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Global Reservoirs/Lakes (G-REALM)

Latest Product News

May 6th 2016
  • Water level products for an additional 33 lakes/reservoirs were added to G-REALM in April:
    0349.Ziway (Ethiopia), 1436.Amboseli (Kenya), 0274.Kinkony (Madagascar), 0418.Faguibine (Mali), 0063.Rkiz (Mauritania), 1439.Shiroro (Nigeria), 1431.Sterkfontein (South Africa), 1458.Mtera (Tanzania)
  • ASIA
    1448.Karai (Iran), 0447.Hammar (Iraq), 0121.Pangong (India), 0519.Gandhi (India), 0958.Vallabh (India), 0625.Tarbela (Pakistan), 1503.Manchar (Pakistan), 0980.Noi (Thailand)
    0320.Eyre (Australia), 0332.Torrens (Australia), 0758.Victoria_2 (Australia), 1440.Alexandrina (Australia), 1441.Albert(2) (Australia)
    0718.Badajos (Mexico)
    0111.Ercek (Turkey), 1450.Karakaya (Turkey)
    0345.Sobradinho (Brazil), 0422.Grazau (Brazil), 0448.Vermelha (Brazil), 0786.Araros (Brazil), 0804.Barragem (Brazil), 1419.Emborcacao (Brazil), 1422.Itumbiara (Brazil), 1554.Peixoto (Brazil), 1621.NovaPonte (Brazil)

October 8th 2015 – Upgrades and New Products   
  • The surface water level variation products have recently been upgraded (from TPJO.1.1 to TPJO.1.3 and TPJO.2 to TPJO.2.3). They have been re-calculated based on updated altimetric parameters and a revised reference datum. End users are advised not to mix the old/new product versions.
  • The upgraded TPJO.2.3 products now include correction factors in the ascii text file header which will enable end users to translate the satellite-based time series into various orthometric frames (approximating mean sea level). At the lake product level there is also the option (left-hand panel) to view the reference and geoid height profiles which were used to determine these correction factors. Details can be found in the Q&A link.
  • Additional lake products displaying variations from 2008 to the present day at 10-day resolution are also now available..

NOTE !!!
  1. End users must consider the location of the satellite ground track AND the section of track used to create the products. This is particularly important for complex and drought-prone lakes and reservoirs, where height variability may be location dependent. See the product headers.
  2. End users must also note that the graphs/products associated with the filtered time series are provided as a visualization aid only.

10-day Near Real Time products with datum based on a single satellite overpass (1 day)
10-day Status products with datum based on a 9 year (1993-2001) mean
Click here to add your Comments/Feedback
One-click Options
Click here to download a summary list of lakes relevant to the 10-day near real time products
Click here to download a summary list of lakes relevant to the 10-day Lake Status products
Click here to download the Near Real Time Graph files
Click here to download the Near Real Time Text files
Click here to download the Near Real Time Smoothed Text files
Click here to download the Lake Status Graph files
Click here to download the Lake Status Text files
Click here to download the Lake Status Smoothed Text files
Click here to view the ENVISAT map
Click here to download the 35-day product ENVISAT Lake Graph files
Click here to download the 35-day product ENVISAT Lake Text files
Click here to download the 35-day product ENVISAT Lake Smooth Text files
Click here to download a summary list of 35-day product ENVISAT product lakes

Project Information


Altimetry Data

Ground-based Data and Information

Floods and Droughts

Other Altimetric Products




The U.S. Department of Agriculture's Foreign Agricultural Service (USDA-FAS), in co-operation with the National Aeronautics and Space Administration, and the University of Maryland, are routinely monitoring lake and reservoir height variations for many large lakes around the world. The program utilizes NASA/CNES/ESA/ISRO radar altimeter data over inland water bodies in an operational manner. The surface elevation products are produced via a semi-automated process and placed at this web site for USDA and public viewing. Monitoring height variations will greatly assist the USDA/FAS Office of Global Analysis to quickly locate regional droughts, as well as improve crop production estimates for irrigated regions located downstream from lakes and reservoirs. All targeted lakes and reservoirs are located within major agricultural regions. Reservoir and Lake height variations may be viewed in graphical and text format by placing the cursor on and clicking the continent and lake of interest.

