Tamlin Pavelsky (University of North Carolina, Chapel Hill, United States of America); Claire Pottier (CNES, France); Damien Desroches (CNES, France); Emmanuel Chapron (GEODE, Université Toulouse Jean-Jaures, France); Arthur Compin (LEF, Toulouse, France); Matthias Raynal (CNES, France); Manon Delhoume (CS Soprasteria Group, France); Grant Parkins (University of North Carolina, Chapel Hill, United States of America); Paul-Gérard Gbetkom (CNRS/LEGOS, France); Muriel Berge-Nguyen (CNES/LEGOS, France); Daniel Moreira (SGB, Rio de Janeiro, Brazil); Roger Fjortoft (CNES, France)

The Surface Water and Ocean Topography (SWOT) mission, launched in December 2022 is aiming to produce Water Surface Elevation (WSE) over lakes larger than 250*250 m, globally, with high accuracy better than 25 cm (1σ). Water extent of lakes is also measured with requirements to perform it with a relative accuracy better than 15% of the total lake extent. Calibration / Validation (C/V) of the mission is therefore necessary on a large number of lakes including a high variety of geographical and morphological situations. WSE of lakes in mountain areas are generally hardly measured from satellite, and using SWOT data is a new opportunity to understand how this type of lakes is responding to climate changes and how they are contributing to the water cycle on the Earth. In the framework of the SWOT’s C/V, a set of gauges have been instrumented over lakes in the Pyrenees mountain, with ruler allowing to measure the water level manually and to report them in the C/V database for further comparison with SWOT lake products. For some of the lake chosen for this study, they were observed by SWOT during the one-day (1-D) fast sampling phase of the six first months, while some others were observed by SWOT only when the satellite shifted to its final so-called science orbit with a repeat cycle of 21-days. Data from the rulers were collected and transmitted to the C/V teams by citizens (hikers, fishermen, some local authorities) within a NASA project named LOCSS and fully based on the citizen for science initiative gathering several networks of lakes worldwide. WSE over the lakes in the Pyrenees measured by SWOT meet the requirement for WSE and water extent, with however a rate of lost measurements which is more pronounced during the science phase than the 1-D fast sampling ones. The small size of the lakes chosen in the Pyrenees have led to determine the limit of the performances under such condition: all lakes larger than 6 ha are meeting the requirement once some strict editing was applied (use of several flags contained in the data records). Smaller lakes present all a high rate of missing data and much lower accuracy. Results obtained during the 1-D fast sampling were better than during the science phase.