Integrating Lateral Inflows Into a SWOT Mission River Discharge Algorithm

Estimating river discharge from observed surface water extents and elevations is central to the Surface Water and Ocean Topography (SWOT) mission. Although near global in coverage, SWOT will only observe rivers wider than 50 to 100 m, overlooking smaller tributaries draining into observable river reaches. This is problematic for the Metropolis‐Manning (MetroMan) discharge algorithm, which assumes changes in discharge per location must be balanced by a change in cross‐sectional area, not accounting for potential flow contributions SWOT will not observe within the inversion region analyzed. Here, we quantify the effect of these lateral inflows on the performance of estimated discharges along the Muskingum River using MetroMan. Three scenarios are considered: (1) disregarding lateral inflows, (2) providing MetroMan with observed lateral inflows, and (3) providing MetroMan with uncertain model‐derived lateral inflows to assess the discharge algorithm's effectiveness. Scenarios are expanded to consider multiple lateral inflow magnitudes and distributions. Results indicate discharge retrievals were degraded once unaccounted lateral inflows exceeded 5% of average river discharge. When MetroMan is informed by observed lateral inflows, the derived discharges have a relative root‐mean‐square error (rRMSE) of 23% as compared to 360% when lateral inflows are neglected. More importantly, when MetroMan uses simulated lateral inflows, with peak flow condition percent errors as high as 93%, discharge retrieval performance is similar (rRMSE = 17%). These findings highlight the importance of accounting for lateral flows, even in the absence of perfect measurements. Plain Language Summary The upcoming NASA and CNES Surface Water and Ocean Topography (SWOT) satellite mission will give us the ability to find the flow rate of rivers (discharge) using measurements of river surface height, slope, and width. Though SWOT is near global, it will not see rivers that are smaller than 50–100 m in width. Metropolis‐Manning (MetroMan), a SWOT discharge algorithm, plans to estimate river discharge, assuming contributions from these smaller, unobserved rivers and runoff from lands along rivers do not matter. This study aims to test this assumption. Will smaller, unobserved rivers significantly affect the outcome of discharge estimations? How accurate must estimations of the flow coming in from unobserved rivers be to make a difference in performance? We run simulations on MetroMan to answe