Semi-Automated Data Processing
The project utilizes near-real time radar altimeter data from the Poseidon-3 instrument on-board the Jason-2 (or OSTM) satellite which was launched in June, 2008. In addition, data from the Jason-1 mission (2002-2008) and an historical data archive from the TOPEX/POSEIDON mission (1992-2002) are also used. Data processing procedures closely follow methods developed by the NASA Ocean Altimeter Pathfinder Project (see references). When fully operational, updated products are delivered within 7-10 days after satellite overpass. The resulting time series of height variations are expected to be accurate to better than 10cm rms for the largest (and more open) bodies of water such as The Great Lakes, USA, Lakes Victoria and Tanganyika in Africa etc. Smaller lakes or those that experience more sheltered (from wind) conditions can expect to have accuracy's better than 20cm rms (e.g. Lake Chad, Africa). Satellite passes that cross over narrow reservoir extents in severe terrain will push the limits of the instruments with resulting rms values of many tens of centimeters. Despite limitations, satellite radar altimeters can potentially monitor the variation of surface water height for many large inland water bodies including lake, reservoir, wetland region and river channel. Full details and references can be found by clicking the associated links in the left-hand frame.

Satellite Radar Altimetry Top
In General: A satellite radar altimeter is not an imaging device, but a nadir-pointing instrument continuously recording average surface `spot' heights directly below the satellite, as it transverses over the Earth's surface. Operating at ~13.6GHz, each altimeter emits a series of microwave pulses towards the surface. By noting the two-way time delay between pulse emission and echo reception, the surface height can be deduced. Each returned height value is an average of all surface heights found within the footprint of the altimeter. The diameter of the footprint depends on the surface roughness, but can typically range between 200m (for open pools of water in calm conditions) to a few kilometers (open water with surface waves). Each satellite is placed in a specific repeat orbit, so after a certain number of days the same point (to within 1km), on the Earth's surface is revisited. In this way, time series of surface height changes can be constructed for a particular location along the satellite ground track during the lifetime of the mission.

There have been a number of altimetric satellite missions to date and follow-on missions can be expected (see Figure and Table below).

General Timeline for Satellite Radar Altimeters

Instrument Summary
Satellite Operation Repeat Period
Topex/Poseidon 1992-2002 10 days
Jason-1 2002-2008 10 days
Jason-2/OSTM 2008-2016 10 days
Jason-3 2016- 10 days
HY-2A 2011-current 14 days
Seasat 1985-1989 17 days
Geosat 1985-1989 17 days
GFO 2000-2008 17 days
Sentinel-3A 2016- 27 days
Sentinel-3B 2017- 27 days
ERS-1 1992-1993, 1995-2000 35 days
ERS-2 1995-2003 35 days
ENVISAT 2002-2010 35 days
SARAL 2013-current 35 days
ICESat-1 (laser) 2003-2009 90 days
Cryosat-2 2010-current 365 days
ICESat-2 (laser) 2016- variable
Jason-CS/Sentinel-6 2020- variable
SWOT 2020- variable

Although their primary objectives are ocean and ice studies, altimeters have had considerable success in the monitoring of inland water bodies. In particular, the ability to remotely detect water surface level changes in lakes and inland seas has been demonstrated. Unhindered by time of day, weather, vegetation or canopy cover, the technique has further been applied to a number of rivers, wetlands and floodplains in several test-case studies. In particular, the results demonstrate how submonthly, seasonal, and interannual variations in height can be monitored.

For full details on Satellite Radar Altimetry and the application to inland water see the References section.

Advantages and Limitations of Satellite Radar Altimetry Top


  • Day/night and all weather operation.
  • Generally unhindered by vegetation or canopy cover.
  • All determined surface heights are with respect to one common reference frame.
  • Satellites are placed in repeat orbits (up to 1km either side of a nominal ground track) enabling systematic monitoring of rivers, lakes, wetlands, inland seas and floodplains.
  • Has the potential to contribute height information for any target beneath the satellite overpass, thus contributing information where traditional gauge (stage) data may be absent.
  • Satellite altimetric instruments have been in continuous operation since 1991 and new missions are scheduled for the next decade. There is therefore the ability to monitor seasonal to interannual variations during the lifetime of these satellites.
  • Techniques have been validated and results published in peer-reviewed journals.


