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Ever since the gun toting days of the Old West, Texans and Mexicans have
argued bitterly over the rights to the water flowing in the Rio Grande. The
river is one of the only sources of fresh water in the dusty, sun-scorched,
region. Access to this precious desert resource has an impact on the health,
wealth, agriculture, and industry of communities on both sides of the river’s
||The Rio Grande flows through arid parts of Colorado and New Mexico (map), and along the southern two-thirds of its length forms the border between Texas and Mexico. It is a primary source of fresh water for nearby communities. (Photograph copyright Philip Greenspun)|
More and more, however, a third party is inserting itself into this age-old dispute. Exotic water plants such as the hyacinths and hydrilla have been taking over large sections of the river. Not only do they draw water up in their roots and transpire it into the atmosphere, but these invader species also clog the free flow of the river. The plants have some of the highest growth rates in the world and can double their population in just 12 days. Given the length of the river and the rugged countryside, simply tracking the infestations is a problem.
With help from NASA, both Mexican and U.S. water managers may finally have a
chance at controlling the aquatic annoyance. Mark Jakubauskas, a researcher at
the Kansas Applied Remote Sensing (KARS) project, is developing techniques to
monitor aquatic infestations using data from NASA and other spaceborne remote
sensing satellites. Such data should allow water managers on both sides of the
river to track and thus manage the outbreak of these destructive weeds. In the
future, the lessons learned from this project could assist river communities
worldwide with similar waterborne vegetation problems.
“The problem is so bad the border patrol apparently can’t get their patrol boats down the river,” says Jakubauskas. Since it is an international border, there are no commercial barges or recreational boats, but on other rivers with similar problems, that traffic too is completely stopped. Besides blocking boat traffic and preventing swimming and fishing, dense water hyacinth infestations also prevent sunlight and oxygen from getting into the water. Decaying plant matter reduces dissolved oxygen content of the water, which has potentially lethal ramifications for aquatic life. Exotic plant infestations impair water quality, impede water flow, disrupt fisheries, displace native plant species, and reduce biological diversity.
Water hyacinths are particularly nasty in this regard. “ Water hyacinth
is regarded as one of the most damaging aquatic weeds in the world,” says
Jakubauskas. Introduced from South America in the 1880s, the plants escaped
from gardens and ponds where they were kept as decorative plants. They can reach
biomass densities as high as 200 tons per acre in less than two weeks, and they
are one of the fastest growing plants known.
|The dense red patch in the riverbed is a close-up view of the water hyacinth blockage shown below. This false-color infrared image was taken by the IKONOS satellite near Lloyd Bend on the Rio Grande. Red is healthy vegetation while light blue is water. The image has a resolution of 1 meter. (Image copyright TerraMetrics and Space Imaging.)|
Just as the plants can clog the river, they can clog up water distribution pipelines and aqueducts. Brownsville, Texas, erected fences and gates to keep the vegetation from being washed into the city’s water. But these structures have been overwhelmed, and so much vegetation has piled up around the water intake pumps that some fencing has collapsed from the pressure. The city has sent workers to stand in the river with poles and force floating platforms of plants away from the water intakes while other workers repair the fences and eradicate the infestations.
Another challenge to water management is the flow impediment the plants provide. In the lower Rio Grande below Amistad, water in the river is a “ stock resource” rather than a “flow resource,” according to Carlos Rubinstein, Water Manager for the Rio Grande. Instead of depending on the river’s flow, water is stored, or stocked, in the Amistad and Falcon reservoirs. Water is released from the reservoirs to meet municipal water needs and agricultural and irrigation demands. By impeding the free flow of water, weeds in the river significantly raise the amount of water required for municipal purposes. Since the houses and schools and businesses surrounding the Rio Grande take priority, water managers are facing the prospect of having no water left over for agricultural use in the coming months in some water districts.
One of the challenges of dealing with these plant invasions is detecting
them. The plants can be quite mobile, spreading with the river’s flow. The
problem is made worse by the fact that field researchers cannot easily reach
many of the locations where these species are growing. New outbreaks can
develop rapidly. Thus it can be both challenging and costly to keep track of all
the sites of infestation using on-site inspection techniques.
|The picture at left is a typical field research site. The picture is of the Rio Grande upstream from Brownsville, Texas, near Lloyd Bend, taken on August 26, 2001. The water hyacinths have completely covered the river making it essentially impassible. (Photograph by Mark Jakubauskas.)|
Jakubauskas explains that satellite remote sensors can enable scientists to monitor large areas rather quickly and efficiently and to observe areas that are difficult to reach from the ground. The Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) instrument on NASA’s Terra satellite, for example, can view a roughly 60 kilometers square area at a resolution of up to 15 meters per pixel. Higher-resolution remote sensing imagery can also be acquired by commercial satellites, such as Space Imaging’s IKONOS satellite, which views a much smaller area than ASTER but has approximately 1-meter resolution. Instruments on aircraft may also play an important role in tracking plant infestations as planes can allow scientists to collect high-resolution images while frequently revisiting any given location of interest.
