UKWIR Technical Note hnical Note

Dec 1, 2006 - GCMs included in the UKWIR06 scenarios. ..... HR Wallingford (Project Manager, Steven Wade), supported by Entec, Met Office and Lancaster ...
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UKWIR Technical Note

December 2006

Water resources planning in an uncertain world UKWIR/Environment Agency Research Project

‘Effect of climate change on river flows and groundwater recharge: A practical methodology’ A succession of dry winters and an extended warm summer in 2006 highlighted the importance of understanding the natural variation of climate and the value of methods and tools for assessing the impacts of climate on water resources. As well as dealing with drought conditions water companies need to plan ahead, considering the impacts of both climate variation and climate change in their water resources plans. Companies are now preparing draft plans for the 2010/11 to 2035/36 period that will make use of a number of outputs from this research project. The project outputs include new climate scenarios, revised methods and new tools that are being made available to water resources planners so that they can consider changes in river flows and groundwater recharge as part of their long term business plans. These build on previous UKWIR research but consider a much wider range of climate models and technical approaches in order to meet the needs of both companies and the Environment Agency.

Using the best scientific evidence

better basis for water resources planning than the use of a single GCM or Regional Climate Model e.g. the UKCIP02 scenarios.

In previous water resources plans, climate change impacts assessment was based on a single climate model and three emissions scenarios that provided similar impacts on river flows and recharge. This research used the outputs of six Global Climate Models (Table 1) and a range of new techniques for downscaling climate models to the catchment scale and considering the additional uncertainties related to hydrological models.

Hydrological modelling

Monthly means for precipitation and PET were computed from individual GCM outputs to develop a new set of climate scenarios for the 2020s that are referred to as the ‘UKWIR06 scenarios’. The range of outputs gives an indication of the uncertainty in global warming and provides a much

Seventy UK catchments were modelled with two different model structures: • PDM (Probability Distributed Model), a lumped conceptual model • Catchmod, a semi-distributed conceptual model that is used widely by the Environment Agency. In addition, research methods for dealing with hydrological uncertainty were applied to the catchment models. This enabled the calculation of distributions of flow factors based on the ratios of future/present monthly mean flow that took account of both different climate models and large numbers of hydrological models. continued on page 2

Table 1. GCMs included in the UKWIR06 scenarios. TCR (Transient Climate response) shows the increase in global temperature as response to a standardized 1%/year increase in CO2 at the time of CO2 doubling Model

Research Centre

HadCM3

Hadley Centre for Climate Prediction and Research, UK

CGCM2 CSIRO-mk2

Canadian Center for Climate Modelling and Analysis, Canada 1.92 Commonwealth Scientific and Industrial Research 2.0 Organisation, Australia Geophysical Fluid Dynamics Laboratory, USA 1.96

GFDL-R30

TCR (°C) 2.0

ECHAM4/OPYC3 Max Planck Institute für Meteorologie, Germany

1.4

CCSR/NIES

3.1

Center for Climate System Research / National Institute for Environmental Studies, Japan

Pr oject highlights Project The overall aim of the project is to ‘produce a robust methodology which water companies can use for climate change impacts assessment and which has the support of the Regulator’. Based on the three simple principles of using the best science, developing practical approaches and involving stakeholders, the project has developed revised guidelines and spreadsheet tools to improve water resources planning.

These include: • a framework for climate change

impacts assessment that allows companies to use different ‘flow factor’ and ‘rainfall-runoff’ methods depending on water resource zone characteristics • the UKWIR06 flow factors. A new set

of catchment ‘flow factors’ that consider six Global Climate Models (GCMs) and uncertainty related to hydrological modelling. A simple method was developed to derive flow factors based on climate and catchment characteristics for catchments that were not modelled as part of the project • the UKWIR06 climate change

scenarios. A new set of climate change scenarios that provide changes in rainfall and PET for every Catchment Abstraction Management Strategy (CAMS) basin in England and Wales and every Water Framework Directive (WFD) basin in Scotland and Northern Ireland • A new set of seasonal and annual

recharge estimates for selected catchments in England and a new framework and tools for assessing the impacts of climate change on groundwater recharge and levels. a number of technical reports, guidance and spreadsheet tools are now available from www.ukwir.org.uk and www.futuredrought.org.uk.

UK Water Industry Research Limited, 1 Queen Anne's Gate, London SW1H 9BT, UK tel: +44 (0) 20 7344 1807 fax: +44 (0) 20 7344 1859 email: [email protected], web: www.ukwir.org

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The expected changes in monthly flows for the Itchen at Highbridge, a very permeable catchment, are shown in Figure 1. There is a significant change in seasonal variation of flows. For the Ribble at Arnford, a mountainous and flashy catchment in NorthWest England, Figure 2 shows an increase in winter flows with large uncertainties, and a decrease in late summer and early autumn.

