The Probe Schedule irrigation scheduling program keeps track of the daily water balance,
using daily weather to estimate the ET and then project when the pre-set soil moisture
deficit will be reached and how much irrigation will be required at that time, considering
the system efficiency. In it's simplest form this is what happens:

Opening moisture balance (measured) + rain + irrigation - ET - drainage = closing balance

The closing balance is modeled daily and also checked with an actual probe reading on a regular basis.
We simulate (and calculate) what the closing balance will be before we get there. If after
a week or so we measure the soil moisture (with any callibrateable device) and we find that
the closing balance is not what we have calculated, we do two things. Firstly, correct the
balance and secondly analyze the data for the last measuring interval and find the most
likely cause for this deviation.  For instance, if there was no rain, irrigation or drainage
during the interval, the adjustment should be made to the ET factor.  Moisture measurements
should be done to isolate and calibrate specific incidents such as rain, irrigation and crop ET.

The program is user friendly and quick to use.
Below is a sample input screen on which the daily weather, rain and irrigation is entered.

The program will immediately calculate the new water balance and project when the preset
soil moisture deficit (in kPa, mm, inch or %) will be reached and the next irrigation should
be applied. The information can be sorted to bring the most urgent site to the top of the list.

The Root zone soil moisture graph depicts the daily moisture balance and provides a lot of important information.

On November 5, 1998 the root zone was increased from 50 to 60 cm, increasing the capacity of
the root zone. The Refill line (red line) is at the pre-set, planned depletion level. this was set
to 32 mm (1.26 ") until 26 December 1998. It was then set to 40 mm (1.57")  to end of February
and thereafter 50 mm (2"). On November 23 (labeled A on graph)  and 30 (B) the soil moisture balance was tested with
a neutron probe and the balance adjusted by 2mm. This means the ET was over projected
by 2mm during that period.

In early December the soil moisture was deplete to below the planned level (C). The irrigation of 7 December did not replenish the soil moisture. (Maybe he had a hunch that there is rain on the
way!). Well the rain came and over filled the profile (D), resulting in some drainage. To this point
the simulation and reality match very well instilling confidence in what we are you do.

The irritations of 1999.02.16 and 1999.02.28 was not as large as indicated - note how the actual
water balance is decreased with the next probe reading after the irrigation. (And I as consultants we
will point out to the grower that he has a supply problem or staff problem as of 16 February).

The depth graph depicted below is used to evaluate what happens below surface. The graph
can be viewed as actual readings in mm or as "percentage-from-full" as in the diagram below.
Full (or field capacity) is 0% from full and therefore the full line becomes a straight line and
all other readings are displayed as % from full. This is the same data as above and represents
probe readings number 2 through 6 (A to just after D).

Note how the soil moisture is depleted to a depth of 1 m (39") when the upper zones is
depleted to over 40%. This would suggest that the effective root zone can be set to at least
80 cm. Note further how the rain of 14 December over fills the profile. Drainage will result
from this.

Frequently asked questions:

1. Can this program be used on a 486 computer?
Yes, if it has Windows 95 or higher but it will be slow.

2. Can I use this program for irrigation scheduling if  I do not have a moisture probe?
Yes, but it is equivalent to writing cheques without ever checking what you have in the bank.
It defeats the object of building an accurate simulation of what is happening in your operation. I wouldn't !

3. What can I use to measure the daily evaporation with?
The ideal is a complete weather station on your site that will give you a Penmann-Monteith ET, secondly you could get real time data from a weather station close to you. The cheapest option ($30) with good results is a Mini station. The software will also work with a Minimum-maximum thermometer (this data is used to correlate to evaporation) and lastly long term data for your area. Accurate modeling is not possible with historic data.

4. How is the full level determined?
It can be done in a laboratory but for typical farm application the following will do:  Calibrate your  probe to read in the unit you are working (mm/m or inch/foot) (counts should be converted to a unit). Saturate the soil you plan to monitor and allow to drain (1 day for sand and up to 5 days for clay soil). Cover the surface to avoid excess evaporation. This is particularly important in hot climates.
Now drill a hole for the access tube, inspecting every 10 cm (or 6") sample to verify that it was saturated and that all free water has now drained out. Insert your access tube and take readings. These readings are the full level at each depth.

5. How is the Refill line determined or planned?
The Refill point is initially planned rather determined. The refill level is preset for each growth stage of the crop (and can be altered at any time). The SMD  (soil moisture deficit) can be set in kPa, mm, inch or percent. A specific export table grape farmer I deal with sets his SMD as follows:   10 mm (0.4") from stem growth until after flowering, then 20 mm (0.8") to full color, then 40 mm (1.5") to harvest and then 60 mm (2.4") into winter. with this recipe he gets a large crop of crisp berries with excellent shelf life. This setting is very site and grower specific. The software comes complete with preset levels that will work for any crop.

6. What material is best for access tubes ?
Firstly you should use just one diameter size.  50 mm (2")  OD is a very common size. Aluminum is the best for neutron probe work because it does not absorb any neutron radiation.  PVC  (class 4 or 125psi) is cheap and easy to use but it does absorb approximately 15%  of the radiation.  It is therefore most important to use a calibration specific to the PVC and never to mix tube materials.

7. What is the best kind of  moisture probe to use?
This program is designed to work with simulation and spot readings, such as will be taken with a portable device (as opposed to  continuous logging). Any device that can be calibrated and can give repeatable readings can be used.  The CPN 503DR neutron probe is in my opinion the most accurate but any TDR or capacitive unit with sufficient accuracy could be used.  Once the program has been set up, the moisture checking could also be done with a tensiometer or gypsum block  device.

8. Can NPS data be used in  PS?
Yes,   rain, irrigation, probe readings, full and refill  levels  can be converted and used in PS. 
Depth and Probe readings time  graphs based on this data can be viewed.

Pricing

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