Probabilistic assessment of the impact of climate change on UK river flows Jean-Philippe Vidal and Steven Wade, HR Wallingford Introduction
UKWIR06 scenarios
Although there is now strong evidence of the intensification of the global hydrological cycle, there is still considerable uncertainty related to the impact of climate change on UK river flow regime. This work updates previous research on the impacts of climate change on UK river flows (UKWIR, 2002) by incorporating uncertainties in the global model configuration and in the hydrological modelling process.
Effect of Climate Change on River Flows and Groundwater Recharge, A Practical Methodology The overall aim of this project funded by UK Water Industry Research and the Environment Agency is to “produce a robust methodology which water companies can use for climate change impacts assessment and which has the support of the Regulator”. This three-year research programme is to be completed in 2007.
Six scenarios of changes in precipitation and potential evapotranspiration (PET) have been derived from outputs of 6 GCMs listed in Table 1 and run under the A2 emissions scenario for the 2020s. The method leading to the UKWIR06 scenarios builds on a gridded data set for the UK (Perry and Hollis, 2005) and consists of the following steps described by Vidal and Wade (submitted): 1. Building appropriate precipitation and temperature time series from land areas covered by GCM sea cells; 2. Correction of GCM outputs inherent biases through “quantile-based mapping” (Wood et al., 2004);
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3. Disaggregation of bias-corrected outputs with monthly spatial anomalies between GCM-specific and observed spatial scales.
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Figure 1 and 2 shows respectively percent change for precipitation and absolute change in temperature as obtained from the 6 GCMs ensemble mean values.
+ 22 %
+ 1.6 oC + 20 % + 18 %
+ 1.5 oC
+ 16 %
+ 1.4 oC
+ 14 % + 12 % May
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+ 1.3 oC
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+ 10 % +8%
Potential evapotranspiration was obtained through a simple and efficient formula derived by Oudin et al. (2005). Individual GCM-time series were then used for hydrological modelling of 70 UK catchments. In addition, monthly means for precipitation and PET were computed to develop a new set of climate scenarios for the 2020s that are referred to as the ‘UKWIR06 scenarios’. They constitute an ensemble of climate change scenarios whose range gives an indication of the uncertainty on global model configuration and provide a better basis for water resources planning than the use of a single GCM or Regional Climate Model e.g. the UKCIP02 scenarios (Hulme, et al., 2002).
+ 1.2 oC
+6% o
+4%
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Table 1. GCMs included in the UKWIR06 scenarios. TCR (Transient Climate response) shows the increase in global temperature as response to a standardised 1%/year increase in CO2 at the time of CO2 doubling (IPCC, 2001).
+2%
+ 1 oC 0 −2 % Sep
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−4 %
Model
Research Centre
HadCM3
Hadley Centre for Climate Prediction and Research, UK
2.0
CGCM2
Canadian Center for Climate Modelling and Analysis, Canada
1.92
CSIRO-mk2
Commonwealth Scientific and Industrial Research Organisation, Australia
2.0
GFDL-R30
Geophysical Fluid Dynamics Laboratory, USA
1.96
CCSR/NIES
Center for Climate System Research / National Institute for Environmental Studies, Japan
+ 0.9 oC Sep
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−6 %
+ 0.8 C
−8 %
+ 0.7 oC −10 % −12 %
+ 0.6 oC
−14 %
Figure 1. Ensemble mean changes in monthly mean precipitation between 2011-2040 and 1961-1990.
Hydrological modelling
Figure 2. Ensemble mean changes in monthly mean temperature between 2011-2040 and 1961-1990.
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Catchmod, a semi-distributed conceptual model currently promoted as a standard by the Environment Agency (EA, 2002).
The GLUE (Generalised Likelihood Uncertainty Estimation) methodology (Beven and Binley, 1992) was used with initial samples of 10000 parameter sets. Behavioural models were retained on the basis of the reproduction of the observed 1961-1990 daily flows.
30 %
20 %
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Aug 10 %
For each catchment, a seasonal analogue method was used to derive GCMspecific future daily time series for precipitation and PET, which served as inputs to the behavioural models. Distributions of flow factors were finally computed by weighting each ratio future/present monthly mean by the likelihoods of the behavioural models. The expected change shown in Figure 3 for the Itchen@AllbrookHighbridge, a very permeable catchment in South-East England, is a significant increase in flow seasonality. For the Ribble@Arnford, a mountainous and flashy catchment in NorthWest England, Figure 4 shows an increase in Winter with large uncertainties, and a decrease in late Summer and early Autumn.