Each method for climate change impacts assessment involves ‘perturbing’ the historic data with change factors, which simply means multiplying rainfall, PET, river flow or recharge by monthly factors that are provided on a set of ‘simple to use’ spreadsheets. There are three main approaches that break down further into seven methods as follows:

Figure 3 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 SouthEast England and no change or a limited increase in the west of the country.

1. Flow Factor methods a) Application of the UKWIR06 Flow Factors for study catchments. Perturbation of historic river flows b) Estimation of flow factors for other catchments based on the UKWIR06 scenarios. Perturbation of historic river flows c) Application of re-sampling methods to river flows (for catchments with short flow records) prior to perturbation using flow factors. 2. Rainfall-Runoff methods a) Perturbation of precipitation and PET series based on UKCIP02 scenarios (optional) b) Perturbation of precipitation and PET series based on UKWIR06 scenarios.

Practical approaches The ‘Guidelines Report’ sets out a framework and ‘how to’ guidance on applying the new flow factors and climate change scenarios. Activity charts are provided to map out the different methods and project data sets and offer some suggestions of ‘where and when’ different approaches are appropriate.

Figure 1. Monthly flow regime for the Itchen at Highbridge, as observed during the 1961-1990 period (blue lines) and predicted for the 2020s (grey boxes)

Figure 4 provides an example of an activity chart, mapping out different steps and approaches to impacts assessment using flow factors.

The UKWIR06 Flow Factors Flow factor methods allow the rapid determination of the impacts of climate change on monthly river flows. Their advantage is that they consider climate change and hydrological uncertainties in a format that can be efficiently applied to any UK catchment. However, they should not be used if a suitable catchment model is available. Figure 5 shows an example of the flow factors using direct outputs of the project’s hydrological modelling, or a simplified approach, which allows the user to select the amount of climate change uncertainty considered in the impacts assessment.

Figure 2. Monthly flow regime for the Ribble at Arnford. Notation are the same as for Figure 1 [email protected] (71011)

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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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Monthly flows (m3/s)

Monthly flows (m3/s)

3. Advanced methods including the application of weather generators

[email protected] (42010)

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c) Re-sampling of climate data to extend records and account for natural variability prior to application of climate factors.

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Figure 3. Median values of per cent changes in mean monthly flows for 70 modelled catchments Jan

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Is there an appropriate catchment model?

Was the catchment modelled in this project 20 %

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Yes FLOW FACTOR METHOD 1a Apply flow/recharge factors for modelled catchments

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Use hydrological or hydrogeological models

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No FLOW FACTOR METHOD 1b Apply ‘regionalised’ flow/recharge factors

Simplified Method Option to use modelled percentiles or ‘simplified method’ based on mean change and standard deviation

FLOW FACTOR METHOD 1c Apply re-sampling techniques Calculate flow/recharge indicators Select flow/recharge time series

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More detailed work-select appropriate methods?

Water Resource Planning processes (See Environment Agency guidelines)

Does climate change trigger major investment?

No

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Figure 4. Climate change impacts assessment based on the application of the UKWIR06 Flow Factors

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Consult with EA

‘Park’ impacts assessment

GR1 is a statistical technique based on long term rainfall and groundwater level records, GR2 is a lumped calculation that uses catchment averaged hydraulic variables and GR3 applies GCM derived rainfall and PET factors to existing groundwater models. Worked examples of GR3 have been undertaken in two geographical areas – the River Test in Hampshire and the River Thet in Norfolk, and Figure 8 shows typical output from the River Test groundwater model. It shows a modelled increase in groundwater level under the HadCM3 GCM scenario, distributed across the catchment, for a period of low groundwater level. The GR2 technique has also been applied to the two catchments and Figure 9 shows typical output from this method for the Thet catchment. The recharge has been calculated using rainfall and PET factors derived from a range of scenarios.

Work in progress

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25th percentile 95th percentile User selected WET SCENARIO

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5th percentile 75th percentile User selected DRY SCENARIO

50th percentile Simplified MEDIAN SCENARIO

Figure 6. Change in monthly and seasonal precipitation for Witham CAMS Change in precipitation (2020s) 40 30 20 10 0 -10 -20 -30 -40

HadCM3 GFDL-R30

CGCM2 CCSR/NIES

CSIRO-mk2 ECHAM4/OPYC3

Figure 7. Proposed framework for assessing the impact of climate change on groundwater recharge and deployable output Assessments of impacts of climate change on groundwater recharge. Based on changes in Rainfall and PET from the UKWIR06 scenarios GR1 GR02 GR3 Increasing complexity

GR1 Key: NOT COVERED IN DETAIL IN THIS PROJECT

GR2

Statistical method based on longterm rainfall and groundwater records

GR3 Distributed hydrometric and hydraulic variables

Catchmentaveraged hyrdrometric and hydraulic variables

“Hindcasting” based on analysis of historic rainfall and historic GWL data.

