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Determining your Irrigation Schedule

By Tim Smith, WSU Extension Specialist Emeritus. Compiled by Tianna DuPont, WSU Extension Specialist, Tree Fruit.

Ag Weathernet has an irrigation scheduler tool which allows you to plan your irrigation schedule based on the actual precipitation and evapotranspiration at a weather station near you. It takes into account your soil type, your soil’s water holding capacity, and the evapotranspiration rate of fruit trees. You can set up multiple fields and track them on your computer, or phone.

Alternatively follow the steps below to calculate your water needs.

How Much Water Do I Need?

The amount of water that trees need in a given week is calculated based on the evapotranspiration rate (ET). You can use data from Ag Weathernet to find apple ET in your area. Login and click on water use. Select your crop and a station near you. The water use model will predict tree water use over the prior week.

Tim Smith, WSU Extension Emeritus put together this nice table which gives you some average numbers to work with.

Tree Water Use per Week

Irrigation season …cooler …average …warmer
inches/day inches/day inches/day
early April 0.04 0.05 0.06
late April 0.08 0.09 0.1
early May 0.12 0.13 0.16
late May 0.17 0.2 0.25
early June 0.2 0.23 0.25
late June 0.25 0.29 0.32
July 0.27 0.32 0.38
early August 0.27 0.31 0.35
late August 0.21 0.24 0.3
early September 0.15 0.18 0.22
late September 0.08 0.1 0.15
October 0.05 0.07 0.09

***note for standard older style orchards

Don’t forget to adjust the tree water needs by the efficiency of your irrigation system. For example, in an overhead sprinkler system you might assume only about 75% efficiency. For drip irrigation you can assume almost 100% efficiency. Your system may be somewhere in between.

How Much Water Can I Apply at Once?

Now that you know how much water you need to apply you need to think about how much of this water you can put on at once.

How much you can irrigate at once depends on how much water your soil can hold and how much is usable to plants. A soil’s water holding capacity is the difference between the field capacity (water in the soil after a soaking rain that has been allowed to drain) and the permanent wilting point (where a plant growing in that soil would wilt and never recover). The first 50 percent of that water used by the trees is the amount you should count on. We like to call this amount the “usable water”.

The amount of “usable water” held in the tree’s root zone depends on the depth of the soil in the root zone, the texture of the soil, the soil’s percentage of rock and gravel, textural layers, and compaction.

To correct for the root zone depth we assume two feet of root depth in young or dwarf rootstock orchards, and about 3 feet of root depth in older, vigorous rooted orchards. In very high quality soil or older trees you may assume up to 3.5 or 4 feet. Root studies have determined that 2/3 of the root volume of large, old trees is in the top two feet of soil. About 80 percent of the roots are in the top three feet. Thus, even for large trees it is generally recommended to assume no more than 3 feet of rooting depth.

To determine your soil type the web soil survey is a useful tool. You can also send a soil sample to your local lab for textural analysis.

  • Determine your soil type.
  • Estimate the ‘usable water’.
  • Divide inches needed per week by the amount your soil can hold in one irrigation to get the number of times you need to irrigate per week.

Usable Water**  in Common Soil Types

soil texture: — 2 ft. root zone — 3 ft. root zone
acre inch acre inch
coarse sand 0.92 1.25
sand 1.25 1.74
fine sand 1.37 1.95
loamy sand 1.41 2.03
loamy fine sand 1.47 2.08
sandy loam 1.54 2.2
fine sandy loam 1.87 2.65
very fine s. loam 2 2.82
loam 2.2 3.15
silt loam 2.41 3.4
sandy clay loam 2.2 3.11
silty clay loam 2.03 2.91
clay loam 2.03 2.95
sitly clay, clay 1.87 2.81

**Usable water is 50% of the water holding capacity.

How Does the Water I Need Compare with the Water I Have Been Applying?

Step 1. Determine the number of sprinkler heads per acre.

To follow the operation outlined below, you’ll need a tape measure, a gallon milk jug, a watch, and a calculator.

  1. Measure the number of feet between heads down the row. ___________
  2. Measure the distance between laterals (crossways). ______________    This is usually the row or every other row spacing.  Don’t measure diagonally between heads.
  3. Multiply these two footages to get the square feet per head. _______
  4. Divide the square feet per head into 43,560 (square feet in an acre). This gives you the number of heads per acre.
  5. Write your answer here. ________________ heads per acre.

Step 2. Determine the gallons of water applied per set.

  1. Catch water in a gallon milk jug until it is full. Time the number of seconds it takes to fill it. This will take from about 20 seconds to about three minutes, depending on the size of the nozzle. Do this timing at several nozzles around the block. There may be some variation. Fix the problem if the variation is more than 10 percent down the lateral. Pressure and nozzle problems are the most common reasons for variation.
  2. Divide the average number of seconds it took to fill your gallon into 60. This will give you the number of gallons the heads are putting out per minute.
  3. Write that number right here. ______________________(gal/minute/head)
  4. Now multiply the number of heads per acre by the average gallons each head applies per minute to get the gallons applied per acre per minute.
  5. Write that number right here.  ________________________(gal/min/acre)
  6. Multiply that number by 60 to determine the gallons applied per hour.
  7. Write that number right here.  ________________________(gal/hour/acre)
  8. Now multiply the gallons per hour by the number of hours you actually irrigate:
  9. Write that number here __________________________(gallons per set per acre)

Step 3. Estimate net gallons per acre:

Water lost to uneven application, evaporation, and runoff can’t be counted towards your trees use. Losses can vary from 10 to 50 percent, depending on the time of season and the system design. The wider the head spacing, the lower the efficiency. Over-tree systems can lose a great percentage of water to evaporation on windy days, especially in mid-day and mid-summer. A normal under tree system will be about 70-80 percent efficient.

  1. Take a stab at estimating your system efficiency. If you just don’t know, figure 70 percent.
  2. Convert percent into a decimal (70 percent is 0.70).
  3. Multiply the gallons per acre per set by this efficiency decimal. For example: 54,880 x 0.7 = 38416
  4. This equals the:  _______________________ (net gallons per acre per set).

Step 4. Determine the net acre inches applied per set:

  1. Divide the net gallons applied per acre per set by 27,000 gall per acre inch. This will give you the net acre inches applied per acre per set. For example: 38,416 net gallons per set per acre divided by 27,000 = 1.42 acre inches.
  2. This is the amount of usable water you have applied.
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