Analysis of CYGNSS coherent reflectivity over land for the characterization of pan-tropical inundation dynamics

International audience ; The monitoring of flood and wetland dynamics at global scale is hampered by several limitations, including a reduced data availability in tropical areas due to the presence of clouds affecting visible and infrared imagery, or low spatial and/or temporal resolutions affecting passive and active microwave Earth Observation (EO) data. As a consequence, surface water extent estimates and their temporal variations remain challenging especially in equatorial river basins. Global Navigation Satellite System Reflectometry (GNSS-R) L-band signals recorded onboard Cyclone GNSS (CYGNSS) mission, composed of 8 Low Elevation Orbit (LEO) satellites, provide information on surface properties at high temporal resolution from 2017 up to now. CYGNSS bistatic observations were analyzed for detecting permanent water and seasonal floodplains over the full coverage of the mission, from 40 • S to 40 • N. We computed CYGNSS reflectivity associated to the coherent component of the received power, that was gridded at 0.1 • spatial resolution with a 7-day time sampling afterwards. Several statistical metrics were derived from CYGNSS reflectivity, including the weighted mean and standard deviation, the median and the 90 th percentile (respectively Γ mean , Γ std , Γ median and Γ 90%) in each pixel. These parameters were clustered using the Kmeans algorithm with an implementation of the Dynamic Time Warping (DTW) similarity measure. They were compared to static inundation maps, and to dynamic estimations of surface water extent both at the global and regional scales, using the Global Inundation Extent from Multi-Satellites (GIEMS) and MODIS-based products. The difference between Γ 90% and Γ median shows the best sensitivity to the presence of water. The river streams and lakes are correctly detected, and a strong seasonality is identified in CYGNSS reflectivity over the largest floodplains, with the exception of the Cuvette Centrale of Congo which is covered by dense vegetation. This seasonal reflectivity signal ...