Making Every Last Drop Count
Reusing Water in the Desert Southwest
A canal near Maricopa, Arizona. ARS scientists have devised
software to automatically control canal gates, giving managers better control
and flexibility and providing better water-use efficiency for
Arizona’s burgeoning population is increasing pressure on
the state’s limited water resources.
Service scientists at the U.S. Arid-Land Agricultural Research Center
(ALARC) in Maricopa, Arizona, are investigating potential solutions.
Reclaim It, Reuse It
Microbiologist Jean McLain and soil chemist Clinton Williams
are examining urban use of “reclaimed” water—which is drawn
from residential and industrial sewage systems and treated to remove
Since September 2006, McLain and Williams have collected soil
and water samples from a municipal park that is being irrigated with reclaimed
water and tested them for harmful shiga toxin-producing Escherichia
coli bacteria. To date, the scientists have not found a single pathogenic
strain of E. coli. They did note a small increase in soil
salinity—a potential downside to using reclaimed water—but the
level observed was too low to harm plant growth.
At the Central Arizona Irrigation and Drainage District,
hydraulic engineer Bert Clemmens (left) and electrical engineer Bob Strand
assess the automatic control of one of the many hydraulic gates operated by
ARS-developed software while student Matt Robbins (background) checks the
Williams has also tested for carbamazepine, an epilepsy drug
that has been detected in low doses in drinking water. His research has shown
that the drug is retained by organic matter found in soil. This suggests that
any carbamazepine in reclaimed irrigation water is unlikely to leach beyond the
In two related studies, the researchers have addressed the
accuracy of the tests used to confirm reclaimed water’s safety—an
important step towards gaining the public’s approval.
In the first study, McLain and Williams found that the quality
of reclaimed water is harder to assess in winter, when EPA-approved tests
return considerably more false positives for E. coli. Over several
months, they drew samples from a wetland fed with reclaimed water and placed
each sample on a culture medium that changes color when E. coli
In December and January, they observed a surprising increase in
positive tests. But genetic assays revealed that about 90 percent of these were
false positives. So why were the media cultures—ordinarily dependable
testing tools—delivering such inaccurate results?
Chemical analysis revealed that the water’s salt content
increased in the winter months, suggesting that the false results may be
related to salt chemistry, McLain says. The ALARC researchers are currently
collaborating with scientists at the University of Arizona to confirm this
hypothesis. This is important not just for the ARS team, but for all people who
rely on the media for accurate assessments.
Microbiologist Jean McLain (left) examines bacterial colonies
in petri dishes as technician Sharette Evans prepares more soil and water
samples for analysis. Dark-blue colonies, presumptive for E. coli,
will be isolated and tested for false positives.
“Not every user has the high-tech assays to verify their
media results,” McLain says.
Ensuring Accurate Tools
Working with researchers from Arizona State University (ASU)
and the University of Arizona, McLain also monitored levels of fecal
Bacteroides, a genus of anaerobic bacteria, in a pond fed with treated
“Because they are prevalent in the feces of warm-blooded
animals, Bacteroides are commonly found in the environment and in
contaminated water,” McLain says. “But the pond levels were a
little higher than we’d expected.”
Initial tests suggested that almost all the
Bacteroides were of human origin, raising concerns about the efficacy
of the wastewater-treatment process. The scientists ran control tests on feces
from dogs, humans, birds, and tilapia, the fish species inhabiting the pond.
When they analyzed the fish samples, they made a surprising discovery.
“The tilapia results were so similar to the human results
that I first thought we’d contaminated the sample,” McLain says.
At the U.S. Arid-Land Agricultural Research Center outside
Maricopa, Arizona, engineering student Matt Robbins (foreground) is being
trained by Bob Strand and Bert Clemmens to operate a canal gate with remote
Instead, something more unusual was at play. Tilapia and humans
carry different strains of Bacteroides, but genetic analysis revealed
that the small segments of DNA used in the assays were not unique to human
Bacteroides, but were exactly the same for both strains.
ARS and ASU are now collaborating to test other fish species to
see if the similarities are widespread or specific to tilapia. They’re
also developing a fish-specific source-tracking tool to confirm that their test
results reflect the pond’s fish population.
“There are hundreds of fish in this pond, so at this
point we can safely assume that they’re the cause of our high test
results,” McLain says. “Once we develop the right tools, we can
confirm that assumption scientifically.”
Soil scientist Clinton Williams prepares for analysis of water
samples from a municipal park being irrigated with reclaimed water. The test
will determine the fate and transport of the human drug carbamazepine.
Improving Water Management
Meticulous water management is essential in arid environments,
where water is a precious and limited commodity. ALARC scientist Bert Clemmens
and his colleagues are exploring ways to improve two areas related to water
management: canal automation and surface-irrigation modeling.
Large water-distribution systems gather water from rivers,
reservoirs, or other suppliers and deliver it to agricultural areas via canals.
