by Bill Pogue
Irrometer Company, Inc.
used with all methods of irrigation with very beneficial results to growers. However, in
microirrigation they are absolutely essential if a grower expects to optimize his
irrigation system to achieve full benefits and avoid problems. There are some special
considerations which need to be taken into account when using IRROMETERS in conjunction
with this method of irrigation.
Why are IRROMETERS so important in
To understand the answer to this question you must realize
the basic "what" and "why" of microirrigation. The objective is to
continuously supply each plant with readily available moisture to meet evapo-transpiration
as it occurs, and to replace the moisture claimed by this process from the soil reservoir
shortly after it has occurred. It is accomplished by supplying filtered water in frequent,
slow applications through mechanical devices known as emitters, drippers, micro-sprinklers
or sprays. One of the major problems faced in microirrigation is poor soil water
distribution because a smaller surface area is often wetted. Distribution of the water
into the effective crop root zone is dependent, to a large extent, on the soil and its
ability to transmit water laterally. The lighter the soil, the bigger the problem. A
designer must not only keep in mind the fact that the engineering may have to provide for
future addition of more emission devices in permanent crops but that the number and
placement of these must match the ability of the soil to move water laterally into the
effective root zone desired. Roots will not grow into dry soil! In climates where seasonal
rainfall causes extensive root development of tree crops, such as citrus in Florida or
California, it must be recognized that the micro system must be capable of replacing soil
water in the entire root system thus developed. During drought periods, trees can suffer
stress if the entire root system is not adequately supplied with water.
In certain areas where salinity is a factor, the outward and
down-ward wetting front serves to push salts away from the effective root zone. If this
process is not continued, salts can readily move back into the root zone resulting in some
rather severe cases of salt damage.
Another major fact about microirrigation is that the
frequent, light applications of water are intended to keep the soil moisture reservoir at
close to field capacity. You must optimize the amount of readily available water and this
is accomplished by keeping things very close to field capacity.
| So we have three basic facts
| 1. Lateral distribution of
water in the soil.
| 2. Maintenance of soil
moisture at or near field capacity.
| 3. Outward and downward
movement of water to prevent salts from accumulating in the root zone.
| IRROMETERS are essential to
monitor these key factors.
IRROMETERS to Monitor Water
IRROMETERS can effectively help you know if your system is
accomplishing adequate distribution by their proper placement in areas where normal root
development must take place. In tree crops, the most active roots will be located at or
near the dripline of the tree so IRROMETERS must be located there. Also, the southwest
side of the tree, in the Northern Hemisphere, receives the hot afternoon sun so this area
would tend to be the quickest to dry. Thus the southwest side of the tree is the spot for
the IRROMETERS. In row and vine crops, locate the instruments in the row between plants. A
key factor to keep in mind on placement, is that the sensing tips of the IRROMETERS must
be in the area representatively wetted by the microirrigation system. With a true drip or
trickle system, the emitter wets a very small surface area, with the subsurface moisture
forming a "wetted onion" as the soil moves the water downward and outward. If
the IRROMETER sensing tips are either too close or too far away from the emitter, you
could be getting false readings on either the wet or dry side. What we suggest is that the
instruments be placed 12"-18" from an emitter. In light soils, the "wetted
onion" is more of a cone due to a lower lateral transmission of water in the soil. In
this case you need to have instruments located 12"-14" from the emitter. In
heavier soils, where lateral water transmission is better, locate instruments
16"-18" from the emitter. With micro sprinklers or sprays, a much larger surface
area is wetted. Experience has shown that IRROMETERS be placed about 24"-36"
from the micro sprinkler or spray. And, that no tree trunks or limbs interfere with the
water pattern to the IRROMETER area. Of course, the basic rule still applies, and that is
the instrument tips be in the active root system of the tree, vine or row crop.
Some growers, in addition to monitoring inside the
"wetted onion", have placed additional instruments to monitor the outside edge
of this "wetted onion". The idea here is that as water is depleted from the
soil, this "wetted onion" begins to shrink, with the outside edge area giving an
early warning from the increasing soil water suction readings on the IRROMETER.
With Subsurface Drip Irrigation (SDI), the water source could
be 9"-18" below the soil surface, but you still may require adequate moisture to
be maintained towards the soil surface in the most active portion of the crop root system.
