Climate Change Projections and Impacts for Tairāwhiti and Hawke's Bay
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Climate change projections and impacts for Tairāwhiti and Hawke's Bay Prepared for Envirolink, Gisborne District Council and Hawke's Bay Regional Council November 2020 Prepared by: John-Mark Woolley (climate change data analysis; report writing) Chris Eager (sea level data analysis; report writing) Javad Jozaei (data analysis; report writing) Vijay Paul (GIS analysis and mapping - climate projections) Ryan Paulik (inundation exposure modelling) Petra Pearce (data analysis; report writing; exposure assessment) Abha Sood (RCM projections; RCM bias corrections and downscaling) Stephen Stuart (RCM projections) Alex Vincent (report writing – sea-level rise) Sanjay Wadhwa (GIS analysis and mapping – river flows) Christian Zammit (river flow data analysis; report writing) For any information regarding this report please contact: Petra Pearce Manager - Climate, Atmosphere and Hazards Climate and Environmental Applications +64-9-375 2052 [email protected] National Institute of Water & Atmospheric Research Ltd Private Bag 99940 Viaduct Harbour Auckland 1010 Phone +64 9 375 2050 NIWA CLIENT REPORT No: 2020298AK Report date: November 2020 NIWA Project: ELF20104 / GDC20101 Quality Assurance Statement Dr Daithi Stone Reviewed by: Climate Scientist – NIWA Wellington Formatting checked by: Petra Pearce Dr Andrew Tait Chief Scientist – Climate, Atmosphere Approved for release by: and Hazards NIWA Wellington © All rights reserved. This publication may not be reproduced or copied in any form without the permission of the copyright owner(s). Such permission is only to be given in accordance with the terms of the client’s contract with NIWA. This copyright extends to all forms of copying and any storage of material in any kind of information retrieval system. Whilst NIWA has used all reasonable endeavours to ensure that the information contained in this document is accurate, NIWA does not give any express or implied warranty as to the completeness of the information contained herein, or that it will be suitable for any purpose(s) other than those specifically contemplated during the Project or agreed by NIWA and the Client. Contents Executive summary ........................................................................................................... 14 1 Introduction ............................................................................................................ 18 1.1 Global and New Zealand climate change................................................................ 21 1.2 Year to year climate variability and climate change ............................................... 23 2 Methodology ........................................................................................................... 26 2.1 Climate modelling ................................................................................................... 26 2.2 Maps and tabulated climate projections ................................................................ 30 2.3 Hydrological modelling ........................................................................................... 31 2.4 Sea-level rise data analysis ..................................................................................... 33 2.5 Inundation exposure modelling .............................................................................. 34 2.6 Limitations .............................................................................................................. 36 3 Current and future climate of Tairāwhiti/Hawke’s Bay .............................................. 38 4 Temperature ........................................................................................................... 38 4.1 Mean temperature ................................................................................................. 38 4.2 Minimum temperature ........................................................................................... 54 4.3 Frosts....................................................................................................................... 69 4.4 Heatwaves .............................................................................................................. 74 5 Rainfall .................................................................................................................... 79 5.1 Rainfall totals .......................................................................................................... 79 5.2 Extreme, rare rainfall events .................................................................................. 94 5.3 Maximum 1-day rainfall ........................................................................................ 115 5.4 Maximum 5-day rainfall ........................................................................................ 122 5.5 Snow ..................................................................................................................... 129 6 Drought ................................................................................................................. 130 6.1 Potential evapotranspiration deficit ..................................................................... 130 6.2 Extreme potential evapotranspiration deficit (>300 mm) .................................... 136 6.3 Soil moisture deficit .............................................................................................. 141 7 Impacts on river flows ............................................................................................ 157 7.1 Mean annual discharge ......................................................................................... 157 Climate change projections and impacts for Tairāwhiti and Hawke's Bay 2 November 2020 10.48 am 7.2 Mean annual low flow .......................................................................................... 158 7.3 High flows ............................................................................................................. 161 7.4 Mean Annual Flood ............................................................................................... 163 8 Sea-level rise and coastal erosion ........................................................................... 166 8.1 Datums and mean sea level .................................................................................. 166 8.2 Impacts of sea-level rise ....................................................................................... 168 8.3 Historic trend in SLR focused on Gisborne and Hawke’s Bay ............................... 169 8.4 Projections for New Zealand sea-level rise. .......................................................... 173 8.5 Tides and the effect of rising sea level ................................................................. 177 8.6 Storm-tide elevations and effect of SLR ............................................................... 181 8.7 Generic impacts of climate change on coastal erosion ........................................ 186 9 Exposure analysis – coastal and fluvial inundation .................................................. 189 9.1 Coastal inundation exposure ................................................................................ 190 9.2 Fluvial exposure .................................................................................................... 191 10 Impacts of climate change on key sectors ............................................................... 192 10.1 Forestry 192 10.2 Horticulture ........................................................................................................... 194 10.3 Agriculture ............................................................................................................ 197 10.4 Tourism 198 10.5 Ecosystem health .................................................................................................. 199 10.6 Human health ....................................................................................................... 208 10.7 Infrastructure and the built environment ............................................................ 211 11 Summary and conclusions ...................................................................................... 212 11.1 Recommendations for future work ...................................................................... 218 12 Acknowledgements ............................................................................................... 220 13 Glossary of abbreviations and terms ...................................................................... 220 Appendix A Coastal and fluvial inundation exposure ......................................... 223 14 References ............................................................................................................. 240 Climate change projections and impacts for Tairāwhiti and Hawke's Bay 2 November 2020 10.48 am Tables Table 2-1: Projected change in global mean surface air temperature for the mid- and late- 21st century relative to the reference period of 1986-2005 for different RCPs. 29 Table 2-2: Approximate years (nearest 5 years) when specific SLR increments would be reached for wider NZ. 35 Table 5-1: Modelled historical and projected rainfall depths (mm) for Waikura Valley for different event durations with a 50-year return period (ARI) 96 Table 5-2: Modelled historical and projected rainfall depths (mm) for Waikura Valley for different event durations with a 100-year return period (ARI) 96 Table 5-3: Modelled historical and projected rainfall depths (mm) for