Probing Plants' Water Needs
Every home gardener can appreciate the desire to put on only enough
waterno more, no lessto grow the best tomatoes or the prettiest
rose bush on the block. For companies that grow roses or tomatoes for a living,
that desire becomes a critical need.
Steven R. Evett and Robert J. Lascano are collaborating to put together a
computerized irrigation system that measures and calculates how much water to
apply every half hour.
It's a simple concept, Lascano says: "Replace just as much water as the
plants have used and you'll get maximum yield with minimum water." He is a
soil physicist at Texas A&M University in Lubbock who is working with
Evett, other scientists at Texas A&M University, and Dynamax, Inc., of
Houston, Texas. Lascano designed the automated system for applying irrigation
Software is available for download on the USDA-ARS Conservation and
Production Research Laboratory web site at
TACQ.EXE - time domain reflectometry system control software. Also look for
the system documentation in file TACQ_WPD.ZIP. You will need the PKUNZIP.EXE
program to unzip the documentation into WordPerfect files.
ENWATBAL.EXE - predicts crop water use. Available, along with source code
and example data files and documentation, in the self-extracting file
Note: These programs are supplied for information purposes solely. No
warranty is intended, nor is any service or support for these programs
Evett is a soil physicist at the USDA-Agricultural Research Service Conservation
and Production Research Laboratory in Bushland, Texas. He designed the
automated time domain reflectometry (TDR) system needed to determine when to
The measurement is done with stainless steel probes placed in the ground at
varying depths, from a few inches down to several feet.
A computer-controlled TDR instrument sends an electronic pulse through a
buried cable to the probes. The longer it takes for the pulse to travel through
the probe, the more soil water. The probes work in most irrigated agricultural
soils, and one TDR system can handle up to 241 probes, although the cable
length is limited to about 100 feet from the instrument.
USDA has signed a cooperative research and development agreement with
Dynamax, which is now manufacturing the TDR system.
Evett says the probes are key because they directly measure how much water
is left in the soil for plants to use. He also designed the software that
controls the TDR system and translates the probe signals into water
The computer turns the water pumps on and off at pre-determined soil
moisture levels, using software written by Lascano.
Evett and Lascano have received inquiries about the system from managers of
greenhouses, tree nurseries, orchards, and vineyards.
The fully automated system may be suited only for horticultural use, but
other farmers could use the probes manually.
"They can install the probes permanently anywhere they want, with no
limit on numbers of probes," says Evett. "They can drive around their
fields, stop at each probe and connect the cable tester to each probe for a
reading. A laptop computer translates the readings. This is fast and easy. It's
sometimes a practical alternative to neutron probes that require a license and
special training because of their radioactive content."
Lascano has grown cotton with an automated drip irrigation system for 4
years. He says the tests have been very successful so far and that automation
could be used with other crops and other irrigation systems. It could also be
used to study the effects of pests and farm practices on crop water use.
By Don Comis, Agricultural
Research Service Information Staff, 6303 Ivy Lane, Greenbelt, Maryland 20770,
phone (301) 344-2748.
Steven R. Evett is at the USDA-ARS
Conservation and Production Research
Laboratory, P.O. Drawer 10, Bushland, TX 79012; phone (806)356-5775, fax
"Probing Plants' Water Needs" was published in the January
1998 issue of Agricultural Research magazine. Click
here to see this issue's table of