How many inundations are detectable in Europe using Sentinel-1 and Sentinel-2?

Inundation is one of the major natural hazards in Europe. After a number of dramatic floods, the Member States agreed to draw up guidelines to develop a flood risk assessment, flood hazard and risk maps and flood risk management plans (Directive 2007/60/EC) with the aim to reduce the adverse consequences on the human health and the environment. Flood hazard and risk evaluation is not straightforward and it is traditionally based on hydro-monitoring systems  not adequately distributed in the territory or on hydrodynamic models as a tool for delineating flooded areas. In the last decades, the satellite sensors launched for Earth Observation represent a valid support for early warning systems and for mitigating the impact of future flooding. The ESA Earth Observation Program includes a series of satellites, Sentinels, for the operative observation of the natural phenomena and, in particular, Sentinel-1 (SAR) and Sentinel-2 (optical) are more suitable for mapping flooded areas. The two instruments assurance an almost global coverage for free. However, the spatial resolution (10 – 20 m) and the revisit time (5 – 6 days) of the sensors do not always guarantee a full mapping of inundated territories.

Here, we proposed a study to evaluate the effectiveness of the Sentinel-1 and Sentinel-2 in the mapping of floods in Europe, where the flood events have duration ranging from some hours to a few days. To reach the target, we analyzed ten years of river discharge data over almost 2000 sites in Europe and we simulated flood riverine inundations selecting flood events over three established thresholds (97^th, 99^th and 99.5^th percentile). Based on the revisit time of both the satellites constellations and the cloud coverage for the optical sensors, we derived the percentage of potential inundation events detectable from Sentinel-1 and Sentinel-2. Assuming the configuration of a constellation of two satellites for each mission and considering the ascending and descending orbit, we find that on average the 58 % of flood events were potentially observable by Sentinel-1 and only the 28 % by Sentinel-2.