Which Water Bodies does SWOT Observe? A Case Study of Orange County, North Carolina.

<span data-olk-copy-source="MessageBody">In December 2022, NASA launched the Surface Water and Ocean Topography (SWOT) satellite to conduct the first global study of Earth's water resources. SWOT uses interferometric synthetic aperture radar to measure terrestrial water features such as lakes and rivers. This study evaluates SWOT's performance in a regional case study of Orange County, North Carolina, from January 2024 to January 2025. According to the United States Geological Survey's National Hydrography Dataset, Orange County contains 1,400 permanently recorded water bodies, ranging from less than 0.01 to over 1.5 square kilometers in surface area. Using SWOT's Level 2 High Rate Raster product at 100-meter resolution, 53 observations were compared against these water bodies to determine the ratio of observed to possible observations. Larger water bodies are consistently observed more frequently than smaller ones, suggesting that the size of the surface area plays an important role in observation likelihood. Water bodies start to be more likely observed than not around 0.04 square kilometers. Still, smaller features have a non-zero chance of detection, indicating a probabilistic decline in observation likelihood with decreasing size rather than a strict cutoff size. Cross-track distance of the water bodies from the nadir position showed a weak positive correlation with observation ratio, however, the relationship is secondary to size. This study provides a quantitative baseline for SWOT's performance with inland water body observation. The limited detection of smaller water bodies is not necessarily a failure of SWOT, but rather a reflection of design trade-offs. These findings suggest that size should be a primary consideration when applying SWOT data to future hydrological studies, and it helps inform next-generation satellite missions that increased spatial resolution ought to be a priority to observe the smallest water bodies.</span>