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Water Quality Data
IRRIGATION SCHEDULING
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BACKUP SOIL/PLANT MOISTURE MEASUREMENTS
As you might guess, there can be errors made during water budget scheduling calculations. Normally some sort of direct measurement of soil and/or plant moisture will be made on a regular basis to ensure that the scheduling system is working. There needs to be a check between the calculated SMDend and what is the real field SMD.
From Soil Moisture Status section, you saw that there were two ways to measure soil moisture. One is volumetrically. This is a measure of the actual amount of water in the ground. It corresponds to the results of the water-budget equation. That is, inches of water in the effective root zone. For that reason, many consultants will use a neutron probe in checking their calculations. The probe reads out in inches of water per inch of soil and thus provides a direct check of the calculated soil moisture depletions.
Experienced growers and consultants (especially those that have worked on the same ground for many years) will use the feel method . This entails using a soil sampler to pull up samples of soil from various depths. The feel and appearance of the soil indicates how much moisture is present. Most tables used with this method are calibrated in inches of water.
The advantage of using a volumetric back-up measurement is that it provides a direct check on the calculations. But remember also that the plant doesn't care how much water is in the soil. It only cares how hard the soil is holding on to that water. Thus it is also a good idea to take tension readings somehow. Readings of soil/plant moisture tension are an indication of how much stress the crop is under. They can help in determining a suitable allowable depletion to use in timing irrigations.
For example, the UC Extension Cotton Specialists now recommend using the leaf pressure chamber to time irrigations on cotton. They feel it is especially important for the first seasonal irrigation. A grower could be computing the water-budget while taking leaf pressure readings. When the plant moisture tension indicates an irrigation (the WHEN to irrigate), the water-budget equation will help in determining HOW MUCH water to apply.
When taking these backup measurements there is also the question of whether to take them before an irrigation or after. Taken before an irrigation they can verify the need for the application and provide an estimate of how much water is needed. Taken after an irrigation they can tell you if the irrigation was sufficient.
Through years of experience, Growers have learned to judge the timing of irrigations through the appearance and development of the crop. Modern irrigation scheduling methods do not ignore this experience. Experience will be essential in the use of modern techniques. However, new varieties can require new strategies on when and when not to irrigate. And modern techniques help planning. They indicate when and how many fields are getting close to an irrigation. And most important, they provide an estimate of HOW MUCH to irrigate.
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