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.