Global Reservoirs/Lakes (G-REALM)


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
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Recent Updates

  • August 18th 2017: Operational updates of the lake water level products has resumed.
  • July 2017: Weekly updates to the lake products have been halted due to technical issues. Operations will resume in August.
  • December 2016: Operations have resumed with data from the NASA/CNES Jason-3 mission. New operational products (e.g. lake0314.TPJOJ.2.txt) are now available. The corresponding status products (e.g. lake0314.TPJOJ.1.txt) will follow shortly. Note the product filename change to "TPJOJ". The merger of Jason-3 results to Jason-2 may be poor where there is little overlapping Jason-3 data, and such products will be reassessed in 2017. The G-REALM team have noted that for a few lakes/reservoirs (e.g. 1619.Three Gorges Dam) there has been no Jason-3 data available so far, and this has been due to a change in surface tracking algorithm. Both the team and the mission agency's are striving to rectify this as soon as possible.
  • A table depicting which lake/reservoir products are available, and over what time period, can now be viewed. Look to the clickable link within the Project Information section below-left.
  • 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.
  • End users must also note that the graphs/products associated with the filtered time series are provided as a visualization aid only.
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Background

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. 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.
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Semi-Automated Data Processing

The project currently utilizes near-real time data from the Jason-3 mission, and archive data from the Jason-2/OSTM, Jason-1, Topex/Poseidon, and ENVISAT missions. 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.
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Satellite Radar Altimetry

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 13.6 or 35.75 GHz, 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-current 10 days
HY-2A 2011-current 14 days
Seasat 1978 17 days
Geosat 1985-1989 17 days
GFO 2000-2008 17 days
Sentinel-3A 2016-current 27 days
Sentinel-3B 2018-current 27 days
ERS-1 1992-1993, 1995-1996 35 days
ERS-2 1996-2003 35 days
ENVISAT 2002-2010 35 days
SARAL 2013-2016 35 days
ICESat-1 (laser) 2003-2009 90 days
Cryosat-2 2010-current 365 days
ICESat-2 (laser) 2018-current variable
Jason-CS/Sentinel-6 Launch 2020 variable
SWOT Launch 2021 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.
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Advantages

  • 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.

Limitations

  • 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.
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Datasets

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), Jason-2 (2008 to 2016), and Jason-3 (2016 to present), and utilizes a mix of both IGDR and GDR data sets. 2) ENVISAT data with a 35-day resolution and extending to North/South latitude 81 degrees. Currently Jason-3 IGDR data are being used for near real time operational monitoring at 10-day resolution. The ENVISAT products will be extended in time with data from the historical ERS and SARAL missions, and they will be appended with Sentinel-3A measurements which will provide near real time monitoring at 27-day resolution.
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Products

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, lakes ≥ 100km2 are included but future project phases will aim to include those in the 50-100km2 size range.
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.
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For More Information Contact:

  • Dr. Charon Birkett
    ESSIC
    University of Maryland
    College Park, USA
    Tel: (301) 405 9296
    Fax: (301) 405 8468
    cmb@essic.umd.edu
  • 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
    USA
    Tel: (202) 690-0134
    Fax: (202) 720-1158
    Curt.reynolds@fas.usda.gov
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Funding Acknowledgement

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 https://ipad.fas.usda.gov/cropexplorer/global_reservoir/.
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Disclaimer

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.