In the United States, such systems may serve hundreds of thousands of acres.
“There are physical limitations to the service that can
be provided to farmers by a canal system,” Clemmens says.
“We’re working with individual districts to improve canal
operations, and thus service to farmers, through automation.”
In a field of guayule at the University of Arizona
Maricopa Agricultural Center, agricultural engineers Doug Hunsaker (right) and
Kelly Thorp (far left) collect remote-sensing data while technician Dick Simer
(right center) and physical scientist Andy French (left center) monitor
soil-water depletion and crop evapotranspiration. To improve irrigation
scheduling efficiency, the group is developing remote-sensing tools and
evapotranspiration models to estimate real-time crop water use.
Clemmens and his colleagues have developed software logic that
can control the position of canal gates. Sensors detect water-level changes
within the canals and respond accordingly, moving the gates to increase or
decrease the water flow. This would improve the efficiency and give land
managers greater control and flexibility.
During a 30-day trial run in 2004, the ARS researchers set up
automatic software to operate the gates on a small lateral canal in the
Maricopa Stanfield Irrigation and Drainage District.
“The software successfully made 48 changes in water
delivery and controlled water levels upstream from farm-delivery gates to keep
flows to irrigators constant,” Clemmens says.
The scientists tested 19 different downstream water-level
feedback controllers, enabling them to make recommendations for improved water
Technicians Allan Knopf and Sharette Evans collect water
samples from a pond that supplies irrigation water to Pacana Park in Maricopa,
Arizona. In the laboratory, the pond-water samples are filtered to isolate
microbes. DNA extracted from the filters is then screened for the presence of
human-specific Bacteroides molecular markers.
The researchers are currently applying the technology in the
Central Arizona Irrigation and Drainage District—which covers about
87,000 acres of irrigated farmland—and developing training software for
In related work, Clemmens and his colleagues have updated a
software program called WinSRFR that simulates, designs, and evaluates
“Surface irrigation doesn’t have the best
reputation for efficiency, but the majority of the world’s acreage is
surface irrigated,” Clemmens says. “The WinSRFR model is a tool
that can help maximize that efficiency.”
The USDA Natural Resources Conservation Service uses the
program to evaluate proposed conservation practices and install more efficient
The model has modules that enable users to simulate the results
of various management strategies. One module evaluates irrigation events.
Another shows how different design options affect a field’s irrigation
efficiency. A third shows the effects of various operational choices.
Though each module operates separately, information can be
transferred between them. Together, they can help land managers make decisions
about how to arrange and water their fields.
The model is available at
and has users throughout the United States and in 14 countries around the
world.—By Laura McGinnis,
Agricultural Research Service Information Staff.
|One alternative for
arid lands is to grow crops that require less water. For example, in Arizona,
alfalfa is a major commercial crop that needs about 72 inches of water per acre
per year. Instead, growing the crops listed below would offer significant
water-conservation benefits. Exact water use varies from year to year,
depending on weather, irrigation scheduling, and system efficiency.
Seasonal Water Use
||Biofuel and other biobased products
||January to late May
|| Clothing and medical supplies
||April to October
||Cosmetics, plastics, and lubricants
||October to early June
||December to late May
From canals that run along the fields, farmers use siphon hoses
to deliver water to thirsty crops.
Research in the Public Eye
You can lead a horse to water, but you can’t make it
drink—and its owners will be equally reluctant to irrigate with that
water until they’re sure it’s cost effective and safe.
Education is an essential part of gaining the general
public’s acceptance of new technology. How much will it cost? Is it safe
and easy to use? What are the short- and long-term effects? These and other
questions about reclaimed-water technology will be addressed at a new
research-and-education location slated to open in Maricopa, Arizona, later this
The 1.5-acre site is the result of a collaborative effort
between ARS and Global Water, a private water utility based in Phoenix,
In Pacana Park, Clinton Williams draws water samples from large
lysimeters installed under the turf while Jean McLain collects soil samples to
assess downward bacterial transport from the surface.
At the site, scientists will irrigate vegetation with either
recycled or potable water and monitor the soil for chemical and biological
changes. Stainless steel lysimeters—subterranean water-collecting
instruments—will gather water samples 100 centimeters (about 39 inches)
below the surface. Sensors within the lysimeters will collect information on
soil moisture, temperature, and salinity.
“This work will reveal long-term environmental effects of
irrigation with recycled water,” says microbiologist Jean McLain.
“It will also provide the information water utilities need to develop
guidelines and make informed decisions for safe recycling of water for
This research is part of Water Availability and Watershed
Management, an ARS national program (#211) described on the World Wide Web at
To reach scientists mentioned in this article, contact
Laura McGinnis, USDA-ARS
Information Staff, 5601
Sunnyside Ave., Beltsville, MD 20705-5129; phone (301) 504-1654, fax (301)
"Reusing Water in the Desert Southwest"
was published in the January 2009
issue of Agricultural Research magazine.