An Impacts-Based Flood Decision Support System for a Tropical Pacific Island Catchment with Short Warnings Lead Time  
[PERSON], [PERSON]  
1 NIWA Tainoro Nakurangi, Christchurch 8011, New Zealand; [PERSON] (J.G.); [PERSON] (B.M.); [EMAIL_ADDRESS] (M.O.); [EMAIL_ADDRESS] (R.P.); [EMAIL_ADDRESS] (G.E.)  
2 Water Resources Division, Ministry of Natural Resources and Environment, Apia WS1338, Samoa; [EMAIL_ADDRESS] (E.R.); [EMAIL_ADDRESS] (M.L.); [EMAIL_ADDRESS] (M.L.  
Flash flooding in particular poses significant early warning challenges due to short catchment response times of <2-3 h compared with the time required for making informed decisions on emergency response and resourcing e.g., [12; 13]. These challenges are exacerbated in ungauged catchments where people, lifelines and infrastructure are exposed e.g., [13]. While much progress has been made in weather forecasting over the last few decades [12], challenges remain in forecasting flash floods which are influenced by a combination ofomorphic, hydrological, soil moisture capacity and antecedent trigger conditions [14; 15; 16].  
In the Pacific Small Islands Developing States (SIDS) region, weather forecasting and early warning systems (EWS) have typically taken a hazard-centric approach in terms of tracking, estimating and alerting affected people of what a specific event'might be' in terms of magnitude and extent (with several hours to days lead time). Over the last decade, however, Pacific SIDS are increasingly integrating impacts-based approaches into EWSs (i.e., what a specific weather event'might do' in terms of exposure and losses) [16]. These shifts reflect responses by Pacific SIDS in delivering on global and regional framework targets such as the 2015-2030 Sendai Framework targets [17], and the Framework for Resilient Development in the Pacific [18].  
Extreme rainfall-induced fluvial and pluvial flooding are an annual risk faced by many populated centres in high volcanic islands across the tropical Pacific SIDS region (e.g., [19]). Many catchments in these settings are typically characterized as short and steep, with rapid critical rainfall to flood peak durations. This results in extremely limited observations-based (or nowcast) early warnings and response lead times, and motivates the shortening of alerting lead times through the incorporation of forecast-based techniques. For example, the Vaisigano River catchment in Samoa, has an extreme critical peak duration of approximately 1-h resulting in very short warnings and response lead times.  
In this paper, we present a case study of a pilot impacts-based flood early warning decision support system developed for the Vaisigano River which flows through the Samoa capital of Apia. The aim of the pilot was to implement a context-specific, real-time, automated river monitoring and forecast-based flood early warnings decision support system capable of alerting hazard monitoring and emergency responders of imminent flooding with up to 24-48 h lead time. We describe the methods and analysis used in the development of different system components, including a web-based information portal which enables interaction with the decision support information tools. We discuss the practical implications of the developed system based on operational testing during the 2020/2021 tropical cyclone season in Samoa, with recommendations offered for future research.