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IRRIGATION SCHEDULING
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INTRODUCTION
It is said many times in this handbook, the keys to efficient, effective irrigations are knowing WHEN to irrigate, HOW MUCH to irrigate, and HOW to irrigate. This chapter will discuss the irrigation scheduling process. Irrigation scheduling is a generic term, covering a variety of techniques. They all objectively attempt to answer the questions, WHEN and HOW MUCH to irrigate.
All methods of irrigation scheduling will start with some form of the water-budget equation. The water-budget equation is a simplified, mathematical model of water going into and out of the effective root zone.
A key element of irrigation scheduling will depend in large part on your experience. What is the effective root zone? That is, where do you want to control soil moisture? And how dry will you let the root zone get?
The measure of how dry the root zone is before an irrigation is the ALLOWABLE DEPLETION. The choice of AD depends on the crop, the growth stage, and how you want the crop to develop. Sometimes there are detailed recommendations available to help. The leaf pressure chamber readings recommended by UC Extension for cotton are an example. Other times it is your experience alone that will decide.
Water going into the root zone is irrigation, rainfall, or upwards movement of groundwater from a shallow water table. Water coming out of the root zone is crop water use (ETc), or deep percolation. The deep percolation may be a result of excessive irrigation or rainfall.
The water budget equation will be introduced as . . .
SMDend =
SMDstart - IRR - PPT - GW + ETc + DEEP
where: SDMstart = soil moisture depletion at the start of a time period
IRR = any irrigation during that time period
PPT = any rainfall infiltrating during that time period
GW = any upwards movement of groundwater from a shallow water table into the root zone during the time period
ETc = crop water use during the time period
DEEP = deep percolation from excess irrigation or rainfall during the time period
SMDend = soil moisture depletion at the end of the time period
The information needs for, and different methods of using the water-budget equation will be discussed.
The help that is available to you will be identified. These include the weekly and daily Crop Water Use Guide, the Crop Data Sheets, and the Irrigation Scheduling Charts . These were all developed by the District's Water Conservation Program.
IMPORTANT! Irrigation scheduling can be an important tool in increasing irrigation efficiencies. As you will see, it can help to decide the best time to irrigate as well as provide an estimate of how much water to apply. However, it does little to improve the actual application of water in the field. That important factor will be discussed in the "System Management" sections.
Also, in many cases, the traditional water-budget irrigation scheduling is used as an early-warning device. It can alert you that a field is getting close to the allowable depletion so that you can begin looking at it closer. It is never recommended that a water-budget irrigation scheduling system be the sole ruler of when to irrigate. However, water-budget irrigation scheduling will always provide an estimate of how much water to put back into the soil.
One other reason for using some type of irrigation scheduling system is repeatability. If something goes right, if above-average yields are achieved one year, you would like to do the same things next year. Not exactly the same things because Mother Nature is not always the same. But you would like to react in the same manner. Objective methods of irrigation scheduling usually create a record of what happened and when. At the end of a year you can look at this record to see what went wrong and what went right.
Additional related materials are also available from an online training manual provided by the United Nations FAO. Be advised that this publication uses metric measurements. Also, the USBR has supported the creation of the WateRight site which provides an additional source of irrigation scheduling information.
Last updated September 2000