Technical Assistance Consultant’s Report ___________________________________________________________________________________________ Project Number: 48488-001 November 2019 Regional: Strengthening Climate and Disaster Resilience of Investments in the Pacific Hydroclimatic Risk Assessment for Vaisigano River Catchment, Samoa Prepared by Anthony Kiem (Hydroclimatologist) For Asian Development Bank This consultant’s report does not necessarily reflect the views of ADB or the Government concerned, and ADB and the Government cannot be held liable for its contents. (For project preparatory technical assistance: All the views expressed herein may not be incorporated into the proposed project’s design. CURRENCY EQUIVALENTS (as of 6 July 2019) Currency Unit – Samoan Tala (WST) 1 WST = $US0.376297 $US1 = 2.65748 WST ABBREVIATIONS ADB – Asian Development Bank AEP – Annual Exceedance Probability AR4 – Fourth Assessment Report from the IPCC AR5 – Fifth Assessment Report from the IPCC ARI – Average Recurrence Interval (years) CMIP – Coupled Model Intercomparison Project CSIRO – Commonwealth Scientific and Industrial Research Organisation ENSO – El Niño/Southern Oscillation GCF – Green Climate Fund GCM – General Circulation Model (or Global Climate Model) GOS – Government of Samoa IOD – Indian Ocean Dipole IPCC – Intergovernmental Panel on Climate Change IPO – Interdecadal Pacific Oscillation ITCZ – Intertropical Convergence Zone MJO – Madden Julian Oscillation MOF – Samoa Ministry of Finance MNRE – Samoa Ministry of Natural Resources and Environment SWA – Samoa Water Authority EPC – Samoa Electric Power Corporation PCCSP – Pacific Climate Change Science Program PDO – Pacific Decadal Oscillation SAM – Southern Annular Mode SPCZ – South Pacific Convergence Zone SRES – Special Report on Emission Scenarios SST – Sea Surface Temperature UNFCCC – United Nations Framework Convention on Climate Change WMO – World Meteorological Organization WPM – West Pacific Monsoon GLOSSARY Wet (cyclone) season – November to April. Dry season – May to October. NOTES (i) In this report, $ refers to US dollars. CONTENTS EXECUTIVE SUMMARY ............................................................................................................ 4 I. INTRODUCTION ............................................................................................................. 5 I-A. Brief project description........................................................................................ 5 I-B. Objectives and scope of this report ...................................................................... 6 II. CLIMATE SUMMARY FOR SAMOA ............................................................................... 7 II-A. Current (historical) climate for Samoa .................................................................. 7 II-B. Future climate for Samoa ..................................................................................... 9 III. DATA USED IN THIS STUDY ....................................................................................... 10 III-A. Rainfall ............................................................................................................... 10 III-B. Streamflow ......................................................................................................... 10 III-B-1. Observed streamflow ............................................................................. 10 III-B-2. Modelled streamflow .............................................................................. 10 III-C. Historical flood frequency ................................................................................... 16 III-D. Tropical cyclones ............................................................................................... 18 III-E. El Niño/Southern Oscillation (ENSO) conditions ................................................ 18 III-F. Interdecadal Pacific Oscillation (IPO) conditions ................................................ 18 IV. HISTORICAL HYDROCLIMATIC TRENDS AND VARIABILITY IN VAISIGANO RIVER CATCHMENT .................................................................................................... 19 IV-A. Rainfall ............................................................................................................... 19 IV-B. Streamflow ......................................................................................................... 22 IV-C. Tropical cyclones ............................................................................................... 25 V. PROJECTED IMPACTS OF CLIMATE CHANGE ON HYDROCLIMATIC VARIABLES IN VAISIGANO RIVER CATCHMENT ....................................................... 28 V-A. Rainfall ............................................................................................................... 28 V-B. Evaporation ........................................................................................................ 30 V-C. Streamflow ......................................................................................................... 30 V-D. Tropical cyclones ............................................................................................... 30 VI. CURRENT AND FUTURE FLOOD RISK IN VAISIGANO RIVER CATCHMENT ........... 31 VII. CURRENT AND FUTURE WATER AVAILABILITY IN VAISIGANO RIVER CATCHMENT ................................................................................................................ 35 VIII. UNCERTAINTIES AND LIMITATIONS .......................................................................... 38 IX. CONCLUSIONS ............................................................................................................ 39 APPENDIX I – LIST OF DOCUMENTS AND PROJECT REPORTS USED IN THIS STUDY .......................................................................................................................... 40 A. Project documents ............................................................................................. 40 B. ADB reports relevant to this study ...................................................................... 40 C. Other publications relevant to this study ............................................................. 40 4 EXECUTIVE SUMMARY 1. Climate projections for Samoa suggest that climate-related risks are already high and could increase in the future. This has the potential to undermine development progress in urban Apia where most of Samoa’s population and economic activity is located. 2. As part of the Government of Samoa’s (GOS’s) program to address issues associated with climate change, a new impoundment called Alaoa Dam is proposed for the Vaisigano River which flows through central Apia. The new Alaoa Dam will be multipurpose as it will be used for flood prevention, water supply, and providing additional hydropower. 3. The objective of this report is to better understand the historical, present, and future hydroclimatic risks for the Vaisigano River catchment. 4. This report concentrates on the situation without the new Alaoa Dam to provide baseline information with which to assess the socioeconomic and environmental costs and benefits of the proposed Alaoa Dam. 5. As per the overall situation for Samoa, there are no statistically significant trends in annual or seasonal rainfall in the Vaisigano River catchment. However, there is significant interannual variability in annual and seasonal rainfall that is strongly related to the El Niño/Southern Oscillation (ENSO) and the Interdecadal Pacific Oscillation (IPO). This is especially the case in the wet season with increased (decreased) rainfall more likely when ENSO is in its La Niña (El Niño) phase. 6. Based on the observed streamflow data that were available, and the simulated streamflow that was produced based on the hydrological model developed in this study, there is no evidence of statistically significant trends in annual or seasonal streamflow in the Vaisigano River catchment. However, it should be noted that significant issues were identified in the observed streamflow record (discussed in Section III-B) meaning there is low confidence associated with this conclusion. 7. Observed tropical cyclone activity over the last few decades exhibits significant year to year variability. However, reconstructed tropical cyclone counts back to 1855 demonstrate prolonged periods of tropical cyclone activity that are very different to the tropical cyclone activity that has been experienced in the last few decades. It is therefore not recommended, irrespective of what might eventuate due to anthropogenic climate change impacts on tropical cyclones, to assume that tropical cyclone behavior observed in the last few decades is representative of what is possible in the future. El Niño and La Niña phases were also found to result in a significant modulation of tropical cyclone activity near Samoa with elevated tropical cyclone risk during either El Niño or IPO positive years and the greatest tropical cyclone risk associated with co-occurrence of El Niño and IPO positive conditions. 8. Future (2090-2099) flood risk in the Vaisigano River catchment is likely to be significantly elevated compared with the current (historical) situation. It is important to note that the more extreme the flood the greater the reduction in Average Recurrence Interval (ARI). This needs to be considered when designing infrastructure like the proposed Alaoa Dam since it is the floods with the higher ARIs (i.e. the more extreme events) that are most likely to