Exploring the effect of river spatial variability on SWOT discharge accuracy

The Surface Water and Ocean Topography (SWOT) mission measures river width, water surface elevation (WSE), and slope for discharge estimation. In this study, we explore the impact on remotely sensed discharge of within-reach river spatial variability, defined as the coefficient of variation (ε p = σ/μ). After quality control, we analyze 484 representative reaches across the United States with paired SWOT observations and United States Geological Survey (USGS) discharge records from January 2023 to October 2024. Discharge is calibrated using modified Manning equations and evaluated using the Nash-Sutcliffe efficiency (NSE), absolute normalized residuals and correlation coefficient. Reaches with high spatial variability exhibit substantially lower calibration skill than reaches with low variability, with the median NSE reduced by 0.09 (0.78 vs. 0.69). Two possible reasons for this phenomenon are examined: SWOT observation errors and structural limitations of modified Manning’s equations. Regarding observation errors, we apply a river hypsometry constraint to correct width and WSE. Overall calibration accuracy improved, and the median NSE gap between low- and high-variability reaches reduced to 0.05 (0.86 vs. 0.81), emphasizing the significance of observation errors in this degradation. Conversely, evidence linking the remaining performance degradation to structural flow-law error is inconclusive. Although effective resistance changes more for reaches with higher spatial variability, adding the specific slope-variability term tested here does not lead to consistent improvement. This research has important implications for better estimating discharge from SWOT data.