This upward movement of water can be monitored by placing IRROMETERS so the tips are
closer to the soil surface.
With proper placement, as above, your IRROMETERS will allow
you to accurately keep track of how the lateral movement of water is doing and can
indicate how good the system is operating in terms of number and placement of emitters.
You are looking for a nice continuous band of moisture down there in the soil in the
plant, tree or vine row and IRROMETERS will help you accomplish this.
IRROMETERS to Maintain Field Capacity
This is probably the most important aspect of why IRROMETERS
are so critical in microirrigation. In microirrigation, we really do not use the soil as a
"reservoir" for water, as is the case with sprinkler and surface irrigation. The
management objective is to never let the soil dry out very far beyond field capacity,
which, in most soils, is somewhere in the 10-20 centibar range. We don't want to keep the
soil saturated at all times (below 10 cb), because this deprives the root system of its
needed oxygen. But, if we let the soil dry out too much (i.e. 40-60 cb), we may never
"catch up" with a micro system. At peak water use, usually the most critical
time for the crop, we are near the upper threshold of the irrigation system capability in
delivering water, unless the system has been grossly over designed.
Because the IRROMETER measures soil water suction directly,
and because it is most accurate in the "wet" end of the soil water range (10-50
cb), it is by far the first choice in keeping track of available soil water in the
"range" we are trying to maintain. Although our new WATERMARK sensor covers the
0-200 centibar area, the resolution of the readings in the 10-25 cb range is not as good
as the IRROMETER. The WATERMARK however, among the many indirect methods of soil water
measurement (gypsum blocks, neutron probes, etc.), is probably the closest in
"tracking" an IRROMETER with accuracy. And it has many practical advantages in
But the major factor is this - there is no practical way of
keeping track of soil moisture in the range we want to maintain, without doing soil
moisture measurement. There is no way you can "feel" the difference between 10
and 25 centibars. This is why IRROMETERS are so important to good management of
New Model "LT" IRROMETER
For very coarse soils, or non-soil planting mixes, irrigation
may have to be done in the 5-12 centibar range. For maximum precision, the new
"LT" is worth considering. With a full scale of 0-40 centibars it is the
ultimate in precision and the first of its kind. Some researchers in Florida, where there
is a lot of very coarse soils, refer to the "LT" as the "Florida
tensiometer". Ask us for the product flyer (#55).
Automation of Micro Systems
The Model "RA" IRROMETER is gaining ever increasing
use as growers learn the value of IRROMETERS and desire to automate their systems. Since
micro systems are typically low volume you can use rather small valves (1"-3")
to irrigate very large blocks. These valves are readily available with solenoid valves as
their control. Most controllers, or time clocks, can be very simple but must be capable of
being programmed with micro time (hours) rather than sprinkler time (minutes). In fact the
Model "RA", without a controller, can be tied directly to the valve solenoid and
can cause the valve to open and close based strictly on soil moisture conditions. However,
controllers can be very valuable and in some cases essential, when there are several
valves which must run sequentially due to the hydraulic design of the system. There are
also new controllers on the market which can perform many other valuable functions such as
fertilization cycles, emergency shut downs and keeping track of running time. However, the
essence of using a controller, or timer, is that it gives you the opportunity to carefully
control the length of running time on a given valve and thus putting the water at the
exact depth desired. This is done by running short but repeated cycles and letting the
IRROMETER permit as many of these cycles as needed to get the water to a specific depth.
Once that has been achieved, the IRROMETER prevents additional cycles not needed, even
though they have been programmed.
