Lakes and enclosed seas monitoring
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. 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.
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.