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WSU-TFREC Orchard Management Forum

How much water is my system applying?

by Tim Smith, WSU Extension

Below is a quick method that you may use to determine how much water you are applying each time you irrigate. This proceedure takes about 1/2 hour, less when you get on a roll.

Many systems were designed when people believed apple trees rooted 5 or 6 feet deep, rather than the 2 to 3 feet that is much more common in this region. The false root depth assumption lead people to assume a much larger storage capacity for water under the tree. Application rates were commonly designed for 4 to six acre inches of water applied over a 24 hour set.

To keep trees well watered during the stressful part of the year, most systems should apply between one and three inches of water per set, the amount dependant on the soil qualities, the age and type of tree in the planting.

To follow the operation outlined below, you'll need a tape measure, a gallon milk jug, a watch, and a calculator (unless you want to do all the arithmatic longhand-possible, very organic, but time consuming)

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

a. Measure the number of feet between heads down the row___________

b. Measure the distance between laterals (crossways)______________

( this is usually the row or every other row spacing, don't measure on the diamond, diagonally between heads)

c. Multiply these two footages to get the square feet per head_______

d. Divide the square ft. Per head into 43560. This gives you the number of heads per acre.

e. Write your answer here: ( heads per acre)_______________________

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

(you're gonna get wet. Do this on a warm afternoon).

a. Catch water in your 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.

b. 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.

Write that number right here: (gal./minute/head)______________________

c. 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.

Write that number right here: ( gal./min./acre) ________________________

d. Multiply that number by 60 to determine the gallons applied per hour.

Write that number right here: ( gal./hour/acre) ________________________

e. Now multiply the gallons per hour by the number of hours you actually irrigate:

Write that number here ( gallons per set per acre)__________________________

Step 3. Estimate net gallons per acre:

Water lost to uneven application, evaporation, runoff etc. can't be counted towards your trees use. Losses can vary from about 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 undertree system will be about 70-80 percent efficient.

So, take a stab at estimating your system efficiency. If you just don't know, figure 70 percent.

Convert percent into a decimal (70 percent is .70)

Multiply the gallons per acre per set by this efficiency decimal.

example: 54,880 x .7 = 38416

This equals the: (net gallons per acre per set) _______________________

4. Determine the net acre inches applied per set:

Divide the net gallons applied per acre per set by 27000. This will give you the net acre inches applied per acre per set.

Example: 38416 net gallons per set per acre divided by 27000= 1.42 acre inches.

This is the amount of usable water you have applied. There are many questions that you may turn to once you have this number. For instance:

Are you sure that your soil can hold this amount of water?

Do you know how to determine when this amount of water has been taken from the soil by your trees?

Knowing how much water you are applying is an important first step in water management. If you go through the above proceedure for each separate irrigation system on your place, you may then move into the next installments of this irrigation management series.

Estimating soil water holding capacity (relates to how much water you are applying)


How many days between sets?

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