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Rainfall Simulators
Target Runoff
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As simulated rain falls from a
nozzle (upper right), hydrologic
technician Terry Troutman (right)
and soil scientist Andrew Sharpley
collect the runoff for laboratory
testing.
(K9572-1)
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From a distance, the portable
rainfall simulator resembles a large cube jutting out of the earth. At close
range, the simulator appears to be an outdoor shower stall, enclosed in a black
tarpaulin and fed by a hose connected to a trailer-mounted water tank. In use,
the simulator showers a field or pasture, creating runoff that can carry
pesticides, nutrients, pathogens, and soil.
Andrew Sharpley, a soil scientist with USDA's
Agricultural Research Service (ARS),
uses the device to study how soil, manure, field management, and other factors
affect the risk of losing crop nutrients, particularly phosphorus, in runoff
water. This simulator is a portable version of one designed a few years ago by
scientists at the University of Arkansas. |
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Sharpley collects a soil sample
from a test plot. The amount
and type of phosphorus in the
soil affect how much is lost
in runoff.
(K9569-1)
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"We can place the simulators
anywhere in a field and collect runoff for analysis of sediment, nutrients,
pesticides, or other chemicals," says Sharpley, who is in the Pasture
Systems and Watershed Management Research Unit at University Park,
Pennsylvania.
As part of an ARS-coordinated program called the National Phosphorus Research
Project (NPRP), 37 teams of scientists nationwide are using the simulators to
standardize collection of soil and runoff data from agricultural areas.
Currently, 20 ARS labs, 17 state universities, USDA's Natural Resources
Conservation Service (NRCS), and the Environmental Protection Agency (EPA) are
participating in the project, now in the second of 5 years.
"The main questions," says Sharpley, the project's coordinator,
"are how much phosphorus in soil is too much and how can we best manage
soil, commercial fertilizers, manure, and other agricultural phosphorus sources
to maximize farm production while protecting water quality?" |
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Inside the rainfall-simulator frame,
Troutman (left) places metal borders
in the soil to create side-by-side
runoff plots. Soil scientist Peter
Kleinman attaches plastic hose to
direct the runoff into collection bottles.
(K9571-16)
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A Race Against Runoff
Driving this effort is concern about agriculture's contributions to pollution
of lakes, reservoirs, rivers, streams, estuaries, and coastal waters. According
to a recent EPA survey, agricultural pollutants are one of the main causes of
poor water quality for most impaired waters. Runoff, leaching, soil erosion,
and artificial drainage are just some of the ways these pollutants are carried
from land to water.
In recent years, phosphorus has risen to the forefront of water quality
concerns involving agriculture. This is because in fresh water, phosphorus
accelerates eutrophication, a process by which a body of water becomes too
enriched with organic material. This process occurs naturally, but at a very
slow rate. EPA says eutrophication is the most pervasive water-quality
impairment factor nationwide. Eutrophication often shows itself through the
growth of undesirable aquatic weeds and blue-green algae, which can crowd out
beneficial aquatic plants. Only minor concentrations of phosphorus in fresh
water, as little as 0.02 parts per million, are required to induce such a
change. In contrast, soft drinks contain phosphorus in concentrations up to
2,000 parts per million. |
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The rainfall simulator was
designed for transport to
remote locations on a trailer,
along with a 300-gallon tank
of water to supply the “rain.”
(K9573-1)
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Additionally, "algal blooms
resulting from eutrophication clog filters at water treatment plants and reduce
the recreational value of lakes and marinas," notes Sharpley. "The
bloom's subsequent death and decay deprive fish and other aquatic life of
oxygen."
In a recent ARS article on agricultural phosphorus and eutrophication, Sharpley
and co-authors stated: "In many areas of intensive, confined animal
production, manures are applied at rates designed to meet nitrogen
requirements, often resulting in phosphorus being applied beyond crop needs,
increasing phosphorus in surface soil and enriching runoff with enough
phosphorus to accelerate eutrophication."
Until recently, research has been primarily confined to the laboratory and
carried out by scientists using different methods, making results difficult to
compare, says Sharpley. The NPRP is an attempt to coordinate such efforts by
experts at the state, university, and federal levels. One of its goals is to
collect field data in a uniform, comparable manner countrywide so it can be
compiled into a national database.
"If everyone is using the same type of equipment, the resulting data is
easier to share," says Brad Joern, an associate professor and NPRP
coordinator at Purdue University, West Lafayette, Indiana. "Part of my
charge is to help develop a protocol for cropland studies."
To further ensure accurate results, Joern's team is studying the impact of
using different water sources—such as well water—on phosphorus'
movement during rainfall simulations. This could have an important bearing on
how scientists interpret simulator research to establish the phosphorus
threshold levels for 45 to 50 different benchmark soils from across the
country.