Jakubauskas and his group are now using remote sensing instruments such as these to track down hyacinths and other infestations. They are working with Terrametrics, a company that provides data and analysis to water managers, and AquaSolutions, a company specializing in water quality and quantity planning solutions. Together they plan to provide practical solutions for water managers. Their work is supported by a grant from NASA and the Department of Transportation to use remote sensing applications for transportation.
“We brought our first image up on the screenÑa high-resolution image of the Rio Grande from last summer,” says Jakubauskas, “and were shocked at how much of the river was blocked by the water hyacinths. Down on the ground, on the river, you can see perhaps a few hundred feet or yards at most, and much of the river isn't even accessible. You can't get to the bank, or you can't get a boat through all the water hyacinths. But now, with the satellite imagery, we were looking at miles and miles of river blocked by these plants.
“Then, this spring, we used the latest ASTER images to check on the river. Again, we were shockedÑnot only by how much of the river was once again covered in hyacinth, but also by how fast the changes occurred. In a matter of just a few weeks, hundreds of feet of river went from being open water to being completely clogged. In one area [shown in the ASTER images above] in a span of just six weeks, a single blockage grew by over a mile and a half! That's about 200 feet of river per day! There's a reason why they call water hyacinth 'the world's worst weed.' We immediately sent printouts of the ASTER imagery down to the water managers in Texas.”
Not only are remote sensing data valuable for detecting invasions, but also
for monitoring how well control techniques work. Researchers at KARS compare
imagery acquired on different dates to observe changes. This not only allows
detection of outbreaks, but it is useful for gauging how fast the plants are
spreading and for tracking how well control techniques are working.
|These ASTER images demonstrate the potential of remote sensors to monitor outbreaks. The first image was acquired on March 30, 2002, and the second image on May 9, 2002. In the near-infrared region of the spectrum, photosynthetically active vegetation is highly reflective. Consequently, vegetation appears bright to the near-infrared sensors aboard ASTER, and water, which absorbs near-infrared radiation, appears dark. In these false-color images produced from the sensor data, healthy vegetation is shown as bright red while water is blue or black. Notice a water hyacinth infestation is already apparent on March 30 near the center of the image. By May 9 the hyacinth population has exploded to cover over half the river in the scene. (Images based on data provided by the ASTER science team)|
Water managers are reluctant to use herbicides and other chemical controls since farmers and communities downstream draw on the same water being treated with these chemicals. American water managers are experimenting with copper-based chemicals to kill water hyacinths with good results, but some issues with their use are unresolved. Two major non-chemical solutions used to date are mechanical chopping and introducing biological controls.
In the mechanical approach, the water hyacinths are literally chopped to bits. AquaSolutions uses a large boat, the AquaTerminator, with huge blades on the front that churn and spit out destroyed plant material. “ Hyacinth,” says Jakubauskas, “spreads primarily by sending out stolons that bud off new plants. Once chopped, the plant can’t function and dies. The cuttings are flushed down the river by normal flow.”
But the same approach does not work for all other exotic invaders. Hydrilla, in contrast, can resprout from cuttings, so the mechanical approach may actually spread the plant. Instead, biologists have been experimenting with a pilot project to introduce sterile grass carp that eat the plants. The carp’ s sterility prevents them from ending up as a new invader species and displacing native fish. The Texas Natural Resource Commission, which handles the U.S. portion of the Rio Grande water management, is seeking approval to introduce the carp based on this pilot program. The estimate is that it will take about 20,000 fish at an estimated cost of $5 to $10 each to bring the hydrilla under control.
“The original intent of the change detection was to assess how successful the AquaTerminator was at aquatic plant control,” says Jakubauskas. “ The chopping seems to work well for several years on the hyacinth; areas that AquaSolutions had chopped back in ’98 or ’99 were still open when we were down there last summer.” Monitoring the effectiveness of the AquaTerminator remains the central focus of the KARS team.
“But this spring, the change detection became more important for monitoring how much the blockages are increasing. Managers responsible for water and plant management on the Rio Grande were quite shocked by the rapid increases in plant cover.” Once the managers saw the ASTER images, they recognized just how major a problem they had and moved rapidly to control the unwelcome infestations.
The situation in Brownsville is dramatic, but not unique. Using ASTER data, the KARS team have noted similar outbreaks located further upstream in the Rio Grande as well as in a Mexican canal that appears completely blocked. The infestations noted on the Rio Grande are also found in rivers throughout the United States and around the world and include other pest plants like water lettuce and pond lilies. Hyacinths are particularly a problem in tropical nations, especially those where waterways are the primary means for moving goods and people. Jakubauskas hopes that in the future, the work that they are doing now along the Rio Grande may help communities all over the world combat hyacinth and other waterborne infestations.
The AquaTerminator is an ominous looking water vessel, operated by AquaSolutions. The vessel is actually a mechanical chopper. Water hyacinths pass through the rotating blades and are shredded to bits and killed. The river then washes the harmless plant debris downstream, and a channel is cleared through the infestation. (Photograph courtesy AquaSolutions)