RZWQMModeling Effects of Farm
Computer specialist Ken Rojas (left) and range scientist Jon Hanson use the
Root Zone Water Quality Model (RZWQM) to examine nitrate distribution in a
simulated soil profile. The model enables scientists to forecast potential
environmental pollution, such as from excessive nitrate leaching.
A new computer model may soon help farmers and others better understand how
their decisions on tillage method, timing and type of irrigation, pesticide and
fertilizer applications, and selection of crop rotations affect the
"The Root Zone Water Quality Model (RZWQM) was developed over the past
decade by a national team of scientists," says Lajpat R. Ahuja. A soil
scientist with the Agricultural Research
Service, Ahuja coordinates the root zone water quality project and heads
ARS' Great Plains Systems Research
Unit at Fort Collins, Colorado.
"This simulation tool is needed to study the effects of management
practices on soil water and movement of chemicals that may be hazardous to
surface and groundwater quality," says Ahuja. "It's a model that
integrates management practices with separate components dealing with
hydrology, plant growth, nutrients, chemistry, and pesticides."
Actually, RZWQM is a series of compatible modules that can be attached or
detached, as needed. Computer programmers term this flexibility "modular
modeling." It allows developers to test new sections without having to
rework any other portions. They say RZWQM could prove useful in many instances.
For example, major chemical companies ignore so-called minor-use pesticides
because they do not want to spend money proving to the U.S.
Environmental Protection Agency (EPA) that
new compounds are safe. The cost of such testing would exceed the profits
likely to come from sale of the products.
Chemical companies could use RZWQM to get a quick answer on which new
compounds seem safe, then conduct field studies on only those most likely to
gain EPA approval. One chemical company,
Zeneca Ag Products of
Wilmington, Delaware, has shown interest in using the model and assigned a
scientist to work with ARS on the model's evaluation at Fort Collins.
"We hope that this model might be used in the future to replace actual
experiments that involve root growth or water movement," says ARS
rangeland scientist Jon D. Hanson.
Data on rainfall intensities, solar radiation, minimum and maximum air
temperatures, and windspeed gathered from weather stations maintained by
technician Dan Palic are critical to the Root Zone Water Quality Model's
"It could allow research agencies to concentrate scarce money on a few
detailed projects aimed at gathering baseline data. No longer would they have
to measure pesticide movements everywhere," he says. "If the water
movement can be defined, the computer can figure out the rest. This reduces the
need for expensive soil-weighing equipment and drainage solution samplers at
Farmers will also be aided by the model's best management practices mode.
For example, if nitrogen movement to groundwater supplies is a potential
problem in their area, they can discover whether it's better to plow in the
fall or spring. RZWQM might show that a farmer should not apply all the
nitrogen fertilizer at planting time, but instead apply it in two or three
Using nitrogen contentseen as greenness of the cropprovided by
the model as a guide, farmers could also apply nitrogen with irrigation water
when crops need the nutrient.
The model incorporates safeguards to ensure that the input data entered by
users is within a reasonable range. If a range is exceeded, the model explains
what it's looking for, or it instructs the user to refer to tables in the user
ARS computer specialist Ken W. Rojas says, "The scope of science RZWQM
covers is just mind boggling. Some scientists are using it to understand how
the whole plant-soil-hydrologic system works."
"The model can run 20- to 30-year-long simulations of one field,"
says ARS soil scientist Marvin J. Shaffer. "These show how a monoculture
or crop rotation would, under various fertilization rates, contribute to
nitrate leaching and changes in soil organic matter content, soil microbial
populations, and other indicators of soil quality."
And the Refinements Continue
Last summer, ARS agricultural engineer Hamid Farahani ran RZWQM to simulate
no-till dryland corn production in eastern Colorado. He and co-researchers
learned that it overpredicted yields on summit and sideslopes, while
underpredicting them on lower areas. The researchers found this was because the
model failed to accurately account for the amount of water that ran off upper
areas onto lower portions of fields. Current versions can now interpret this
Soil core samples withdrawn from a field near Fort Collins, Colorado, by
technician Mike Murphy (left) and soil scientist Laj Ahuja will yield
information on soil horizons and their physical and chemical properties for the
Root Zone Water Quality Model.
Gerald W. Buchleiter, an agricultural engineer in the ARS
Water Management Research Unit
at Fort Collins, tested RZWQM using data from a commercial farm in eastern
Colorado. He found that the model's performance was acceptable as a research
tool for predicting corn production on sandy soils under center pivot
irrigation. After more testing, scientists will use RZWQM to estimate the
effects of various farming practices on keeping fertilizers close to plant
A larger test of the model was conducted in the Corn Belt. It was part of a
regional study known as the Management Systems Evaluation Area project, a
water-quality initiative involving research at 10 sites in Iowa, Minnesota,
Missouri, Nebraska, and Ohio.
In Missouri, ARS scientists evaluated RZWQM to see how accurately it could
predict crop yields, surface water runoff, and chemical movement through the
"In general, the model was accurate in predicting corn and soybean
yields, surface runoff, and chemical discharges in the runoff. But it
underestimated the movement of chemicals downward through a soil that had a
subsurface layer of high clay content," says soil scientist E. Eugene
Alberts, who leads the ARS
Systems and Water Quality Research Unit at Columbia, Missouri.
The scientists are now modifying the model to make it more accurate for
soils encountered in their tests. It will then be able to predict formation of
the cracks and fractures in high clay soils that permit more rapid chemical
movement. Other sites where RZWQM is being run through its paces include
Tifton, Georgia; Guelph, Ontario, Canada; Lisbon, Portugal; and Bonn, Germany.
The next step is getting the program to users. So far, the scientists have
trained more than 50 during 2- and 3-day sessions. And ARS has entered into a
cooperative research and development agreement for the commercialization of
RZWQM by Water Resources Publications, LLC. This Englewood, Colorado, company
is now enhancing the manual so it is more user-friendly and will make it, the
program documentation, and software package available to customers. By
Dennis Senft and Linda Cooke, ARS, contributed to this article.
R. Ahuja is in the USDA-ARS Great Plains Systems Research Unit, Fort
Collins, CO 80522; (970) 492-7315.
W. Buchleiter is in the USDA-ARS Water Management Research Unit, Fort
Collins, CO 80523; phone (970) 492-7413.
Eugene Alberts is in the USDA-ARS Cropping Systems and Water Quality
Research Unit, University of Missouri, Columbia, MO 65211; phone (573)