August, 2007 — Jason-1 data modification -- Small modifications have recently been made to the lake level variations derived from the NASA/CNES Jason-1 mission. The update more accurately aligns the results of the Jason-1 mission to those of its predecessor Topex/Poseidon during the tandem phase (2002) and enables improved continuity from 2002 to the present day. Users are urged to utilize the revised Topex/Poseidon/Jason graphs and text files (TPJ.2 or Version 2) and to note that that new height offset between the missions (the 'relative bias') has been added to the header of the ascii text files.
	August, 2007 — GFO data added -- Many of the lakes in the database are missing results derived from the Jason-1 mission. This is due to additional on-board and ground-based data filtering and out of the control of the reservoir project team. However, additional USDA and NASA funding is enabling the exploration of data from the US Navy's Geosat Follow On Mission (GFO). Preliminary GFO results are now provided here (TPJG.1 or Version 1) for a sub-set of the lakes. Users can access these results via clicking on the appropriate text next to the TPJ graphic products. Note that GFO samples each lake at a different location than Jason-1. Research continues to improve accuracy and consistency with respect to the NASA/CNES satellite data.

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 approximately 100 lakes located around the world. This project is unique, being the first of its kind to utilize near-real time radar altimeter data over inland water bodies in an operational manner. Surface elevation products are produced via a semi-automated process and placed at this web site for USDA and public viewing. Monitoring heights for ~100 reservoirs and lakes around the world will greatly assist the USDA/FAS/PECAD 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 around the world. Reservoir and Lake height variations may be viewed by placing the cursor on and clicking the continent of interest.

Semi-Automated Data Processing
The project utilizes near-real time radar altimeter data from the Poseidon-2 instrument on-board the Jason-1 satellite which was launched in December, 2001. In addition, historical archive data is used from the TOPEX/POSEIDON mission (1992-2002). Data processing procedures closely follow methods developed by the NASA Ocean Altimeter Pathfinder Project. When fully operational, products should be delivered within 7-10 days after satellite overpass and the resulting time series of height variations are expected to be accurate to better than 10cm rms.

Satellite radar altimeters can potentially monitor the variation of surface water height for many large inland water bodies (lake,wetland or river) Topex/Poseidon measurement system. However, there are limitations which affect both the size and number of observable targets. The complexity and severity of the surrounding topography also plays a role. These details and more can be found by clicking the associated links in the left-hand frame. For example, these instruments are nadir pointing (non-imaging) and so only those targets directly beneath the satellite overpass are potentially observable e.g. Topex/Poseidon ground tracks.

Satellite Radar Altimetry

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In General: A satellite radar altimeter is not an imaging device, but continuously records average surface `spot' heights 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.During 1993 for example, 4 are operating: T/P, Jason, ENVISAT and GFO.

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 (Birkett 1994, Morris and Gill 1994, Birkett 1995a, Dalton and Kite 1995, Cazenave et al. 1997, Birkett et al. 1999). 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 (Rapley et al. 1987, Cudlip et al. 1990, Koblinsky et al. 1993, Birkett, 1995b, Birkett 1998, Rosenqvist et al. 1999, Birkett, 2000). In particular, the results demonstrate how submonthly, seasonal,and interannual variations in height can be monitored.

For full details on Satellite Radar Altimetry, see the References section. Particularly useful are the Chelton et al., references.

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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Two 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 The follow-on mission to T/P, the Jason 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 orbital repeatability is 10-days with the same global coverage. The Jason IGDR datasets are available via ftp at podaac.jpl.nasa.gov.

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 TOPEX (the NASA ALT or SSALT) altimeters, the red symbol denotes results from the Jason POSEIDON-2 altimeter. 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 Mailcode 923 NASA/GSFC Greenbelt, MD 20771 USA Tel:(301)614-6643 Fax:(301)614-6699 cmb@nemo.gsfc.nasa.gov	Dr. Curt Reynolds USDA-FAS-CMP-PECAD 1400 Independence Ave, SW South Building Stop 1045, Room 6053, Washington, DC 20250 USA Tel:(202)690-0134 Fax:(202)720-8880 Curt.Reynolds@fas.usda.gov

Acknowledgement

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These lake products exist in the public domain. However, the following general acknowledgement of this database should be made if the information presented here is used for further scientific purposes and/or additional applications:

Radar altimeter data from the NASA/CNES Topex/Poseidon and Jason-1 satellite missions. Time series of altimetric lake level variations from the USDA Reservoir Database at http://www.pecad.fas.usda.gov/cropexplorer/global_reservoir

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/Raytheon Project Investigators accept no responsibility for the accuracy and application of the lake leve products held in this database.