Tools provided by this project

Spreadsheet based tool provided with this project

Further refinement of project tools and groundwater modelling tasks will continue until March 2007. In the meantime, guidance is available on the application of the new flow factors. Scenarios and tools to assist water companies in climate change impacts assessment are also available on the project website.

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Figure 7 shows the framework for assessing groundwater recharge, and this framework identifies three techniques of increasing complexity that can be used for assessing the impacts of climate change.

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Estimating the impacts of climate change on groundwater levels is a challenging task. The previous UKWIR Report 03/CL/04/2 provided annual recharge factors but this project goes further by developing a framework of different methods, offering different levels of analysis through to detailed groundwater modelling. The following tasks have been completed: • an overall framework for assessing the effects of climate change has been developed through consultation with the Environment Agency and water companies • techniques for using the output from GCMs have been developed and worked examples of these techniques have been undertaken.

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Impacts on groundwater recharge and levels

Change in flow (2020s) 60

Percent change

Rainfall-runoff methods can allow more detailed assessment of the potential impacts of climate change than flow factor methods. The UKWIR06 scenarios provide changes in rainfall and PET for the 2020s that can be used with rainfall-runoff models to estimate the impacts of climate change on river flows. Figure 6 shows an example of the catchment scale climate scenario data.

Figure 5. Percent change in river flows for the 2020s: A comparison of UKWIR06 flow factor percentiles (black) and ‘simplified’ flow factors for Harper’s Brook at Old Mill in Anglian Region

Percent change

Rainfall-runoff methods

Estimated changes in rainfall used to predict changes in groundwater level

Lumped spreadsheet recharge calculator

Distributed run-off and recharge model

Change in recharge (lumped)

Change in recharge (distributed)

Lumped storativity estimation

Groundwater Model

Transient change in catchment scale/ lumped water level (transient)

Distributed transient change in GW level due to climate change

Is the source hydraulically limited? No

Yes

Catchment wide change in recharge:

Source specific assessment:





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Reduction in recharge may have implications on catchment wide licensing Key variable depends on aquifer type Sandstone Long Term Average recharge Chalk Two year drought period

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Assess the impact of change in water level on Deployable Output of source Transform catchment scale change in water level to change in water level at source May consider multiple sources

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Project outputs

Figure 8. Test & Itchen catchment. Difference in modelled groundwater level: scenario – baseline (m)

UKWIR (2005), Trends in UK River Flows 1970-2002. UKWIR Report 05/CL/04/5. ISBN 1-84057387-2

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Te st Riv er

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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

165000

Riv

UKWIR (2006), Interim Report on Rainfall-Runoff Modelling. UKWIR Report 06/CL/04/7. ISBN 1-84057-421-6

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UKWIR (forthcoming), Guidelines for Resource Assessment and UKWIR06 Scenarios. UKWIR Report 06/CL/04/8.

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Reports:

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River Itchen

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UKWIR (2005), Use of Climate Change Scenario Data at a Catchment Level. UKWIR Report 05/CL/04/3. ISBN 1-84057-373-2

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0m Scale 1:300,000 @ A3

Data:

• UKWIR06 factors: probabilistic monthly flow factors for 70 UK catchments

Spreadsheet tools: • Selection of precipitation and PET changes for a given catchment

Figure 9. Lumped Thet Catchment model. Average Monthly Recharge under 3 scenarios 30

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Average Recharge (mm/month)

• UKWIR06 scenarios: sets of Rainfall and PET monthly factors from 6 GCMs for every CAMS (Catchment Abstraction Management Strategy) in England and Wales and WFD river basin in Scotland and Northern Ireland

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• Selection of flow factors corresponding to a given probability for modelled catchments • Multiple linear regression for estimating monthly flow factors for ‘unmodelled’ catchments.

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UKCIP Medium

AUG HadCM3 (UK)

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CGCM2 (Canada)

Pr oject par tner s Project tners and fur ther inf or ma tion infor mation Project Sponsors:

UKWIR and Environment Agency

Contractors:

HR Wallingford (Project Manager, Steven Wade), supported by Entec, Met Office and Lancaster University

Project Management:

UKWIR (client manager Dave Cook, project manager Richard Kirby)

Project Steering Group: Anglian Water, Environment Agency, Scottish Water, Southern Water, South West Water, Thames Water, United Utilities, Veolia Water, Yorkshire Water For queries concerning the project contact [email protected] All report outputs are available directly from UKWIR www.ukwir.org.uk Information on the project, including newsletters and related publications from the project team, is available on www.futuredrought.org.uk 4