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Irrigation Guide

USBR Water Manage- 
ment Plan

Water Quality Data

IRRIGATION SCHEDULING

Information requirements for Water-Budget Irrigation Scheduling

The actual water budget equation is simple, a starting soil moisture depletion and five numbers to add or subtract to determine the ending soil moisture depletion. The problems, and the costs, of water budget scheduling come with identifying those numbers. Accurate water budget irrigation scheduling requires knowledge of the following . . .

• The soil's available water holding capacity throughout the season.
• Agronomic factors that determine how much to stress the crop between irrigations, the choice of allowable depletions.
• Effective rainfall, that is, rain that actually soaks into the soil and is not runoff.
• Effective irrigation depths, how much water delivered to a field stays in the effective root zone, available to the crop.
• Upwards water movement from any shallow water-table.

Most people doing water budget scheduling will also take (or they should be taking) regular measurements of soil and/or plant moisture. This will make sure that the estimates of the numbers are accurate.

This looks like a lot of information and it is. But there are many sources available to you. Also, scheduling consultants will develop the required numbers as part of their service.

Looking at the help available to you in finding the information needed (by category):

SOILS - the Reference section of this handbook describes important properties of most important soils in the District. The UC Extension specialists also will have information, including Extension pamphlet #21463, "Holding Capacities of California Soils".

Knowledge of the soil will provide estimates of field capacities and permanent wilting points as well as any restrictions to root zone development and salt levels.

A very fine point of irrigation scheduling concerns the estimate of available water holding capacity, AWHC. We've said that AWHC is equal to the field capacity minus the permanent wilting point. And we've also said that it does no good to soak water into a soil already at field capacity as it will just drain through the root zone. However, depending on the soil and the depth of the root zone, it may take 2 or more days for this excess water to soak through. In this time the crop can be using some of the excess. In real terms, the AWHC may be somewhat more than the accepted value of field capacity minus permanent wilting point.

The effect of this slow drainage in increasing the practical soil moisture reservoir vary with the soil. The effect will be greater on finer soils and deeper root zones than on coarse soils and shallower root zones.

ROOT ZONES - it can be very difficult to set effective root zones and may take a couple of years of experience, especially with annual crops. Your UC Cooperative Extension specialist or consulting agronomist will have information for you.

Make sure you consider any restrictions on the root zones due to hard pans or high water tables. The root zone of an annual is going to change constantly up to plant maturity. But the effective root zone does not have to be the full depth of the rooting system. For example, in cotton it is common to choose an effective root zone of 4 feet. This is done even though the full system might go to 6 foot or more. Remember, the effective root zone is where you want to control soil moisture.

ALLOWABLE DEPLETIONS - AD's are a measure of how much stress is to be applied to a crop. AD's may change with the season. With cotton it was common (although not recommended with newer varieties) to put heavy water stress on early and then go to a more regular routine once the plant was fruiting. Be aware of the different growth stages of your crops and how they should be manipulated during these stages.

Also, if you have fields with high salinity, the AD is likely to be lower than normal (keeping more moisture in the root zone). The Crop Data Sheets in the Reference chapter contain recommended water stress levels for the different growth stages of most important District crops.

Many times, allowable depletions are "backed-in to" when first starting irrigation scheduling. The Grower is checking a field and finally decides to irrigate. The irrigation scheduling system is checked for the soil moisture depletion at the time of the irrigation. This is then converted to an allowable depletion for future use.

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Last updated September 2000