Assessing the potential global extent of SWOT river discharge observations

- The SWOT satellite would likely observe 50-100 m wide rivers, globally.
- We compare potential SWOT observation extent to the current river gauge network.
- SWOT detection of 50 m rivers (vs. 100 m) would yield substantial resolution gains.
- SWOT would improve observation of global runoff at scales >10-50,000 km2.
- SWOT and gauges combined would most improve understanding of global runoff patterns.

SummaryDespite its importance as a major element of the global hydrologic cycle, runoff remains poorly constrained except at the largest spatial scales due to limitations of the global stream gauge network and inadequate data sharing. Efforts using remote sensing to infer runoff from discharge estimates are limited by characteristics of present-day sensors. The proposed Surface Water and Ocean Topography (SWOT) mission, a joint project between the United States and France, aims to substantially improve space-based estimates of river discharge. However, the extent of rivers observable by SWOT, likely limited to those wider than 50-100 m, remains unknown. Here, we estimate the extent of SWOT river observability globally using a downstream hydraulic geometry (DHG) approach combining basin areas from the Hydro1k and Hydrosheds elevation products, discharge from the Global Runoff Data Centre (GRDC), and width estimates from a global width-discharge relationship. We do not explicitly consider SWOT-specific errors associated with layover and other phenomena in this analysis, although they have been considered in formulation of the 50-100 m width thresholds. We compare the extent of SWOT-observable rivers with GRDC and USGS gauge datasets, the most complete datasets freely available to the global scientific community. In the continental US, SWOT would match USGS river basin coverage only at large scales (>25,000 km2). Globally, SWOT would substantially improve on GRDC observation extent: SWOT observation of 100 m (50 m) rivers will allow discharge estimation in >60% of 50,000 km2 (10,000 km2) river basins. In contrast, the GRDC observes fewer than 30% (15%) of these basins. SWOT could improve characterization of global runoff processes, especially with a 50 m observability threshold, but in situ gauge data remains essential and must be shared more freely with the international scientific community.