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WSU-TFREC Orchard Management Forum

Irrigation scheduling

Determining the approximate number of days that should pass between sets

Tim Smith, WSU Extension

Before you can carry out this process you need to determine how much water your system applies per set, and how much water your soil can hold in the trees effective root zone. When you compare those two values, the lowest amount of the two is the amount that you can let the trees use between irrigation sets.

To determine the approximate number of days that may pass safely between sets:

Look up the average daily water use that relates to the time of season and weather conditions in the table of values below. Divide this number into the orchard blocks net applied water per set, or the soils usable moisture holding capacity, whichever is least. The resulting number is the number of days that can pass safely between sets.

Irrigation season ...cooler ...average ...warmer

early April =.04 in/day =.05 in/day =.06 in/day

late April =.08 =.09 =.10

early May =.12 =.13 =.16

late May =.17 =.20 =.25

early June =.20 =.23 =.25

late June =.25 =.29 =.32

July =.27 =.32 =.38

early August =.27 =.31 =.35

late August =.21 =.24 =.30

early September =.15 =.18 =.22

late September =.08 =.10 =.15

October =.05 =.07 =.09

Probe your orchard soil prior to each set. If your soil seems too moist prior to your estimated set timing, you may have underestimated the soils water holding capacity or the net amount of water your irrigation system applies. Add 10 percent to your estimated stored water (multiply by 1.1). If the soil seems too dry prior to the set, reduce your estimated stored water amount by 10 percent (multiply by .9). By making these adjustments up or down, you will soon have a good, practical value to work with in that block. This working value wont change through time, except in blocks of immature trees, where root depth and tree size is increasing.


Your orchard soil has the ability to store 2.4 inches of usable water in the trees effective root zone. You have evaluated your irrigation system, and have found that it applies a net amount of 2.2 acre inches per irrigation set. It is early June, the weather is warmer and windier than normal, and you need to determine how many days your block can go between sets.

You look at the table across from "Early June"' and in the "Warmer" column and determine that the average daily water use under these conditions is .25 inches per day. You divide that number into your 2.2 inch net application per set (since that is less than the soils water holding capacity) and you determine that about 8.8 days can pass between sets under these conditions. So you begin your next irrigation set 9 days after you began the last one. (You probed the top three feet of your soil just before you started irrigating, and it looked pretty good: top foot fairly dry, second foot moderately moist, third foot plenty moist.)

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