How might climate change affect river flows across the Thames Basin? An area-wide analysis using the UKCP09 Regional Climate Model ensemble

SummaryThe Thames Basin drains an area of over 10,000 km2 through London to the North Sea. It encompasses both rural and heavily urbanised areas overlying a spatially-varied and complex geology. Historically, the lower Thames has proved resilient to climate variability, and careful river management in recent years has helped protect the region from flooding. However, recent climate projections for the region indicate that over the next century winter rainfall might increase by 10–15%, potentially leading to higher flows than the Thames can accommodate. This study uses a distributed hydrological model, the Grid-to-Grid (G2G), to assess future changes in peak river flows for a range of catchments across the Thames Basin. The G2G model has used as input an ensemble from the UK Climate Projections (UKCP09) Regional Climate Model (RCM), under the A1B emissions scenario, to analyse changes in flood frequency between two 30-year time-slices (October 1960–September 1990 and October 2069–September 2099). The RCM ensemble uses a perturbed-parameter approach to address uncertainty in climate projections.Results indicate considerable spatial variation in projected changes in peak flows. Towards the downstream end of the fluvial Thames, the average estimated change in modelled 20-year return period flood peaks by the 2080s is 36% with a range of −11% to +68%, which is broadly in line with recent government guidance for the Thames Basin. A key question that arises is whether these estimated changes fall within the range of natural variability and would therefore be indistinguishable from the effects of typical weather patterns in the current climate. Comparison of the modelled changes in flood frequency with an RCM-based estimate of current natural variability shows that, whilst for some rivers (or parts of rivers) there are few changes outside the range of current natural variability, for other rivers there are more changes outside of this range. The latter locations could be considered as sites where further monitoring/modelling may provide early warning of statistically significant changes in observed flows, due to climate change.

► A distributed hydrological model (G2G) estimates projected future change in flows.
► Changes in flood frequency are estimated for rivers across the Thames Basin (UK).
► Most areas see increased flood peaks, with less change over chalk than elsewhere.
► Importance of including the effect of CO2 on stomatal closure and PE is shown.
► Areas where future change may exceed estimated natural variability are identified.