SMAP Passive Microwave Radiometer for Global River Flow Monitoring

River flow is a fundamental observable in hydrology, but there is no consistent global ground measurement network. Various types of orbital remote sensing are, therefore, well positioned to meet an important observational need, including for hydrological modeling and for understanding trends through time. In previous studies, we showed that passive microwave radiometry (PMR) can measure streamflow over selected locations around the globe with a high correlation to colocated in situ discharge observations. This article demonstrates the potential of low-frequency, L-band NASA Soil Moisture Active and Passive (SMAP) satellite observations for streamflow measurement: an unanticipated but exceptionally valuable use of this sensor. By using the fully polarimetric capability of SMAP with full Stokes parameters, we optimize the polarization combinations of the observations to retrieve accurate river hydrographs from space. Flow measurements over 150 satellite gauging reaches (SGR) are retrieved over different continents, and 14 SGRs provide comparisons to available in situ river gauging data. Results from linear correlation calculations provide coefficients of determination $r<^>{2}$ of approximately 0.75 for SMAP-based discharge measurements when compared to in situ streamflow observations. SMAP river observations thereby improve river gauging results compared to ESA's Soil Moisture Ocean Salinity (SMOS) satellite L-band PMR as the analysis indicates typically lower $r<^>{2}$ values of approximately 0.68 for SMOS.