Water companies in the United Kingdom have considered climate change in their water resources plans for more than a decade through studies funded by UK Water Industry Research (UKWIR). This paper presents an initial assessment of the impact of the UK Climate Projections 2009 (UKCP09) on river flows at a national scale for the 2020s under the A1B scenario and the implications for water resource planning. A daily hydrological modelling framework based on two conceptual model structures and the Generalized Likelihood Uncertainty Estimation (GLUE) methodology has been applied to 70 catchments across the UK. A Latin Hypercube Sampling approach was adopted to handle the probabilistic nature of UKCP09. Results show a decrease in mean annual flow over most of the UK, with negative median values of all monthly changes except in winter over the western and northern mountainous areas. Furthermore the results indicate a high likelihood of a significant decline in summer flows. An analysis of variance shows that the main uncertainty in river flow changes comes from the spread in climate projections. Finally results are found to be consistent with a previous UKWIR assessment based on individual projections from 6 Global Climate Models (GCMs) under the A2 scenario. The reduction in summer low-flows, critical for water resources, appears however more limited with UKCP09. Although most expected changes are within natural variability, the drier conditions overall and the greater spread of results with respect to previous assessments indicates a need for testing the robustness of water resource plans.

The NERC and CEH trademarks and logos ('the Trademarks') are registered trademarks of NERC in the UK and other countries, and may not be used without the prior written consent of the Trademark owner. Public water supply systems are designed to maintain water supply through 21 extended periods of dry weather without excessive cost or environmental 22 damage. During a drought, water suppliers can take further measures to 23 enhance supplies or reduce demand. The introduction of drought measures is 24 usually formalised in a drought plan, but there is often little evidence that the 25 plan will prove successful during a range of feasible droughts. As the climate 26 changes, recent hydrological data may be a poor guide to future drought, and 27 planned actions may prove insufficient to maintain adequate water supplies. 28 29 This paper describes a method for testing the resilience of water company 30 drought plans to droughts that are outside recent hydrological experience. 31Long severe droughts of the nineteenth century provide an opportunity to test 32 water supply system behaviour in a range of realistic droughts. The method 33 developed combines system modelling with an interactive approach that asks 34 water system managers to work through the actions that they would take at 35 different stages of the drought, without knowledge of subsequent drought 36 development. 37 38The approach was tested for two contrasting English water resource systems. 39In both cases, the existing water supply and drought planning measures 40 succeeded in maintaining water supply, but significant demand restrictions 41 and engineering measures had to be introduced. Wider use of the method by 42 water supply planners should allow the refinement of drought and water 43 supply plans, and will also create increased awareness of the actions 44 necessary to manage a range of droughts.

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