Evaluating Earth observation products for Catchment-Scale operational flood monitoring and risk management in a sparsely gauged to ungauged river basin in Nigeria

• The devastating 2012 and 2022 Nigeria floods along the Niger and Benue Rivers are estimated 14- and 30-year floods. • Predicted 100-year flow magnitude suggests a potential use for delineating regulatory floodway and zoning. • River discharge or streamflow changes can be monitored using PMR in ungauged river basins and cross-border locations. • FwDET is suitable for large-scale operational flood damage assessment and response. • Satellite products can potentially serve as components of an Early Flood Warning System. With the persistent rise in intensity and magnitude of hydrological extremes globally, timely information from operational early flood warning systems provide lead times that translate into actionable strategies to monitor and mitigate flood risk. However, the situation is often different for flood-prone regions of the global south with sparse to no ground flood monitoring systems, where flood management practices are inadequate or lacking, and floods are further exacerbated due to cross-border water infrastructure management practices. With the unprecedented volume of data from Earth Observation Satellites (EOS), such as MODIS (Moderate Resolution Imaging Spectroradiometer) and passive microwave radiometry (PMR) river discharges, insights are being provided into different catchment hydrologic variables required for early flood detection and risk management. This work presents a geospatial and satellite-based heuristic that will be of use for operational flood risk management, which ties together PMR river discharges, the Floodwater Depth Estimation Tool (FwDET), and MODIS-based inundation detection. Case studies of the 2012, 2018, 2020, and 2022 floods in the Lower Niger River Basin in Nigeria are presented, with emphasis placed on the devastating 2012 and 2022 floods. Furthermore, using the time-series from the PMR river discharges, a flood frequency analysis was performed. The analysis was found to capture peak discharges corresponding to the flood events. The result of the flood frequency analysis shows that the return periods of all the flood events combined are approximately equal to or below 30-year, with the 2012 and 2022 catastrophic floods approximately 14- and 20-year floods at satellite gauging reach (SGR) 1438, while 14- and 30-year floods at SGR 1441. From the perspective of a 100-year flood regulatory magnitude, these return periods are an indication of possible river flooding in its natural floodplain. The predicted flow magnitude of a 100-year flood at SGR 1438 was 44% and 38% greater than the 2012 and 2022 flood magnitudes, respectively; 18% and 12% greater at SGR 1441. The results show that operational flood monitoring and risk management or assessment are possible in sparse to ungauged river basins using these products, especially with appropriate predictive algorithms to enable incorporation in an early flood warning system. Since satellite-based measurements have no regard for political boundaries, possible effects of cross-border water infrastructure management practices within the basin could also be assessed.