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When to Irrigate

Program Contact: Tianna DuPont, Tree Fruit Extension Specialist
(509) 663-8181 •


By Timothy J. Smith, WSU Extension

To determine the approximate number of days that may pass between irrigation sets:
Divide the number from the table that relates to the time of season and the weather conditions into the number of acre inches of usable water stored in your orchards soil, or the net water applied per set, whichever is least.

EXAMPLE: Your orchard soil has the ability to store 2.4 acre inches of water in the trees effective root zone. You have evaluated the irrigation system, and have determined that it applies a net 2.2 acre inches of water per irrigation set. It is early June, the weather is warmer than normal, and you want to determine the approximate number of days that should pass between sets in your orchard. Look to the table under the Early June-warm column. The average daily use under these conditions is about .26 inches per day. Divide that value into the net 2.2 inches of water you apply per set ( which is less than your soils’ usable water holding capacity). You determine that about 8.5 days should pass between irrigation sets in that block. So, in this example, you would start irrigating as soon as 8 or 9 days have passed since the day you last started irrigating.




For each irrigation unit:

Step 1: Choose the orchard “Usable water”.

 What is your soils’ estimated “Usable water”?  ___________
 What is your “Net applied water per set”?  ___________
 Which of these two amounts is the least?  A.__________


Step 2: Determine the approximate set interval.

Do you think the weather over the next several days will be cooler than average, average, or warmer than average? What is the average “Water use per day” for this time of season under these general weather conditions?

Put the daily use rate from the proper column here:                          B._________

Divide the amount in space “A” by the decimal in space “B” to determine the approximate number of days that may pass between sets in that irrigation unit.

Example: It is late May, and the weather is cooler than average. Your estimated usable water is 1.9 inches. Your system applies 2.1 inches per set. Referring to the Average water use chart, you see that about .17 acre inches of water are used by the orchard daily under the present weather conditions. You divide .17 into 1.9 usable inches of water, (which is less than the 2.1″ net applied water) and determine that about 11 days may pass between sets.

1.9 inches usable water
 __________________ = 11.2 days set interval
 .17 inches use per day


You probe the soil after 11 days and feel that it is too moist to require irrigation for another day or two. You irrigate when ready, but add another 10% to your usable water estimate, bringing it up to 2.1 inches. It is now early June, and the weather is about normal. Divide .23 ( the daily use rate for average-early June) into your new estimate of 2.1 inches of usable water. You find that about 9 days can pass between sets. After 9 days, you check the soil prior to irrigating, and find it in the moisture condition you prefer. You irrigate, and continue to use 2.1 inches as your usable water estimate during the rest of the season. Your set interval becomes shorter during July, then lengthens as Fall approaches, but you feel that the soil is staying adequately moist between sets.