  • These instruments are primarily designed to operate over uniform surfaces such as oceans and ice-sheets. Highly undulating or complex topography may cause data loss or non-interpretation of data.
  • Retrieved heights are an "average" of all topography within the instrument footprint. Such values are further averaged in the direction of the satellite motion, giving, for example, one final height value every 580m (TOPEX/POSEIDON) or 350m (ERS) along the ground track. Altimetric values therefore differ from traditional gauge measurements which offer "spot" heights at specific locations.
  • The height accuracy is dominated by knowledge of the satellite orbit, the altimetric range (distance between antenna and target), the geophysical range corrections and the size and type of the target.
  • Unlike imaging instruments, altimeters only retrieve heights along a narrow swath determined by the instrument's footprint size. The effective footprint diameter can vary depending on the nature of the target, and can potentially range from several hundred meters to many kilometers.
  • Minimum target size is controlled by the instrument footprint size and the telemetry/data rates, and also on the surrounding topography and the target-tracking method used.
  • The satellite orbit scenario and target size also determine the spatial and temporal coverage. Improved temporal coverage is gained at the expense of spatial coverage for a single satellite mission.
  • Major wind events, heavy precipitation, tidal effects and the presence of ice will effect data quality and accuracy.

Datasets Top
Several altimetric datasets are currently being exploited: 1) The Topex/Jason series in a 10-day repeat orbit and with global coverage extending to North/South latitude 66 degrees). This comprises T/P (1992-2002), Jason-1 (2002-2008), and Jason-2 (2008 to present day) and a mix of both IGDR and GDR data sets. 2) NRL Geosat Follow On mission (or GFO) data, with a 17-day resolution and extending to North/South latitude 72 degrees, this data was used as an occasional substitute for missing Jason-1 data. 3) ESA/ENVISAT data with a 35-day resolution and extending to North/South latitude 81 degrees. Currently Jason-2/OSTM IGDR data are being used for near real time operational monitoring.

Additional information on the Jason-2/OSTM mission can be found at:

Products Top
The aim of this web site is to provide time-series of water level variations for some of the world's largest lakes and reservoirs. Currently, large (>100 km²) lakes in important agricultural regions are the main targets.
The main database products are graphs and associated information in tabular form. For the Graphs, changes in water level are real but the y-scale is arbitrary (relative) and given in meters. The x-axis refers to time with intervals of several months. The blue symbol represents results from the Topex/Poseidon satellite (the NASA-ALT and SSALT/Poseidon-1 altimeters), the red symbol denotes results from the Jason-1 mission (the Poseidon-2 altimeter), the purple symbol denotes the OSTM or Jason-2 mission (the Poseidon-3 altimeter). Additional graphs may also depict results (green symbol) from the GFO mission.
The Results Table gives heights, associated errors and date/time of the observation. Note that a geographical extent across the lake has been used to derive the time series - rather than a spot measurement which is more typical of a traditional gauge. A discussion on altimetric height accuracy can be found in the Accuracy+Validation section.

For more information contact: Top

  Dr. Charon Birkett
University of Maryland
College Park, USA
Tel: (301) 405 9296
Fax: (301) 405 8468
Robert Tetrault
USDA, Foreign Agricultural Service
Office of Global Analysis, International Production Assessment Division (OGA-IPAD)
1400 Independence Ave, SW
South Building Stop 1051, Room 4651,
Washington, DC 20250
  Dr. Curt Reynolds
USDA, Foreign Agricultural Service
Office of Global Analysis, International Production Assessment Division (OGA-IPAD)
1400 Independence Ave, SW
South Building Stop 1051, Room 4609,
Washington, DC 20250
Tel: (202) 690-0134
Fax: (202) 720-1158

Funding Acknowledgement Top
These lake products exist in the public domain and their creation was funded as part of the USDA/FAS/OGA and NASA Global Agriculture Monitoring (GLAM) Project and the NASA/Applied Sciences/Water Resources Program. The following general acknowledgement of this database should be made if the information presented here is used in publications for further scientific purposes and/or additional applications:

  Lake products courtesy of the USDA/NASA G-REALM program at

Disclaimer Top
Users of these datasets must carefully note the information given in the Accuracy+Validation and Advantages and Limitations sections.

This is an on-going project with elements that reside in the research domain. We therefore reserve the right to state the following liability disclaimer:

The USDA/NASA/UMD/SGT Project Investigators accept no responsibility for the accuracy and application of the lake level products held in this database.

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