A national flood awareness system for ungauged catchments in complex topography for Aoetearoa New Zealand.

Floods cause over $40 Billion of damage worldwide every year. In Aotearoa New Zealand, it is the most frequent natural disaster, with an average annual cost of NZ$100 million for residential properties. Effectively forecasting and communicating flood hazards at national or continental scales is critical to reducing the impacts of flooding. However, developing national-scale river flow forecasting systems remains a challenge due to the predominance of ungauged catchments in often complex and steep terrain. We will present the model development, communication, and evaluation of New Zealand’s first national flood awareness system prototype, the Aotearoa Flood Awareness System, AFAS (Cattoën et al., 2022). To produce river forecasts, a high-resolution convective-scale atmospheric model drives an uncalibrated and semi-distributed hydrological model. The system includes statistical perturbations in rainfall, soil moisture and baseflow to generate a 50-member ensemble. We implemented a relative flow and flood exceedance threshold framework to evaluate hourly forecasts across six categories from below normal to extremely high. We assessed forecast performance categorically against observations, for a 2.5-year reforecast period, at 272 flow sites nationwide, up to 48 hours ahead. AFAS produces skilful streamflow forecasts in catchments with complex topography, even with operational delays ingesting observations. We explored a novel approach to river forecast communication using daily videos and will present feedback gathered from stakeholder workshops and semi-structured interviews. Finally, we will share our experience providing real-time AFAS forecast information during flood responses on the West Coast in 2021 and 2022. AFAS appears to be the first river forecasting system to produce public-friendly videos to communicate streamflow forecasts in their topographical context. Further development of AFAS would benefit from a federated approach across national and regional agencies, including sharing real time weather observations, forecasting tools and expertise.

Cattoën, C., Conway, J., Fedaeff, N., Lagrava, D., Blackett, P., Montgomery, K., Shankar, U., Carey-Smith, T., Moore, S., Mari, A., Steinmetz, T., & Dean, S. (2022). A national flood awareness system for ungauged catchments in complex topography: The case of development, communication and evaluation in New Zealand. Journal of Flood Risk Management, e12864. https://doi.org/10.1111/jfr3.12864