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.
Instrument Summary
Satellite |
Operation |
Repeat Period |
SEASAT |
1978 |
17 days |
GEOSAT |
1986-1989 |
17 days |
ERS-1 |
1991-1996 |
35 days (phases C+G) |
T/P |
1992-2002 |
10 days |
ERS-2 |
1995-2003 |
35 days |
GFO |
post 2001 |
17 days |
ENVISAT |
post 2002 |
35 days |
JASON-1 |
post 2002 |
10 days |
T/P(new orbit) |
post 2002 |
10 days |
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.
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.
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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|
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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 |
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
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.