"By carrying out on-site studies," adds Sharpley, "we're looking
at something that's much closer to real-world losses."
Developing Tools To Help Farmers
For several years, researchers have known that most phosphorus lost from
agricultural lands comes from only a small area of a watershed, during a few
storms each year. This fact, coupled with the need to manage farm nutrients for
water quality as well as productivity, led NRCS, ARS, and university scientists
to develop a phosphorus index that assesses and ranks a farm field's
vulnerability to phosphorus runoff.
"The phosphorus index pinpoints hot spots for phosphorus runoff and is
based on how much phosphorus is in soil, how much was recently applied to the
soil, and how likely that phosphorus is to move and enter a sensitive water
body," notes Robert Wright, ARS national program leader for
soil-management research.
"It's a decisionmaking tool that can be used by farmers, consultants, farm
advisers, and landscape planners," he adds. "It will let them find
areas vulnerable to nutrient losses and develop appropriate, cost-effective
management practices."
The NPRP's rainfall studies will help researchers provide technical support for
the phosphorus index. Over the next several years, the research findings from
the NPRP will provide the scientific backbone for state, NRCS, and EPA
nutrient-management policies to guide farmers' use of fertilizer and manure.
Most importantly, the index will provide farmers with options on how best to
achieve their production and water-quality goals on a site-specific basis.
Showcasing Science
Showing science in action is yet another facet of NPRP. Last summer, for
example, Sharpley and colleagues used the rainfall simulators to illustrate the
effect of conservation tillage on runoff in a field demonstration at Cedar
Meadow Farm, operated by Steve Groff near Lancaster, Pennsylvania.
"The rainfall simulator can easily show farmers the benefits of
conservation practices such as no-till," says Sharpley. "The other
great thing about these demos is that farmers often come up afterwards and say,
'If you need them, I've got some good sites for rain simulator research on my
fields.'"
For studies and demos, scientists generally set up two, 3-by-6-foot plots, with
one plot serving as a control. "The first simulators were nowhere near as
portable or easy to use as the simulators we now have," says Tommy Daniel,
a professor at the University of Arkansas in Fayetteville and one of the
original developers of the NPRP simulator.
"Today's simulator is a beautiful thing to behold," he continues.
With the pull of a cord, a pump begins forcing water through the simulator's
plumbing system and out a nozzle that converts it to rain, which falls onto the
plots at about 3 inches per hour. After awhile, the first signs of
runoff—usually small puddles—appear on the soil. Metal borders
located downslope from the plots collect the runoff and direct it into gutters
that empty into a plastic bottle. Devices called lysimeters are sometimes
inserted into the ground to collect subsurface water, called leachate.
Scientists weigh the collected runoff at the site to determine how much has
occurred. Back at the laboratory, they measure concentrations of various forms
of phosphorus that are important to water quality. For instance, they can
measure the specific forms available to algae.
Using the devices, scientists can also:
- Compare sediment losses from plots having grass filter strips to those
lacking filter strips.
- Evaluate the effect of no-till and conventional tillage on erosion and
phosphorus loss.
- Examine how intensive grazing or trampling affects runoff from pastures.
- Study the effect of manure type, for example, poultry versus hog, on
potential phosphorus loss.
"In each case," Sharpley notes, "we have a standardized protocol
for using the rainfall simulators."
State and Federal Partnership
According to Wright, the project's participating universities and state and
federal agencies offer unique expertise and resources. For example, EPA's
involvement "helps agricultural scientists and the EPA itself understand
what scientific information is needed for establishing federal
regulations" regarding manure handling, transport, and use, he says. State
extension services and NRCS help put research findings into practice.
"They transfer the technology and management practices developed by ARS
and university scientists to producers," he adds.
But no single tactic or technology is likely to curb phosphorus runoff losses
alone. Rather, the emphasis is on combining "source and transport"
control strategies. At the source level, this could involve refining feed
rations, adding substances like phytase to improve animal absorption of
phosphorus, or finding alternative uses for manure. On the transport front, it
could be limiting runoff by reduced tillage, use of buffer strips or cover
crops, or other techniques.—By
Jan Suszkiw,
Agricultural Research Service Information Staff.
This research is part of Water Quality and Management, an ARS National
Program (#201) described on the World Wide Web at
http://www.nps.ars.usda.gov.
Andrew N. Sharpley is at the USDA-ARS U.S.
Pasture Systems and Watershed Research Unit, Bldg. 3702, Curtin Rd., University
Park, PA 16802-3702; phone (814) 863-0948, fax (814) 863-0935. |
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"Rainfall Simulators Target Runoff " was
published in the October 2001
issue of Agricultural Research magazine.
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