Forecasting the Flood in 2024 in SW Poland on Virtual Stations of Altimetry Satellites Based on the AltHydro System

This study describes the AltHydro system, which is the first approach to calculate water level predictions at virtual stations (VS) of altimetry satellites.It has been developed for the middle Odra River basin, where 8 VS of the Sentinel-3A satellite have been selected.The system is based on a Vector Autoregressive Model, which is utilized to issue water level prognoses for gauge stations.The forecasts are later transferred to the neighbouring VS using (1) linear regression (vertical shift) updated whenever a new satellite measurement is available, as well as (2) flow velocity estimates (temporal shift), calculated hourly for each prediction.The system operates in real-time and its results are presented on the AltHydro map portal (http://althydro.uwr.edu.pl/).The accuracy assessment can only be based on nadir (Sentinel-3A) and wide-swath (SWOT) altimetry observations, as these are the only water level measurements available for the virtual stations.In this study we used the system to forecast water levels during the flood in September 2024 in SW Poland.A good accuracy of the AltHydro progoses was observed, with absolute error values ranging from 4 to 45 cm for forecasts with a lead time of 24 hours.