October, 2009 —  New time-series graphs for lake and reservoir water-levels measured by the Jason-2/OSTM satellite are now available. These new time-series graphs are preliminary and will be revised as the USDA/NASA GRLM project continues. The new time-series graphs have filenames called TPJO.1 (which stands for Topex/Poseidon/Jason/OSTM version 1). Users please note these TPJO.1 products also contain revised lake level estimates from the T/P and Jason-1 missions, i.e. there has been a revision of the old TPJ.2 products. See the reference section for details. The Jason-2 or Ocean Surface Topography Mission (OSTM) was launched on June 20, 2008. It is the follow-on satellite to the Topex/Poseidon (1992-2002) and Jason-1 (2002-2008) missions, and continues the observations of surface water levels. The OSTM mission is a joint effort between NASA, CNES, NOAA and EUMETSAT with science objectives that focus on the ocean, as well as coastal and inland water regions. The new lake and reservoir water levels observed by the OSTM radar altimeter instrument will be continuously added to the existing time series on a weekly basis. Nevertheless, validation exercises, comparing TPJO.1 with ground based daily gauge data on the Great Lakes, USA are showing that the root mean square (rms) error on the current products are better than 10cm and this accuracy can be assumed for similar sized large lakes around the world. Additional information on the Jason-2/OSTM mission can be found at:

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 currently utilizes near-real time NASA, CNES and NRL 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. Currently ~75 lakes are included but his will expand to ~600 as the program incorporates data from the ESA ERS and ENVISAT missions. 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

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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).

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

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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.

Datasets

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Several altimetric datasets are currently being exploited: 1) TOPEX/POSEIDON (T/P) In its original orbit (1992-2002) this satellite operated with a 10-day repeat orbit, with global coverage extending to North/South latitude 66 degrees. 10 years of archived GDR data (from September 1992) are utilized. T/P datasets have been provided by AVISO/CNES (Version C) and the NASA Physical Oceanography DAAC at the Jet Propulsion Laboratory, California Institute of Technology. 2) Jason-1 The follow-on mission to T/P, the Jason-1 satellite was launched on December 7th 2001. This project utilizes the IGDR Jason data which is typically available within 3-4 days after satellite overpass. Like T/P, the Jason-1 orbital repeatability is 10-days with the same global coverage. The Jason-1 IGDR datasets are available via ftp at podaac.jpl.nasa.gov. 3) Jason-2 or OSTM, the follow on mission to Jason-1 launched in June 2008. OSTM IGDR data are utilized and can obtained from both AVISO and NOAA. 4) The project also made use of data from the NRL's Geosat Follow On mission (or GFO). In this case GDR data from NOAA was acquired.

Products

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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:

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	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-FAS-OGA-IPAD 1400 Independence Ave, SW South Building Stop 1051, Room 4649, Washington, DC 20250 USA Tel:(202)690-0134 Fax:(202)720-8880 Curt.Reynolds@fas.usda.gov

Funding Acknowledgement

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These lake products exist in the public domain and were funded as part of the USDA/FAS/OGA and NASA Global Agriculture Monitoring (GLAM) Project. 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:

USDA/FAS/OGA and NASA Global Agriculture Monitoring (GLAM) Project. Lake and reservoir surface height variations from the USDA’s Global Reservoir and Lake (GRLM) web site at: http://www.pecad.fas.usda.gov/cropexplorer/global_reservoir/. Altimetric lake level time-series variations from the Topex/Poseidon, Jason-1, Jason-2/OSTM, and Geosat Follow-On (GFO) missions.

Disclaimer

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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.