Each pass of the satellite provides a swath of new data. These are downloaded and then processed: each swath pixel is compared with the location of over 2500 gauging sites and retained as an observation in case it is closer than 0.1 decimal degrees. There are two types of sites: calibration (C) or dry site and measurement (M) or wet site.

The ratio of the dry (C) and the wet (M) site is sensitive to surface water change. The unfiltered ratio is the ratio of C and M for each observation. Because of spatial variability, averaging is needed to get a stable signal: this is done by averaging the C and M values over 4 days and then taking the ratio. The signal should be sensitive to surface water change, but not to the geometry of the swaths.

A preliminary method to detect potential floods uses the statistics (average and standard deviation) of the signal per site; only after one year of data has been gathered this method will be somewhat reliable. The flood level is set to major flood (3) for the ~95 percentile (2 stdev), to flood (2) for the ~85 percentile (1.5 stdev) and to normal (1) below that. Low flow is currently not considered.

Publications:

• De Groeve, T., Z. Kugler, G. R. Brakenridge, 2007. Near Real Time Flood Alerting for the Global Distaser Alert and Coordination System. Proceedings of the 4^th International ISCRAM Conference (B. Van de Walle, P. Burghardt and C. Nieuwenhuis, eds.) Delft, the Netherlands, May 2007, in press
• Brakenridge, G. R., Nghiem, S.V., Anderson, E., Mic, R. (2007) Orbital microwave measurement of river discharge and ice status, Water Resources Research, in press