204 Bill Moons, Potato Specialist Lynn Gilmore, Fruit Specialist

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Irrigation Scheduling Using C-Probes
Bill Moons, Potato Specialist
Lynn Gilmore, Fruit Specialist
Manitoba Agriculture and Food, Carman, MB
Dolf Feddes, Agrometeorological Centre of Excellence, Carman, MB
Irrigation scheduling is a process required to determine when and how much irrigation water is required
for your crop. To be successful with your scheduling you must monitor and measure soil moisture levels
in the active root zone as well keeping a good record of crop water use based on evapotranspiration rates.
Current irrigation scheduling methods include; experience and intuition, feel method, checkbook method
and soil moisture sensors. Experience and intuition is understandably the least accurate method. The feel
method is used to determine % field capacity and requires an experienced manager with a good
knowledge of soil types. Soil moisture sensors are widely used, with a variety of sensors available that
include; tensiometers, gypsum blocks, water marks and the C-Probe.
The C-Probe is a soil moisture probe consisting of capacitance sensors that can be set at multiple depths
on a column installed in the soil, within a waterproof access tube. The sensor depths are chosen relative
to the crops root system and soil structure. The sensors utilize the capacitance technique, which measures
the time it takes for a signal to be sent out and returned to the sensor. Water being an excellent conductor,
the longer it takes the signal to return, the drier the soil. Sensors can be set at ten centimetre intervals and
cover a ten centimetre radius of the soil. Based on these facts, it is absolutely imperative that the growers
have the C-Probe installed in a representative spot in the field. Keep in mind that irrigation scheduling
decisions are made based on this single location.
The C-Probe utilizes radio telemetry, attached to a weather station, to log and transmit the data to a
computer base station. This eliminates the labour and costs of installing cables and digging trenches.
Along with soil moisture, irrigation volume and rain events are also recorded. The data is then transferred
directly to the main computers for storage. Updates can be accessed, via the Internet, every fifteen
minutes.
In 2001, in Manitoba, eleven potato fields and one carrot field had the C-Probe installed. The remainder
of this paper will refer to potatoes only. As an initial introduction to this new technology, there were two
C-Probes installed under each central pivot (1/4 section). Each C-Probe had five sensors, at 10, 20, 30, 50
and 80 centimetre depths. The 80 centimetre sensor is well beyond the active root zone, however, it
provided valuable information as to how much water, either as rains or irrigation, is required for leaching
to occur.
The benefits to using the C-Probe can be placed in three categories; quality, savings and yield. Quality
has numerous parameters that are affected. Tuber size uniformity can be better managed by ensuring a
uniform moisture supply during the entire growing season. Avoiding access moisture conditions can
ensure high specific gravity, which is an important component for processors. High sugars, again an issue
for the processors, can be avoided by not allowing a moisture deficiency to occur. Potato defects, such as
knobby potatoes, can also be avoided by providing sufficient moisture to avoid stress.
Savings can be measured in a number of ways as well. Growers discovered that they were irrigating to
excess early in the growing season when the root system was quite shallow in the soil profile. Having the
data sent to their office computer allowed the grower to make irrigation decisions for remote fields
without having to physically go out and see them. Avoiding excessive moisture conditions in the fall,
results in a more efficient harvest, saving time, fuel and machinery wear.
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Higher yields were accomplished by providing sufficient water to the plants during tuber initiation.
Having a soil moisture capacity of less than 65% results in a reduced number of tubers under each plant.
Providing sufficient water to the potatoes when they are in the bulking stage resulted in increased tuber
size.
As previously noted, the rain, irrigation and soil moisture can be updated every fifteen minutes. Once the
data has been downloaded, to your own computer, it is graphically displayed to show trends in crop water
use and potential plant stress. Two graphs are utilized, the first is a Separate Layer Graph, which displays
the water use at each depth. This graph is used to determine how much to irrigate, see Illustration 1.
The second is the Summed Graph which provides a total of the individual sensors. This graph will help
in determining how much irrigation to apply. Water volume decisions are also made by determining
where the Summed Graph is positioned in relation to the Refill Point and Full Point. The Refill Point is
positioned on the sum graph as a point that could result in water stress due to lack of moisture. The Full
Point is set at a value below saturation to avoid excess moisture and potential leaching of nitrates. See
Illustration 2 as an example of the Summed Graph, Refill Point (lower horizontal line) and the Full Point
(upper horizontal line). Note how the Grower is managing the soil moisture to remain within the Refill
Point and Full Point.
There were a number of positive results that came from the C-Probe program in 2001. The growers
determined that this technology is a good option to use as an irrigation scheduling tool. The C-Probe’s
accuracy was confirmed by the growers by looking at real time data on their computer and then
immediately going out to the field to validate that data. Growers were able to avoid gross error in
irrigation scheduling by managing their soil moisture within the Refill and Full Points. Finally, water was
used more efficiently by the growers throughout the growing season.
In 2002, growers using the C-Probe again will be able to fine tune their water management and irrigation
scheduling to ensure a good yield of high quality potatoes.
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Illustration 1.
Illustration 2.