A satellite-based technique that measures water surface elevation by timing radar pulse reflections from the Earth's surface. Radar altimetry enables monitoring of lake levels, river stages, and reservoir storage from space.
Radar altimetry uses nadir-looking radar instruments on satellites to measure the distance between the satellite and the Earth's surface by precisely timing the round-trip travel of radar pulses. Combined with accurate satellite orbit determination, this yields water surface elevations with centimeter-level precision. Originally developed for ocean surface topography (Topex/Poseidon, Jason series), radar altimetry has been increasingly applied to inland water bodies including large rivers, lakes, and reservoirs. The SWOT (Surface Water and Ocean Topography) mission, launched in 2022, represents a major advance with its wide-swath interferometric altimeter capable of measuring water surface elevations for rivers wider than 100 m and lakes larger than 250 m x 250 m. Altimetry-derived water level time series are critical for monitoring ungauged river basins, validating hydrological models, and tracking reservoir operations in data-sparse regions. The Hydroweb database (LEGOS/CNES) provides freely available water level time series derived from multiple altimetry missions for thousands of lakes and river reaches worldwide. Combining altimetric water levels with satellite imagery-derived surface areas enables estimation of reservoir and lake storage changes without in-situ data.