response patterns identified nine flood response types for all flood indicators that can be described by five main families of behaviour: Neutral catchments, for which the changes in flood peak magnitude are of similar magnitude to the maximum change in monthly rainfall; damping catchments, which are relatively resilient to small changes in rainfall (3 subgroups); enhancing catchments, which are relatively vulnerable to small increases in rainfall (3 sub-groups); mixed catchments, which are both vulnerable and resilient to changes in rainfall, depending on the magnitude and seasonal pattern of the rainfall changes; and sensitive catchments, which are very vulnerable to most increases in rainfall.\ud \ud These nine flood response types fully describe the range of responses in the flood regime to climate change in Britain. They characterise the vulnerability of a catchment’s flood regime to changes in climate.\u
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