A national flood awareness system for ungauged catchments in complex topography: The case of development, communication and evaluation in New Zealand

Abstract Effectively forecasting and communicating flood hazards at national or continental scales is critical to reducing impacts of flooding. Yet, it remains a challenge due to the predominance of ungauged catchments in often complex and steep terrain. We present the development, communication, and evaluation of a national flood awareness system, the Aotearoa (New Zealand) Flood Awareness System, AFAS. Forecasts are produced with an uncalibrated, semi‐distributed hydrological model, driven by a high‐resolution convective‐scale atmospheric model with statistical perturbations in rainfall, soil moisture and baseflow to generate a 50‐member ensemble. We implement a relative flow and flood exceedance threshold framework to evaluate hourly forecasts across six categories from below normal to extremely high. Forecast performance is categorically assessed against observations, for a 2.5‐year reforecast, at 272 sites nationwide, up to 48 h ahead. Overall, AFAS produces skilful streamflow forecasts in catchments with complex topography, even with operational delays ingesting observations. We explore a novel approach to river forecast communication using daily videos. We suggest rethinking large‐scale streamflow forecast communication by balancing a depth with breadth approach (pointwise absolute flows versus distributed relative flows), to raise collective awareness before and during natural disasters. AFAS appears to be the first system producing public‐friendly videos to communicate streamflow forecasts in their topographical context. Future development of AFAS will benefit from a federated approach across national and regional agencies, including sharing of real‐time weather observations, forecasting tools and expertise.