Orbital Microwave Measurement of River Discharge: Technical Note
G. Robert Brakenridge
Son V. Nghiem
Satellite microwave sensors provide global coverage of the Earth’s land surface on a near-daily basis and without interference from cloud cover. Using a strategy first developed for wide-area optical sensors (see for example: Brakenridge, and others, Space-based measurement of river runoff, EOS, Transactions of the American Geophysical Union, 86, 2005), such data can be used to measure river discharge changes. As rivers rise and discharge increases, floodplain water surface area increases, and microwave emission or backscatter can monitor such changes without interference by cloud cover.
The spatial imaging resolution obtained by the sensor is less important than scene-to-scene calibration and the contrast between water and land signal returns. We use the Advanced Scanning Microradiometer (AMSR-E) band at 36.5 Ghz, descending orbit only, horizontal polarization, and the resampled and gridded level 3 daily global data product. The discharge estimator is a ratio of: C, calibration target brightness temperature (for a local land parcel unaffected by river changes), to M, measurement target brightness temperature (for a pixel centered over the river reach). At mid-latitudes, pixel dimensions are approximately 25 km. Due to low emission from water surfaces, C/M values increase with river discharge, and especially as overbank flow is approached and exceeded. In contrast, the ratio is unaffected by soil moisture, vegetation, or other changes affecting both land parcels. The initiation and removal of river ice cover can also be detected: ice breakup immediately increases the C/M ratio.
Transformation of the remote sensing signal to actual discharge values on an international basis is accomplished by comparison of monthly means where at least intermittent gaging station data are available. The accuracy, and precision of such orbital measurements is tested along U.S. and European rivers where daily in situ discharge data are also available (see additonal note).
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