Satellite microwave sensors provide global coverage of the Earth’s land surface on a near-daily basis and, at certain wavelengths, without significant interference from cloud cover. Using a strategy first developed for wide-area optical sensors (see for example: Brakenridge and others, 2005), such data can be used to measure river discharge changes. As rivers rise and discharge increases, floodplain water surface area increases. Microwave emission or backscatter observed from space can monitor such changes: just as ground stations track discharge by measuring river level (stage) changes. Both methods use a high frequency proxy measurement (water surface area or water level) to provide the needed temporal sampling; both require the observational data to be calibrated to discharge units. If accurate discharge values can be obtained and the contributing watershed area is known, then runoff volume (expressed as mm/contributing area) can also be computed.

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 NASA/Japanese Space AgencyAdvanced Scanning Microradiometer (AMSR-E) band at 36.5 GHz (descending orbit only, horizontal polarization) and also the NASA/Japanese Space Agency TRMM 37 GHz channel. The discharge estimator is a ratio of: C/M. C is the 95th percentile value of the brightest (driest) calibration pixel brightness temperature for a 7x7 pixel array of calibration pixels centered on M, the measurement target. M is the brightness temperature for the river measurement pixel, centered over the river reach. At mid-latitudes, pixel dimensions are approximately 10 km.

Due to low emission from water surfaces compared to land surfaces, and depending on channel and floodplain morphology, this ratio responds strongly to river discharge changes in the measurement pixel. Modeling and empirical studies demonstrate that the ratio is, in contrast, relatively 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 changes the C/M ratio as low-emission water replaces ice within the pixel.

Transformation of the remote sensing signal to actual discharge values is accomplished via use of a rating equation (as the case for stage-based gauging stations on the ground). The calibrating discharge values are obtained via runs of a global runoff model (WBMsed). The years 2003-2006 were chosen as the calibration period, and the model was run to produce daily discharge values at each measurement site. As shown on the measurement site displays, a signal-discharge rating curve is constructed for each site based on the resulting monthly mean, maximum, and minimum discharge values for this time as compared to the identical statistics for the M/C ratio. The rating curve is then used on an ongoing basis to transform the incoming signal data to discharge.

This method can also make use of the planned NASA GPM mission data. The approach is novel in that microwave sensors used, mainly, to monitor the atmosphere and precipitation are here employed to directly measure surface runoff as well. The accuracy and precision of such orbital measurements have been tested along U.S. and European rivers.

This is a cooperative project between the University of Colorado, Boulder, CO, USA and GDACS-GFDS (Global Disaster Alert Coordination System, Global Flood Detection System), European Commission Joint Research Centre, Ispra, Italy. The Dartmouth Flood Observatory at the University of Colorado is supported in part by grants from NASA. Co-Investigators: G. R. Brakenridge*, S. Cohen*, A. J. Kettner*, J.P.M. Syvitski*, T. De Groeve**, S.V. Nghiem***

*CSDMS/INSTAAR, University of Colorado;Boulder, Colorado; **Joint Research Centre, Ispra, Italy; ***Jet Propulsion Laboratory, California

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