The Global Reservoirs and Lakes Monitor (G-REALM) provides time-series data of water level variations, as measured by radar altimeter data, for some of the world's largest lakes and reservoirs, allowing users to monitor water supply used for irrigation, power generation, and consumption. By harnessing satellite altimetry data in an operational monitoring system, G-REALM provides a valuable tool for tracking water resources globally and assessing related impacts on food security.
G-REALM is a program that uses satellite data to routinely track water levels of major lakes and reservoirs around the world. It is a collaboration between the U.S. Department of Agriculture's Foreign Agricultural Service (USDA-FAS) and NASA.
G-REALM processes near-real time measurements from the Jason-3 satellite mission, as well as archived data from several earlier altimetry missions including Jason-2, Jason-1, and Envisat. These satellites carry instruments called radar altimeters that continuously measure the height of water surfaces below the satellite as it passes overhead. By analyzing the satellite height measurements over time, G-REALM can detect variations and trends in water levels for monitored lakes and reservoirs. This information helps users identify developing drought conditions and improve crop production estimates in irrigated agricultural areas supplied by that water.
G-REALM aims to provide updated lake and reservoir height products within 7-10 days of the satellite overpass. Surface height accuracy is estimated to be better than 10cm for large, open water bodies like the African Great Lakes. Smaller or more sheltered lakes have expected accuracy of 20cm or more. Despite limitations over narrow reservoirs, the technique allows large-scale monitoring of inland surface waters including lakes, reservoirs, wetlands, and rivers.