Temporal variations of river basin waters from Topex/Poseidon satellite altimetry. Application to the Amazon basin

Although developed and optimised for open oceans, satellite altimetry has the potential to monitor level variations of inland surface waters such as lakes and rivers. Here we present results of water level variations of the Amazon River based on eight years (1993–2000) of altimetry data of the Topex/Poseidon satellite. We first discuss methods to detect wet surfaces from the altimetric measurements, discriminate between water and dry land, and quantify the accuracy of altimetric measurements over water. Then we show water level fluctuations at selected locations where the satellite crosses the Amazon River. The dominant signal is seasonal, mostly annual, with an amplitude of up to 10–15 m peak to peak. Comparison with in situ measurements indicates that water levels are well measured by Topex/Poseidon during high-water season, unlike low-water season, which suffers from data gaps. We further discuss the interannual component of the signal, which shows two marked minima in 1995 and 1998. The 1998 minimum is interpreted as an effect of the 1997–1998 ENSO event, causing rainfall deficit in the central part of the Amazon basin, hence decrease in water levels. An EOF analysis of precipitation fields over the basin during the 1993–1999 period confirms the rainfall minimum by the end of 1997 for this region.