Evaluation using in-situ observations from national governments and Citizen Scientists suggests nadir altimeters can accurately measure water level changes regardless of lake area

Water level and water level changes of lentic water bodies such as lakes, wetlands, and reservoirs are rarely available, despite the relevance of these to maintain ecosystems (e.g., by providing drinking water, food, and cultural activities; and by supporting biodiversity). One alternative is to measure water surface elevation with radar nadir altimeters, whose use has increased as the capabilities of measuring inland water bodies have improved. However, their effectiveness has primarily focused on lakes larger than ~100 km2, and the analysis of contributing factors to obtaining high accuracies is typically performed for a few lakes with special cases. We evaluated water surface elevation change from Sentinel-3 and Jason-3 nadir altimeter missions in lakes and reservoirs using in-situ observations from national governmental institutions and the citizen science network from the project Lake Observations by Citizen Scientists and Satellites. Utilizing the accuracy metrics of Pearson correlation coefficient, R, and the unbiased Root Mean Square Error, ubRMSE, over 27 waterbodies, we found a median ubRMSE of 0.15 m and R of 0.88. We combined the results from 61 additional lakes in a Random Forest algorithm with a permutation of importance to evaluate the impact of 7 factors on the accuracy metrics. Although water surface variability was the most contributing factor to the accuracy, we found differences in the order of variables depending on the evaluation metric used. Our results contribute to show the potential of the nadir altimeters to estimate water surface elevation changes in small lakes, present the advantages of using citizen science monitoring, and the relevance of water surface variability in relation to other factors contributing to high altimeter accuracy. Our implementation of the level of importance algorithm shows potential to systematically evaluate the nadir altimeter validation work available in the literature.