Using Wireless Sensors To Schedule Irrigation
PORTAGEVILLE, MO.
Missouri has become a leader in wireless
soil moisture sensors use by farmers. The
benefit of wireless soil moisture monitoring
is that a farmer can readily “see” the amount
of moisture in his field to determine if irrigation
is needed or not. He can view soil moisture
graphs on his computer, or even see it on his
iPhone.
Cotton, a crop that suffers if either too little or
too much water is applied, is a prime candidate
for wireless monitoring in Missouri. Data from
the annual Bootheel Irrigation Survey since
1997 shows that nearly 10 percent of the time
irrigators’ dryland cotton make as much or more
than comparable irrigated fields!
Engineers at Cotton Incorporated (CI) have felt
that wireless soil moisture data may be the key
to assist cotton irrigators throughout the mid-
South in fine tuning their irrigation management
skills. To this end, CI has provided grant
funds to researchers in states in the mid-South
to install wireless systems in individual cotton
growers’ fields. The Delta Center has been involved
with this program for two seasons.
Two pivot-irrigated cotton fields near
Hornersville, MO were equipped with Decagon
soil moisture sensors provided by CI. In the first
year, the 6-, 12-, and 18-inch depths were monitored
at three locations placed laterally in the
field (so they would all be watered at the same
time) in the 3rd, 5th, and 7th span of the pivot.
Results from the first year resulted in an additional
sensor being installed at the 24-inch
depth. Inexpensive rain gauges were also installed.
Data were sent by cell phone modem to
a WWW site and the farmers and MU
personnel could observe the conditions
which were updated about every four
hours. For the first year total cost was
$3,943 and the annual cost was
$7.40/ac/yr. One of the goals of the test
was to see if, instead of the three locations
under the pivot, two or even one location
would suffice. Results from Year
1 showed that irrigation would have
been called for at about the same time
whether three, two, or just one site was
present. This being the case, then the
investment cost would have $1,769, or
$3.77/ac/yr. To be careful a farmer
might want to use less expensive hand-read
sensors in two other satellite locations.
This would then bring the cost to
$4.83/ac/yr.
In the second year, one notable change was
that the equipment was purchased from a local
farmer/dealer instead of from the manufacturer
in Washington state. The local, farm- savvy
dealer was able to address cooperators’ questions
and concerns more realistically then university
experts. The cooperators, local dealer
(Below Ag Services, LCC [Parma, MO]), and Dr.
Henggeler could individually observe the plotted
data from the sensors and text one another on
observations.
In Year 1 the various strategies of using data to
trigger irrigation were evaluated; these included:
1) Changes in the extraction curve.
2) Not allowing the 18-inch depth to start to
dry down.
3) Irrigating when 65 percent of total available
water (TAW) was gone.
Deficit irrigation was practiced in Year 1, making
options 1 & 2 difficult to use. Thus, the best
strategy was the last one, which was irrigating
when the percentage moisture remaining, p, of
TAW was only 35 percent (Fig. 1). Note that the
field was not allowed to go beyond the 35 percent
level until the last week of August when few
blooms after that would become harvestable
bolls.
First year cooperators reported that this field
was the second highest yielding one among all
their 17 pivots. One of the changes the sensors
caused was having them apply more water than
they would have normally done. Δ
DR. JOE HENGGELER: Irrigation Specialist,
University of Missouri