Le prochain Vendredi Découverte aura lieu vendredi 06 juin 2025 à 11h à la fois en présentiel en salle Aquadémie (Bât. Confluence - SIte Hydropolis Lavalette - 361, rue JF Breton - 34090 Montpellier) et en distanciel via le lien suivant :  https://ird-fr.zoom.us/j/92505334421?pwd=VO5egHPqcS7WnkvENUTsCSUUlHCd9n.1

Il sera consacré à une séance spéciale Axe 3 - Cette séance porte sur la présentation d’une des 15 planches de BD qui alimenteront le futur ouvrage dans le cadre de l’axe 3 : Informations, Incertitudes, Décisions. Elle vise la mise en discussion du message porté par cette BD et invite chacun à se positionner en rapport avec ses propres recherches. La discussion est ouverte à tous.

Thème de la séance : « Une bonne eau : une eau qui vit ! », planche présenté par Caroline Lejars et Olga Peytavi.

Dans le cadre du projet GOUTTE (https://www.cresica.nc/projet/goutte), Caroline Lejars a souhaité partager son expérience en Nouvelle Calédonie relative aux différentes perceptions de la qualité de l’eau en lien avec les solutions de gestion (co-auteurs : Olga Peytavi, Severine Bouard et Pierre Yves Le Meur).

David Dorchies sera dans un premier temps rapporteur de cette planche, puis Caroline nous présentera le message qu’elle a voulu transmettre. Enfin, une discussion sera ouverte à tous les participants qui seront libres d’exprimer leurs ressentis ainsi que les expériences ou travaux auxquels cela fait écho. 

Vous êtes invités à prendre connaissance de la planche ci-après, avant la séance :

Venez nombreux ! 

Lors du Vendredi Découverte du vendredi 16 mai 2025, Hind Oubanas nous a présentéle sujet suivant : SWOT - Discharge from Reach to Basin Scale

Résumé : Modeling river discharge at a global scale is vital for understanding water fluxes, managing water resources, and mitigating flood risks. Traditionally, hydraulic and hydrological models have depended on in-situ observations collected at strategic points along major rivers and their tributaries. These ground-based measurements are essential for calibrating and validating models, ensuring that the simulated discharge closely aligns with observed river flows. However, river gauges have inherent limitations, including sparse spatial coverage and a decline in availability due to practical, economic, or political factors.

Remote sensing has emerged as a transformative tool to address these challenges, providing systematic observations over extensive regions. Various methods have been developed to estimate river discharge using satellite data, typically requiring calibration with ground-based data or models. The launch of the Surface Water and Ocean Topography (SWOT) satellite mission in December 2022 marked a significant advancement in surface hydrology, particularly for discharge retrievals. SWOT employs two Synthetic Aperture Radar antennas to provide the first-ever concurrent two-dimensional, high-resolution measurements of water surface elevation and extent in rivers, lakes and wetlands. These measurements can offer a new perspective on how surface water is monitored and quantified, with initial results suggesting that SWOT is capable of meeting and even exceeding its science requirements. Among the products of the SWOT mission is a river discharge estimate, the first global discharge mission product. It is produced through operational discharge estimation algorithms that leverage the unique capability of the satellite to simultaneously observe river water surface elevation, width, and slope. Discharge algorithms solve hydraulic inverse problems at the reach scale, approximately every 10 km, as defined by the SWOT River Database hydrographic network, where the satellite observes the river during each overpass. The wide-swath nature of SWOT offers a fundamentally novel discharge product, providing comprehensive spatial coverage.

In this presentation, we first analyze SWOT discharge product and its spatiotemporal patterns across entire basins, such as the Mississippi, a well-monitored basin, the Congo, a poorly gaged basin and the Mekong, a transboundary basin. We will focus on the discharge algorithm, SIC4DVAR, one of the six official SWOT algorithms, developed by INRAE’s team. We will present the latest available discharge products, discussing their spatiotemporal coverage and the status of validation at the time of the meeting. Our focus will be on the product derived from the nominal orbit with a 21-day cycle, which provides global coverage. Differences between the constrained and unconstrained products will be explored. Preliminary results have also examined the impact of data quality filters on discharge performance, showing that stricter filters bring performance closer to pre-launch expectations. Remaining challenges related to data and algorithms will be discussed.

Next, we extend SWOT discharge's capabilities from the reach to the basin level. The analysis at the basin scale reveals sub-daily coverage of basins from upstream to downstream, with different overpasses capturing multiple tributaries simultaneously, thereby enabling a comprehensive examination of discharge variability in space and time. Next, we enhance large-scale basin discharge estimation by assimilating SWOT reach-scale data into the RAPID river network model. For the Mississippi basin, SWOT observes the basin an average of three times daily, with each overpass covering different rivers and tributaries. This assimilation process leverages SWOT's spatial and temporal coverage to integrate observed and propagated dynamics within a single cycle. Preliminary results indicate that while SWOT directly observes 10% of the basin, assimilation propagates this information to 82% of the basin, including unobserved tributaries. These findings demonstrate that integrating data assimilation, river modeling, and scaling can overcome temporal and spatial coverage limitations, significantly enhancing our ability to estimate basin-scale discharge.