The level of lakes (such as the American and African Great Lakes, etc) varies through the seasons according to inputs (rain rates, snow melting, etc) and outputs (evaporation, withdrawal, etc), and is thus a very sensitive indicator of regional climate variations.
fig 1. Level of the African Great Lakes, as seen by altimeters. (Credits De Montfort University)
Moreover, the level of enclosed seas (Aral Sea, Caspian Sea, etc) is a major indicator of their good (or bad) health. Altimetry enables us to continually monitor these levels, even in areas which are difficult to access.
|fig 2. Sea level in both basins of the Aral Sea measured by altimeters (multi-satellite data collected between 1993 and 2012 and some in situ measurements) (left: North Aral, right: South Aral). (Credits CNES/Legos)|
Studying altimetry over lakes was first undertaken to validate altimeter measurements, lakes having few dynamics compared to the ocean, and many of them being monitored. Today, a great number of lakes of all sizes are monitored by altimetry. However, in situ data (river runoff, temperature, or precipitation) are still critically needed for studying the evolution of each lake’s water mass balance. 43 lake systems can be observed by Topex/Poseidon or Jason-1, and 215 by ERS-2 or Envisat, out of a total global population of 842 lake systems of more than 100 km².