MEASUREMENT OF RIVER LEVEL VARIATIONS WITH SATELLITE ALTIMETRY

Quantitative assessment of water levels and river discharge is often m ade difficult by large distances, limited access, and low population d ensities in remote areas. Satellite altimetry provides a repetitive re mote sensing approach to determining river levels at a number of locat ions within a river system, providing the orbital repeat cycle is shor t enough in time, the ground track maintains a stable repeat over prev ious locations, and the return power of the altimeter signal can be re adily identified and located. The U.S. Navy's Geosat radar altimeter m ission between 1985 and 1989 provided the first altimeter measurements with sufficient precision and extended duration to examine the utilit y of such measurements for long-term monitoring of inland waters. Thes e measurements have been examined over the Amazon basin. Satellite obs ervations are retrieved at four locations that overlap with river gaug e measurements. A technique is developed to isolate radar return signa ls from the river. Two years of satellite measurements are compared wi th the river gauge retrievals. The overall level of comparison is 0.7 m rms when the technique is applied manually, and 1.2 m rms when an au tomated version of the method is applied. At one location the average difference is 0.2 m rms. This level of accuracy may not be useful for routine hydrological measurements. However, there are a variety of dif ficulties that are specific to the Geosat altimeter measurement over r ough terrain. Present altimeter satellites, ERS 1 (launched June 1991) and TOPEX/Poseidon (launched August 1992), correct many of these prob lems. This study suggests that the prospect for obtaining useful measu rements of river level from space is promising.