The best way to use Model "RA" IRROMETERS in an
automatic system is to wire them in to control individual valves rather than overriding
the entire controller. Due to the fact that one valve could be irrigating 20-30 acres
under a micro system, it is advisable to have more than one "location" of
IRROMETERS to monitor the variables of soil type, topography and sun exposure as they may
exist in that block. The instruments are wired in parallel, with "locations"
wired in parallel, so that any single instrument which senses the need for water can call
the system to run. Only needed irrigations will take place with unnecessary irrigations
When a controller or time clock, is used, it should be
programmed so that the system has an "opportunity" to run every day. It is
preferable to program station times to be short, but have the "opportunity" for
repeats of these cycles. As an example, if it is necessary for a 6-hour run per day of a
given valve to apply the peak consumptive use required, then you would program that valve
to run for 2 hours at a time, but have an "opportunity" for this cycle to occur
three times that day. Only the needed cycles would run since the IRROMETERS would override
any cycle not needed. These shorter cycles also give water a chance to penetrate and move
through the soil profile more efficiently. In some heavier soils, this "pulsing"
method of microirrigation systems has greatly aided in deeper penetration, where run off
was occurring with longer cycles. One note of caution is in order when utilizing a
controller. The controller resets itself every 24 hours and you can only control as much
valve time as there are hours in a day. If your hydraulics dictate that only one valve can
run at any given time the valves must run sequentially. If total time per valve per day
had to allow for a maximum of eight hours of run time, you could only use three valves on
that controller (8 hours x 3 valves = 24 hours).
Some very simple automatic systems have utilized a single
valve with DC power to activate the valve solenoid. The IRROMETERS are wired in to
interrupt the current to the valve. When the instrument calls for water, the IRROMETER
switch closes allowing current to flow from the battery to the valve and the valve opens.
Irrigation takes place until water penetrates to the sensing tip of the instrument calling
for water, the switch opens and valve closes to terminate irrigation. One point here is
that we need to know if you are going to use a DC current system, since our standard
switch is AC. We can provide special DC switches when requested for battery controlled DC
One final note of caution on our automatic switching type
instruments (Model "RA", "LTA" and "TGA") is that our switch
capacity is limited to 30 volts, 4 Amps. DO NOT EXCEED THIS LIMITATION.
| Frequency of Monitoring Locations
| The major variables which determine the
number of monitoring locations are:
| 1. Soil Type
| 2. Topography
| 3. Sun Exposure
Remember that you are monitoring the soil moisture status for
a given tree, vine or plant and using that as an "indictor" for a given area.
Don't make a mistake by spreading your instruments over too large an area as this can do
more harm than good. One extreme which can be cited would be some of the avocado plantings
in North San Diego County in California. These situations have all the variables - soils,
topography and sun exposure. In most cases, a monitoring location would be present every
1-2 acres in order to do an adequate job. Where soils are more uniform, topography level
and sun exposure are the same, you could do nicely with a monitoring location every 10-15
acres. The point is this, don't skimp on such an important practice. It is far better to
use a greater number of instruments on a smaller area and do a good job there, then add
instruments and move locations as you begin to get a good control on this situation.
Typical Monitoring Location
It is important to note where in the root system a crop
actually takes its moisture. The top 1/4 of the root system extracts 40% of the total
moisture, second 1/4 extracts 30%, third 1/4 extracts 20% and the bottom 1/4 extracts 10%.
Thus 70% of the total moisture is extracted in the upper 1/2 of the root system. It is for
this reason that it is imperative to monitor soil moisture in at least two depths in the
root horizon. The typical monitoring location with a micro system utilizes a 12" and
24" instrument, with a 36" additional setting for the deeper rooted tree and
vine crops. With shallow rooted (<15") veg crops, one instrument 1/2 way down the
root system is fine. In automatic systems, the 12" and 24" are the automatic
Model "RA", with the 36" often being the manually read Model "R".
Where a rising water table is a problem, the use of a 4' or 5' Model "R" can be
helpful in keeping track of the water table, or in verifying that a "leaching"
type irrigation has driven any accumulated salts down below the active root system.
These days, most microirrigation systems are used to apply
nutrients (principally nitrogen) in soluable form directly with irrigation events. We
"spoon-feed" water and we "spoon-feed" nutrients. We can use the
IRROMETER to keep track of our water, so why not measure our nutrients in the soil? We
have a piece of technology here also - Model "SSAT" (Soil Solution Access Tube).
Without going into further detail here, why not just ask us to send you the complete
details on our Model "SSAT", if you feel this may be of interest in your
Whether you automate or not, if you microirrigate, you really
need to consider measuring your soil moisture if you expect to achieve top results from
your irrigation system investment. Your "investment" in IRROMETERS is the least
expensive insurance policy you could ever purchase. The economics of your
"investment" are rather simple ------- for example"
- 2 IRROMETERS every 10 Acres (Model "R")
- IRROMETER "life" of 5 years (Normal) About