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What is Surface Water?
Water occupies about 71% of the Earth's surface, of which only 2.5% is freshwater in the form of lakes, reservoirs, and rivers. Hydrologic observations of rivers, lakes, and reservoirs are extremely important given humanity's reliance on freshwater for survival, and are of major importance to understand the global water cycle. Existing stream gauge systems are limited in many areas, especially remote and politically unstable environments where people are often most vulnerable to floods and droughts. Remote sensing techniques, such as altimetry and interferometry, enable measurements of surface water to understand the global water cycle on land, as well as provide a global inventory of water resources.
How is Surface Water Measured?
Altimetric missions, such as TOPEX/Poseidon and the Jason-series, enable monitoring of major inland waters. However, systematic, global monitoring of inland waters with altimeters has limitations as they provide measurements directly at the satellite’s nadir, greatly restricting spatial sampling. Nadir altimeters miss more than 60% of lakes, thus only measuring a small fraction of the global lake storage change. The utilization of interferometry, such as the SWOT mission, will provide high-resolution observations over nearly all surface water bodies (e.g., lakes, rivers, reservoirs). Contrary to nadir altimetry, SWOT should measure 65% of the global lake/reservoir storage change. SWOT will provide the very first comprehensive view of Earth's surface inland water bodies from space and will allow scientists to determine changing volumes of inland water across the globe at an unprecedented resolution. Hydrologists will use the data to calculate the rate of water gained or lost in lakes, reservoirs, as well as discharge variations in rivers, globally.