Orbital microwave measurement of river discharge and ice status

Abstract

Satellite passive microwave sensors provide global coverage of the Earth's land surface on a near‐daily basis without severe interference from cloud cover. Using a strategy first developed for wide‐area optical sensors, and in conjunction with even limited ground‐based discharge information, such microwave data can be used to estimate river discharge changes, river ice status, and watershed runoff. Water surface area in a river reach increases as flow widens, and any temporally calibrated observation sensitive to changing water area monitors discharge. The sensor spatial resolution is less important than the scene‐to‐scene calibration and the contrast in upwelling radiance between water and land. We use the Advanced Microwave Scanning Radiometer (AMSR‐E) band at 36.5 GHz, descending orbit, horizontal polarization, and the resampled Level‐3 daily global data product. The discharge estimator HR is a ratio of calibration‐target radiance (expressed as brightness temperature), for a local land parcel unaffected by the river, to measurement‐target brightness temperature, for a pixel centered over the river. At midlatitudes, pixel dimensions are approximately 25 km. Because of low emission from water surfaces, HR increases with discharge as in‐pixel water area expands. It increases sharply once overbank flow conditions occur. River ice‐cover is also detectable. The sensitivity and accuracy of the orbital measurements is tested along U.S. rivers monitored by in situ gaging stations, with favorable results. Other tests demonstrate that for seasonally variable rivers, AMSR‐E can provide useful international measurements of daily river discharge even if only fragmentary monthly mean discharge data are available for calibration.