Passive Microwave Radiometry at Different Frequency Bands for River Discharge Retrievals

The present era of climate change and expanding population requires major improvements in sustained observation of global river discharge. Floods and droughts are affecting food supplies, and suspected long-term trends require appropriate data for evaluation. Orbital remote sensing can address this observational need. Here we use satellite Ka- (36.5 GHz) and L-band (1-2 GHz) passive microwave radiometry (PMR) to monitor river discharge changes and determine what size rivers can be measured and the frequencies and polarization configurations that yield the most robust results. Selected satellite gauging reaches (SGRs) can be measured at near-daily intervals from 1998 to present (Ka-band) and 2010 to present (L-band). The SGRs are 10-36 km in length; the dynamic proportion of water surface area within each varies with river discharge. Due to contrasting dielectric properties, water and land emit different intensities of microwave radiation; thus emission from a mixed water/land pixel decreases as the proportion of water within the pixel increases. Depending on the river and floodplain morphology, water flow area can be a robust indicator of discharge and the microwave sensors can retrieve daily discharge to +/- 20%. Instead of spatial resolution, it is the sensor measurement precision, geolocation accuracy, and channel and floodplain morphology that most strongly affect accuracy. Calibration of flow area signals to discharge can be performed using nearby ground stations (even if now discontinued) or by comparison to hydrologic modeling. River discharge (flow rate) measurements are critical to hydrology research and water resource management worldwide. Such data have long been collected on the ground at gauging stations, but environmental monitoring in an era of climate change and expanding population now requires major improvements in its quantity and quality. River flow area can be used to monitor streamflow variation, and such area observations are available from a variety of orbital satellite sensors. Passive microwave information offers the advantage of frequent (near daily) sampling with little interference from cloud cover. Flow area is monitored for defined satellite gauging reaches (SGRs) measuring 10-36 km in length depending on the sensor and band used. As demonstrated by comparison to ground gauging stations, Ka-band and L-band microwave information can both be used successfully. Ka-band data offer higher spatial resolution and a time series extending back to 1998. L-band SGR data from the SMAP polarimetric radiometer are less sensitive to small geolocation errors, and may provide better results especially for the larger rivers or those with forested floodplains.