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Figure 3.
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Ribble@Arnford (71011)
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Figure 5 presents the median values of percent changes in monthly mean flows for the 70 modelled catchments. It shows an overall small increase of Winter flows and a much larger decrease in late Spring and Summer. The picture for Autumn is largely dominated by a decrease of flows in South-east England and no change or a limited increase in the West of the country.
Project outputs
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Monthly flows (m3/s)
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90% confidence intervals 50% confidence intervals median
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90% confidence intervals 50% confidence intervals median
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Figure 5. Median values of monthly flow factors for the 2020s, in percent of 1961-1990 values.
Monthly flows (m3/s)
PDM (Probability Distributed Model), a lumped conceptual model (Senbeta et al., 1999),
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Itchen@AllbrookHighbridge (42010) Jan
Seventy UK catchments were modelled with two different model structures:
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TCR (°C)
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Reports UKWIR (forthcoming), Guidelines for Resource Assessment and UKWIR06 Scenarios. UKWIR Report 06/CL/04/8. UKWIR (2006), Interim Report on Rainfall-Runoff Modelling. UKWIR Report 06/ CL/04/7. ISBN 1-84057-421-6 UKWIR (2006), A Strategy for Evaluating Uncertainty in Assessing the Impacts of Climate Change on Water Resources. UKWIR Report 05/CL/04/6. ISBN 1-84057-396-1 UKWIR (2005), Trends in UK River Flows 1970-2002. UKWIR Report 05/CL/04/5. ISBN 1-84057-387-2 UKWIR (2005), Use of Climate Change Scenario Data at a Catchment Level. UKWIR Report 05/CL/04/3. ISBN 1-84057-373-2
Data
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UKWIR06 scenarios: sets of Rainfall and PET monthly factors for every CAMS (Catchment Abstraction Management Strategy) in England and Wales and WFD river basin in Scotland and Northern Ireland UKWIR06 factors: probabilistic monthly flow factors for 70 UK catchments
Spreadsheet tools
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Selection of precipitation and PET changes for a given catchment Selection of flow factors corresponding to a given probability for modelled catchments Multiple linear regression for estimating monthly flow factors for unmodelled catchments from seasonal climate factors and baseflow index
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References Beven K. J. and Binley A. (1992), The future of distributed models: model calibration and uncertainty prediction. Hydrological Processes, 6 (3): 279-298. EA (2002), Water Resources modelling Strategy and Programme, Catchmod User Manual. May 2002.
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Figure 4. Confidence intervals for mean monthly flows for the 2020s (grey bars) compared to 1961-1990 mean monthly flows (blue lines).
IPCC (2001). Climate Change 2001: The Scientific Basis. Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change [Houghton J. T., Ding Y., Griggs D. J., Noguer M., van der Linden P. J., Dai X., Maskell K., Johnson C. A. (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, 881pp. Oudin L., Hervieu F., Michel C., Perrin C., Andréassian V., Anctil F., and Loumagme C. (2005), Which potential evapotranspiration input for a lumped rainfall-runoff model? Part 2 – Towards a simple and efficient potential evapotranspiration model for rainfall–runoff modelling. Journal of Hydrology 303(1-4): 290-306 Perry M. and Hollis D. (2005). The generation of monthly gridded datasets for a range of climatic variables over the UK. International Journal of Climatology 25(8): 1041-1054.
Contact Jean-Philippe Vidal
Senbeta D. A., Shamsedin A. Y., and O’Connor K. M. (1999), Modification of the probability-distributed interacting storage capacity model. Journal of Hydrology 224(3-4): 149-168.
HR Wallingford, Howbery Park, Wallingford, Oxfordshire OX10 8BA, UK Email:
[email protected], telephone +44 (0)1491 835381
UKWIR (2002), Effect of Climate Change on River Flows and Groundwater Recharge UKCIP 02 Scenarios. UKWIR Report 03/CL/04/2. ISBN 1-84057-286-8
www.hrwallingford.co.uk
Vidal J.-P. and Wade S. D. A framework for developing high-resolution multi-model climate projections: 21st century scenarios for the UK. Submitted to International Journal of Climatology. Wood A. W. , Leung L. R., Sridhar V., Lettenmaier D. P. (2004). Hydrologic implications of dynamical and statistical approaches to downscaling climate model outputs. Climatic Change 62(1-3): 189-216.
Acknowledgements: This research was supported by a European Commission Marie Curie Research Fellowship and additional research funding from UK Water Industry Research Ltd and the UK Environment Agency (project CL\04\C).
