RCM rainfall for UK flood frequency estimation. II. Climate change results

Abstract

The first part of this two-part paper demonstrated the feasibility of the direct use of data from a high resolution (25 km) Regional Climate Model (RCM) to provide inputs for a rainfall-runoff model, in order to obtain estimates of flood frequency. This paper uses data from a climate change experiment with the same RCM (HadRM3H) to provide estimates of change in flood frequency between the 1970s and 2080s, for 15 catchments across Great Britain. This experiment is a rerun, at twice the horizontal resolution, of one of those used by the UK Climate Impacts Programme (UKCIP) in the construction of its UKCIP02 climate change scenarios for the UK. It thus allows an exploration of the implication of these scenarios using a resolution more suited to the detailed hydrological application addressed in this paper. Despite decreases in annual average rainfall in all but one catchment, eight show an increase in flood frequency at most return periods whereas two show substantial decreases. As part I of this paper showed a distinct positive correlation between errors in annual rainfall and errors in flood frequency, the fact that flood frequency can increase despite an overall decrease in rainfall implies a marked change in the distribution of rainfall, either in terms of the probability of rainfall events and/or its seasonal cycle. Decreases in flood peaks are shown for a number of the catchments in the south and east of England, despite an increase in winter mean and extreme rainfall. Increased summer and autumn soil moisture deficits are thought to be the reason for this. Other catchments, further north or west, show an increase in flood peaks, in some cases of over 50% at the 50-year return period. Care needs to be taken when interpreting these results, as they are based on a single RCM experiment (using driving data from one GCM under a single emissions scenario). Other RCM experiments may give quite different results, and ensemble runs would ideally be required to limit sample error.

Introduction

It is widely believed, and quoted in the media, that climate change will result in increased flooding in many areas across the globe, and indeed that we have already experienced such an effect. A number of studies have used data from Global Climate Models (GCMs) in some way, to demonstrate such increased flood risk, e.g. Milly et al., 2002, Ministry of Agriculture, Fisheries and Food 2001, global; Mirza et al. (2003), Bangladesh; Muller-Wohlfeil et al. (2000), Germany; Reynard et al. (2001), UK; Roy et al. (2001), Canada; Schreider et al. (2000), Australia; Tung (2001), China.

Previous research carried out in the UK, using GCM-implied changes applied (in a number of ways) to baseline observed rainfall and potential evaporation (Reynard et al., 1999, Reynard et al., 2001) to generate inputs for a semi-distributed continuous simulation rainfall-runoff model, led to Defra's current policy guidance that flood defence scheme appraisal should include a sensitivity study where peak flows are increased by 20% for the 2050s (MAFF 2001). However, the coarse spatial scale of GCMs and their lower reliability at a fine temporal scale, particularly for precipitation, limits their usefulness for small-scale flood studies, unless skilful fine-scale spatial and/or temporal details can be added to their predictions.

The recent advent of Regional Climate Models (RCMs), limited area models nested within GCMs, has greatly improved spatial representation, and brings outputs much closer to the scale required for reliable flood modelling. A new set of climate scenarios for the UK were recently released by the UK Climate Impacts Programme (Hulme et al., 2002, hereafter referred to as UKCIP02). These scenarios are based on a UK Met Office Hadley Centre RCM (HadRM3) with a grid size of ∼50 km across Europe, and are provided under four emissions scenarios, which relate to those of the IPCC (2000).

Crooks and Reynard (2002) repeat the methods presented in Reynard et al. (2001) for the Thames, but using the new UKCIP02 high emissions scenario for the 2080s, and the results show a decrease in flood peaks, which is contrary to the results using older scenarios. This is despite an increase in winter mean and extreme rainfall predicted in this region of the UK by the RCM (winter being the main flood season). It is thought that the increased temperatures and decreased rainfall in summer and autumn are the reason for this apparent disparity, as these cause increased soil moisture deficits leading into winter, which have to be reduced to zero before flooding can occur. However, the adjustment of baseline rainfall by factors derived from RCMs is not ideal, as the results depend on the way in which the monthly factors are applied to the daily rainfall series (Crooks et al., 1996, Prudhomme et al., 2002, Reynard et al., 2001).

Ideally it would be possible to use RCM outputs directly in rainfall-runoff models, but there has traditionally been more trust in the size and direction of changes than in the magnitudes of RCM (or GCM) output. However, Kay et al. (2005) demonstrated the feasibility of the direct use of RCM data for flood frequency estimation by use of output from a 25 km RCM for the UK driven with boundary conditions derived from re-analysis runs of the European Centre for Medium-Range Forecasting (ECMWF) global model for the period 1979–1993 (ERA-15). As observations of surface and atmospheric weather elements are continuously assimilated into the latter model, the output from the RCM can be expected to match the observed evolution of the weather through this period to a reasonable extent. Use of this ERA-driven RCM data to derive inputs for a rainfall-runoff model can then be assessed against the use of observed inputs, and the results demonstrate that such an RCM has relatively good ability to reproduce flood frequency curves as estimated using observed input data.

This paper will use the same spatially-generalised version of the Probability Distributed Model (PDM; Moore, 1985, Moore, 1999) for the same 15 UK catchments as in Kay et al. (2005), to simulate flows at the catchment outlet using catchment-average rainfall and potential evaporation inputs. However, in this case the inputs will be derived from two 30-year experiments with the 25 km RCM, one a simulation of the recent past centred on the 1970s and the other a prediction of a future climate centred on the 2080s (Table 1). This experiment is a rerun, at twice the horizontal resolution, of one of those used in UKCIP02. It thus allows an exploration of the implication of these scenarios using a resolution more suited to the detailed hydrological application addressed in this paper. Flood frequency curves will be derived from each 30-year time-series of simulated flows, using a standard peaks-over-threshold method, to demonstrate the change in flood frequency that could be expected in future.