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UK CLIMATE PROJECTIONS Future changes to river flood flows in England and Wales Author/Organisation Bill Donovan, Environment Agency, Karl Hardy, Defra and Nick Reynard, Centre for Ecology and Hydrology, Wallingford What is the overall aim of the work? Current Defra/EA guidance requires all flood management plans to allow for climate change by incorporating within a sensitivity analysis an increase in river flows of up 20% over the next 100 years. Although the 20% figure is a memorable precautionary target, there is the risk that it leads to a significant under- or over-estimating of the future risk. The objective of this research is to use the latest evidence, including UKCP09, to re-consider the existing guidance and provide evidence for policy makers to help set new allowances depending on catchment vulnerability. Which data source(s) of UKCIP08 were used? • 11-member RCM What dummy outputs were used? • Observed climate datasets • 11-member RCM Why were these outputs used? The 11 member RCM output was used as this is the first available UKCP09 dataset. It allowed us to consider both changes to monthly mean rainfall, but also the potential changes to the structure of rainfall e.g. changes to the number of rainfall days or changes to the intensity of rainfall in the future. How were the outputs used? The project took a different approach to the normal climate change impact assessment. Rather than taking data from the climate projections to drive a local impact model to determine changes to future flood flows, we explored how arbitary climate changes affected a large number of catchments’ response. This provides us with sensitivity surfaces describing the climate/catchment dynamic in a ‘scenario neutral’ way. This approach is helping us to evaluate which 1 UKCIP08?: Worked examples – Assess adaptation measures catchments are most vulnerable to change and if there are similar patterns in different catchments’ responses. We have then taken the 11 member RCM data and considered where on the sensitivity surface for each catchment these different climate model realisations occur. This helps us understand the likely range of change that these climate models are suggesting. As well as the 11 member RCM data we have also considered all the Global Climate Model (GCM) data from the IPCC 4AR approved model set to evaluate what change they indicate with respect to the sensitivity surfaces. This helps us determine if the potential range of change and where the 11 member RCM set sit compared to the GCM set. This approach of working from arbitary change through the Hadley RCM set to the widest set of different GCM models gives us a broad picture of the vulnerability of a catchment and the potential impact on river flows and some idea of the likelihood of that change occuring. This will provide us with important evidence to reconsider our current guidance and climate change allowances. • Location chosen — 150 catchments across England, Wales and Scotland broadly representative of the different catchments types and locations across these countries. • Climate variable chosen — rainfall and temperature to drive calibrated hydrological models and determine changes to river flows • Averaging period (eg Summer, January) — mean monthly changes, but we also considered the changes to the intra-monthly pattern of rainfall • Emission scenario chosen — RCM ensemble only provides one emission scenarios, but we have also considered four emissions scenarios from the wider IPCC 4AR GCM data set. • Time Period chosen — historic data and future data especially for 2050s and 2080s periods. Describe any difficulties or limitations associated with using probabilistic information such as UKCIP08. The lack of potential evapotranspiration projections in the probabilistic projections is a limiting factor We are not clear yet how we will make best use of the weather generator data in comparison to the RCM data, because of the large spatial information we need to consider, e.g. rainfall over a large catchment How would you communicate the results to your target audience? We are using a graphical representation of the sensitivity space for each catchment with RCM information overlaid to illustrate the potential changes river flood flows. See diagram opposite. 2 UKCIP08?: Worked examples – Assess adaptation measures Change to the 1:20 year river flow for the River Trent. The plot shows changes to flows resulting to changes to mean annaul precipitation (y-axis) against seasonality — ratio of winter rainfall to summer rainfall (x-axis). The coloured bands are change to river flow in 10% increments, grey is no change or reduction. The dots represent the current projections from all IPCC 4AR models (black), UKCIP02 (red) and UKCP09 (blue). What other UKCIP08 information (if any) not included in the dummy ouputs would you have liked to have used? We intend to consider rainfall data from the weather generator when this is available. Links to sources of more information http://www.defra.gov.uk/Environ/Fcd/Research/ Contact Details Bill Donovan, Environment Agency: [email protected] Karl Hardy, Defra Flood Management: [email protected] Nick Reynard, CEH: [email protected] 3