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The earliest altimetry missions were dedicated to studying the open ocean and some ice measurements. However, as scientists always like to see ‘what happens if…’, they began looking at the levels of lakes, then rivers as measured by altimeters. Some experiments have also been conducted over solid land, to observe and analyse the signal sent back to the altimeter.

Altimetry has the advantage of taking being able to take global, homogeneous, repeated measurements (thus enabling systematic monitoring to be carried out over several years), unhindered by clouds, night or even vegetation. The measured surface heights are referenced to the same frame. However, it this technique is mainly optimised for the ocean (but although specific land re-tracking can be applied) and takes measurements only at the nadir (i.e. just under below the satellite), with a rather narrow footprint — and averagiaveraging everything in that footprint. Over non-ocean surfaces (wet or dry), the accuracy of the altimetry measurements is can be degraded to by several centimetres or tens of centimetres, mainly because of the heterogeneity of the reflecting surface (a mix of water and emerged landsland surfaces). Another important source of error lies in the signal’s propagation of the signal through the atmosphere. The satellites’ repeat-orbits are rather long (10 to 35 days), which do not fit withsuit real-time monitoring of river or lake level variations (e.g. flood alerts), but agree do work well with seasonal or interannual monitoring.

References:

Alsdorf, D., Birkett, C.M., Dunne, T., Melack, J., Hess, L., Water level changes in a large Amazon lake measured with spaceborne radar interferometry and altimetry. Geophys. Res. Let., 28 (14), 2671-2674, 2001.
Berry P. A. M., J. D. Garlick, J. A. Freeman, E. L. Mathers, Global inland water monitoring from multi-mission altimetry, Geophys. Res. Lett., 32, L16401, doi:10.1029/2005GL022814, 2005.
Birkett, C.M. The contribution of TOPEX/POSEIDON to the global monitoring of climatically sensitive lakes. J. Geophys. Res. 100 (C12), 25179-25204, 1995.
Birkett, C.M., Contribution of the TOPEX NASA radar altimeter to the global monitoring of large rivers and wetlands. Water Resour. Res., 34 (5), 1223-1239, 1998.
Maheu, C., Cazenave, A., Mechoso, C.R., Water level fluctuations in La Plata basin (South America) from TOPEX/Poseidon satellite altimetry. Geophysical Research Letters, 30.3, 2003.
Mercier, F., Cazenave, A., Interannual lake level fluctuations in Africa (1993-1999) from Topex-Poseidon: connections with ocean-atmosphere interactions over the Indian Ocean. Global and Planetary Change, 32, 141-163, 2002.
Stanev, E.V., Peneva, E.L., Mercier, F., Temporal and spatial patterns of sea level in inland basins : Recent events in the Aral Sea. Geophysical Research Letters Vol 31 N°15, 2004.

Further information:

River and Lake (ESA)
Hydrology from satellite altimetry (Legos/CNRS, France)
Global Reservoir and lake monitor (U.S. Department of Agriculture, USA)