Geometric Controls on SWOT Water Surface Elevation Measurement in Urban Rivers: Cross-Track Position and Along-Track Flow Angle
Abstract
The Surface Water and Ocean Topography (SWOT) mission provides spatially continuous two-dimensional measurements of water surface elevation (WSE) and river width. Signal interference and layover from built structures are expected to degrade its observations, but it is currently unclear whether SWOT can reliably measure urban rivers. We evaluate how urban morphology and satellite viewing geometry jointly control SWOT WSE errors across 409 river reaches in the conterminous United States with impervious surface area (ISA) ranging from 0.6% to 60.5%. For each reach, we classify reach orientation relative to the orbital ground track to determine the satellite-river-building geometry that controls layover. Simple percent ISA poorly predict WSE errors, and existing SWOT quality flags miss measurement degradation from urbanization. Errors instead depend on the interaction between satellite orbit, channel orientation, and urban morphology. In reaches where SWOT flies parallel to the river far bank ISA produces a monotonic error gradient consistent with layover, while near bank ISA shows a weaker, non-monotonic pattern consistent with shadow saturation. Parallel geometry has errors larger by a factor of two than perpendicular geometry. Building height amplifies the layover gradient. We propose a simple method to identify and correct this phenomenon selected by knee-point optimization (£13° flow angle, 8 m building height, 19% ISA). For parallel reaches, the building height filter in this method reduces the 68th-percentile error by 8.9% while flagging 22.7% of observations. The ISA proxy achieves 17.9% error reduction while flagging 21.5% of observations. Taken together, we have quantified an expected source of error for SWOT and identified a simple method for flagging and removal that should improve downstream applications
Kwan-Hyuck Kim
Graduate Student
