SWOT follows reservoir rises and falls in a US ground-data comparison
Satellite observations closely followed reservoir water-level changes in a peer-reviewed comparison with 773 US ground-observation time series. Researchers removed the average satellite–ground offset before calculating relative errors. Their separate downstream-discharge comparison showed weaker agreement, distinguishing the satellite’s ability to follow rising and falling water from its assessment of river flows.
Science··Midday
Water-level changes agree across hundreds of time series
Reservoirs rising and falling in ground observations generally followed the same ordering in measurements from the Surface Water and Ocean Topography satellite mission, SWOT. A peer-reviewed observational study compared 773 water-level time series across the contiguous United States. Median Spearman correlations were 0.96 against the US Geological Survey, USGS, and 0.94 against the US Army Corps of Engineers, USACE. Some reservoirs contributed observations from both agencies.[1]
Downstream flow showed weaker agreement
Discharge data covered fewer sites, comprising 50 time series from USACE and 17 from USGS. The median correlation was 0.57 for USACE, against 0.39 for USGS. Reservoir operators’ release decisions can change how lake level relates to downstream flow. Satellite observations are also intermittent: SWOT follows a 21-day orbital cycle. The analysis used a lake product giving one elevation for an entire reservoir, with quality flags and cross-track position used to filter observations.[1]
The error calculation first removed the average offset
Researchers aligned the means of the satellite and ground series before calculating relative errors. The adjusted differences were below 10 cm in 68 per cent of USGS observations; the USACE threshold for that proportion was 22 cm. Ground elevations had not been converted to a shared vertical datum. The small residual differences therefore describe variation after the adjustment; different elevation reference systems remain part of the unresolved absolute-height comparison.[1]
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