A framework for estimating global river discharge from the Surface Water and Ocean Topography satellite mission

Abstract

The forthcoming Surface Water and Ocean Topography (SWOT) mission will vastly expand measurements of global rivers, providing critical new datasets for both gaged and ungaged basins. SWOT discharge products will provide discharge for all river reaches wider than 100 m, but at lower accuracy and temporal resolution than what is possible in situ. In this paper, we describe how SWOT discharge produced and archived by the US and French space agencies will be computed from measurements of river water surface elevation, width, and slope and ancillary data, along with expected discharge accuracy. We present here for the first time a complete estimate of SWOT discharge uncertainty budget, with separate terms for random (standard error) and systematic (bias) uncertainty components in river discharge timeseries. We expect that discharge uncertainty will be less than 30% for two thirds of global reaches and will be dominated by bias. Separate river discharge estimates will combine both SWOT and in situ data; these "gage constrained" discharge estimates can be expected to have lower systematic uncertainty. Temporal variations in river discharge timeseries will be dominated by random error and are expected to be estimated to within 15% for nearly all reaches, allowing accurate inference of event flow dynamics globally, including in ungaged basins. We believe this level of accuracy lays the groundwork for SWOT to enable breakthroughs in global hydrologic science.

Turning Lakes Into River Gauges Using the LakeFlow Algorithm

“…Since RSQ algorithms are sensitive to prior FLP estimates (Bonnema, Sikder, Hossain, et al., 2016; Durand et al., 2016; Tuozzolo et al., 2019), the more accurate LakeFlow bathymetries could improve the performance and efficiency of other RSQ algorithms near river‐lake interfaces. Thus, there is potential to implement LakeFlow into the SWOT Confluence program (Durand et al., 2023) to inform other SWOT RSQ algorithms while also extending the SWOT discharge product to river‐lake boundaries.…”

Turning lakes into river gauges using the LakeFlow algorithm

“…Since RSQ algorithms are sensitive to prior FLP estimates Durand et al, 2016;, the more accurate LakeFlow bathymetries could improve the performance and efficiency of other RSQ algorithms near river-lake interfaces. Thus, there is potential to implement LakeFlow into the SWOT Confluence program (Durand et al, 2023) to inform other SWOT RSQ algorithms.…”

“…We further compare LakeFlow FLP estimates with the SoS prior estimates to assess LakeFlow's capabilities for informing other SWOT-based RSQ algorithms. The SoS FLPs are chosen for comparison as these are the default prior FLP estimates for SWOT RSQ algorithms (Durand et al, 2023) (see section 1 for more information).…”

“…hydraulic geometry, Manning's equation), mass conservation principles, and a priori estimates of non-SWOT-observable flow-law parameters (FLP) such as frictional resistance (Manning's n) and bathymetry . These SWOT discharge estimates will be practically produced using the Confluence program which houses several different RSQ algorithms (Durand et al, 2023). SWOT RSQ algorithms are sensitive to FLP estimates (Durand et al, 2016) which are provided by the SWORD of Science (SoS) database for all rivers in SWORD .…”

The Surface Water and Ocean Topography Mission: A Breakthrough in Radar Remote Sensing of the Ocean and Land Surface Water

“…Further analysis will be required for robust, quantitative validation. SWOT measurements will allow space‐based inference of lake water storage (Biancamaria et al., 2016) and river discharge (Durand et al., 2023) to complement geographically limited ground‐based measurements. Our preliminary results suggest that SWOT can measure these key variables crucial to the study of the global water cycle for nearly 6 million lakes and approximately 160,000 ∼10 km river reaches (Altenau et al., 2021).…”