A New Remote Sensing Tool for Water Resources
Management
Charon Birkett and Brad Doorn
Traditional ground-based
gauge instruments have been measuring the levels of lakes
and rivers for many decades, serving the scientific
community, by monitoring the ever changing daily, seasonal
and inter annual fluctuations. The gauges do an excellent
job, often recording height measurements (known as
“stage”) hourly or twice daily, with accuracies better
than a centimeter. Some automated gauges transmit their
measurements via satellite for rapid dissemination of
data.
In some regions of the world however, such gauge
information may be classed as restricted, or the data
release may have to be delayed due to national policy.
Many lakes are located in remote regions, which inhibit
gauge deployment and/or regular maintenance. Some gauge
systems may also lack the sophistication of modern
technology (Figure 1). Reductions in national or regional
budgets, even in the industry-rich nations, are leading to
the shutdown of many long-term gauge sites. All of these
factors make satellite remote sensing an attractive tool
to provide additional stage data to enhance the
ground-based networks.
Over the past couple of decades, the use of
satellite radar altimetry, to monitor the variations in
the surface elevations of lakes, rivers, wetlands and
floodplains, has moved from exploratory studies to
near-real time semi-operational programs based on
well-validated techniques. The current NASA/CNES (Centre
National D’Etudes Spatiales, the French counterpart of
NASA), NOAA and ESA satellites all have the ability to
retrieve stage data for many inland waters regions around
the world.
The history of satellite radar altimetry is linked
with early attempts to map the surface of the oceans.
Several generations of instruments have led to both
ocean-dedicated missions, such as Seasat, Geosat, GFO,
Topex/Poseidon, and Jason-1, and to multi-discipline
missions such as ERS and ENVISAT. The technique, based on
the emission of a microwave pulse towards the surface, and
the subsequent capture and interpretation of the returned
echo, has the incredible ability to measure surface water
height over oceans and large lakes to a few centimeters
accuracy. That’s an amazing feat, considering the
satellites are 800-1300 km above the Earth’s surface.
This level of accuracy depends on precise knowledge of the
satellite’s orbit, excellent geophysical auxiliary data,
and robust instrument tracking of the ever-changing
variations in the Earth’s surface.
With the application of satellite radar altimetry
over inland waters at an advanced stage, the US Department
of Agriculture/Foreign Agriculture Service, Production
Estimation and Crop Assessment Division (USDA/FAS/PECAD)
approached members of NASA’s Goddard Space Flight Center
and researchers at the University of Maryland (UMD) to
learn about the possibility of using the technique in a
near-real time operational manner. With fresh drinking
water in short supply throughout much of the world, and
the limited availability of water for irrigation, members
of PECAD stressed the need for determining how much water
is stored in the large lakes and reservoirs around the
world. Although PECAD already relied on sensors on-board
the Terra, Aqua and TRMM satellites to assess current crop
conditions, the satellite radar altimetry offered the
Division a new layer of information to assist agriculture
monitoring on a global scale. In turn, the distribution of
enhanced crop information would aid crop producers,
agribusiness, commodity traders, policy makers, and
international food aid organizations.
Under a USDA, NASA, UMD, and Raytheon ITSS
collaborative effort, a program was launched to monitor
the changing water levels of selected large lakes using
data from the NASA/CNES TOPEX/POSEIDON (T/P) and Jason-1
satellites. Launched in 1992, the T/P mission provides an
historic archive while Jason-1 continues the observing
period from 2002. These satellites cross over some 350 of
the world’s largest lakes, measuring lake level
variations every 10 days with accuracies better than 10 cm
for the larger lakes. With the availability of near-real
time data from the Jason-1 mission, analysts can now
quickly assess drought or high water-level conditions
within the reservoirs and thus determine the effects on
downstream irrigation potential and the consequences on
food trade and subsistence measures. One example from the
database is Lake Tharthar in central Iraq (Figure 2). This
reservoir was intended to provide irrigation water to
downstream regions and also to be a link to the Tigris and
Euphrates rivers via a canal system. Due to a multiple
year drought, the 1999-2001 grain output in Iraq and Syria
was drastically reduced. Increased rainfall, since this
period though, has permitted grain production to recover
and surpass pre-drought levels. Knowledge of water storage
is therefore crucial.
The Global Reservoir and Lake Elevation Database
resulting from the program is the first of its kind to use
near-real time radar altimeter data over inland water
bodies in a semi-operational manner and display the
results on the Internet (http://www.pecad.fas.usda.gov/cropexplorer/global_reservoir)
(Figure 3). Typically, within 1-2 weeks of satellite
overpass, the variations in lake and reservoir levels are
recorded and the database is automatically updated. The
website displays elevation variations together with
appropriate Landsat imagery and MODIS land-cover
classifications. Links to the LakeNet global lakes
database provide an additional range of on-hand lake
information. Currently, the reservoir database holds
records for 70 lakes. This will increase in the 2004/2005
timeframe as additional satellite datasets and refined
techniques are incorporated.
About the Authors
Dr. Charon Birkett is a Research Scientist with the
Earth System Science Interdisciplinary Center (ESSIC) at
the University of Maryland, USA. She specializes in
exploration of satellite radar altimetry over inland
waters for water resources, natural hazards, river
dynamics, and climate change applications. She can be
reached at [email protected].
Dr. Brad Doorn is a remote sensing technical
advisor within the Production Estimation and Crop
Assessment Division of the US Department of
Agriculture’s Foreign Agriculture Service. He can be
reached at [email protected].
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