Government of Western Australia Department of Water Selection of future climate projections for Western Australia Securing Western Australia’s water future Report no. WST 72 September 2015 Selection of future climate projections for Western Australia Department of Water Water Science Technical Series Report no. 72 September 2015 Department of Water 168 St Georges Terrace Perth Western Australia 6000 Telephone +61 8 6364 7600 Facsimile +61 8 6364 7601 National Relay Service 13 36 77 www.water.wa.gov.au © Government of Western Australia September 2015 This work is copyright. You may download, display, print and reproduce this material in unaltered form only (retaining this notice) for your personal, non-commercial use or use within your organisation. Apart from any use as permitted under the Copyright Act 1968, all other rights are reserved. Requests and inquiries concerning reproduction and rights should be addressed to the Department of Water. ISSN 1836-2877 (online) ISBN 978-1-922248-79-4 (online) Acknowledgements This report was prepared by Ben Marillier, Joel Hall and Jacqui Durrant. The Department of Water thanks the following for their contribution to this publication. Steve Charles, Bryson Bates and Carsten Frederiksen from the Indian Ocean Climate Initiative (CSIRO/BoM) for project input and provision of data. Pandora Hope from the Centre for Australian Weather and Climate Research (CSIRO/BoM) for technical review. CLIMSystems Ltd. for software support. For more information about this report, contact: Supervising Engineer, Surface Water Assessment Cover photograph: Cold front in the south-west, courtesy Vanessa Forbes Reference details The recommended reference for this publication is: Department of Water 2015, Selection of future climate projections for Western Australia, Water Science Technical Series, report no. 72, Department of Water, Western Australia. Disclaimer This document has been published by the Department of Water. Any representation, statement, opinion or advice expressed or implied in this publication is made in good faith and on the basis that the Department of Water and its employees are not liable for any damage or loss whatsoever which may occur as a result of action taken or not taken, as the case may be in respect of any representation, statement, opinion or advice referred to herein. Professional advice should be obtained before applying the information contained in this document to particular circumstances. This publication is available at our website <www.water.wa.gov.au> or for those with special needs it can be made available in alternative formats such as audio, large print, or Braille. Selection of future climate projections for Western Australia Water Science Technical Series, report no. 72 Contents Summary ................................................................................................................... vii 1 Introduction and background ................................................................................... 1 1.1 Introduction ......................................................................................................................... 1 How to use this document ....................................................................................................................... 2 1.2 Climatic influences and trends in Western Australia .......................................................... 2 Climatic regions ....................................................................................................................................... 2 Climatic trends and influences ................................................................................................................. 5 Climatic trends and influences by region ................................................................................................. 8 1.3 The need for standard climate projections ....................................................................... 12 Inconsistencies in climate projections between studies ......................................................................... 12 Availability and accessibility of climate projections ................................................................................ 13 1.4 Project scope .................................................................................................................... 15 1.5 Project outputs .................................................................................................................. 15 1.6 National projects ............................................................................................................... 16 2 Selection of emissions scenarios, general circulation models, baseline and downscaling method .............................................................................................. 17 2.1 Emissions scenarios ......................................................................................................... 17 2.2 General circulation models (GCMs) ................................................................................. 21 2.3 Climate historical baseline period ..................................................................................... 24 2.4 Downscaling methodology ................................................................................................ 25 Constant (pattern) scaling using SimCLIM ............................................................................................ 27 3 Selection of wet, median and dry climate scenarios .............................................. 29 3.1 Analysis of GCM results and selection of the wet, median and dry scenarios ................ 29 Climate variables analysed .................................................................................................................... 30 Spatial extent and output format ............................................................................................................ 31 3.2 Climate scenarios for Western Australia .......................................................................... 33 South-west region .................................................................................................................................. 34 Central-west region................................................................................................................................ 37 Pilbara region ........................................................................................................................................ 40 Kimberley region .................................................................................................................................... 43 Central region ........................................................................................................................................ 46 4 Guidance for applying climate scenarios to water resource modelling, planning and management ......................................................................................................... 49 When and where to use the climate scenarios ...................................................................................... 49 How to make future climate scenarios at point locations ....................................................................... 52 Examples of scaled datasets at point locations ..................................................................................... 54 Which scenario to use for water resource applications based on uncertainty and risk management? .. 57 5 Concluding remarks .............................................................................................. 62 Appendices ................................................................................................................ 64 Appendix A: Long term rainfall and temperature record for selected meteorological stations .. 65 Appendix B: Calculation of FAO56 reference evapotranspiration and Penman evaporation ... 73 Appendix C: Assessment for selection of wet, median and dry future climate scenarios ......... 76 Appendix D: GCM agreement for direction of rainfall trends ..................................................... 77 Department of Water iii Water Science Technical Series, report no. 72 Selection of future climate projections for Western Australia Appendix E: Coefficient of variation and rainfall anomaly for selected meteorological stations78 Appendix F: Long-term climate projections and historical annual rainfall at selected rainfall stations.............................................................................................................................. 79 Shortened forms ........................................................................................................ 85 General Circulation Models ....................................................................................... 86 Glossary .................................................................................................................... 87 References ................................................................................................................ 89 Figures Figure 1-1: Seasonal rainfall zones of Australia (source BoM 1999) with the subdivisions used for this study .............................................................................................................. 3 Figure 1-2: Average annual rainfall 1961–90 for Western Australia (source BoM 2013b) ....... 4 Figure 1-3: Average daily temperature 1961–90 for Western