Procedures for Estimating the Amount of “Usable Water” Your Irrigation System Applies
By Timothy J. Smith, WSU Extension
The amount of irrigation water applied or used by the orchard trees is usually measured in “acre inches”. An acre inch is the total water necessary to cover one acre (a 209 foot by 209 foot square) with water one inch deep. There are about 27,000 gallons in one acre inch. To determine the gross amount of water your system applies, you need to determine the number of sprinkler heads per acre and the amount of water each sprinkler puts out per minute.
To carry out the procedure on the following worksheet, you will need a tape measure, a stop watch or watch with a seconds indicator, and a quart or gallon container or 5 gallon bucket calibrated to gallons and quarts.
Measure the water output from a number of sprinklers around the block. It is important to measure sprinklers scattered along the entire length of laterals and at higher and lower elevations in the block. If there is large variation in output, the system may need repair. To determine variability, divide the average output into the individual sprinkler output. If this results in a number over 1.1 or under 0.9, the nozzle should be replaced and retested.
Catch the water from low output systems (such as microsprinklers) into quart or gallon jugs. To determine the gallon per minute output, time the number of seconds it takes to fill the container, then divide the seconds it took to fill the gallon jar into 60. If you use a quart jar, multiply the seconds it took to fill by four, then divide that number into 60. With some very low output microsprinklers, you may wish to determine the number of ounces applied per minute, and divide that number by 128 to determine the gallons applied per minute.
For higher volume systems, catch water for a minute into your calibrated bucket. If you are measuring the output of an overtree system, you will need a 810 foot length of hose, a tall, steady ladder, nerves of steel, and the ability to balance a partlyfull bucket of water while standing near the top of a ladder. (Sometimes it’s helpful to have a longer hose, and a helper to hold the bucket steady on a lower rung while you catch the water from the nozzle into the hose). Do this on a warm day, you’re sure to get soaked.
Estimating efficiency: Some of the water coming from the sprinklers will not be available for tree use. Some water evaporates or runs off, but the largest losses come from uneven application. Water is applied unevenly because of wind, poor sprinkler spacing, or blockage of the water stream by weeds or trees. Excess water must be applied in order to better wet the “light” areas. Some of the water that falls on the “wetter” areas will pass through the root zone, so it can’t be counted as usable. The wider the sprinkler spacing, the more uneven the application. Average efficiency in well designed and maintained solid set undertree systems is about 70% (.7 times your gross applied water = net water). On hot, windy days, efficiency of some overtree systems drops as low as 50%. Most microsystems, weed free and well maintained, operating at proper pressures, may have efficiencies as high as 85%. Efficiency will vary during the season, or even the time of day the system is running. A good estimation of efficiency is sufficient for your working estimate of “usable water”.
ESTIMATING THE NET AMOUNT OF WATER YOUR IRRIGATION SYSTEM APPLIES – Worksheet
by Tim Smith, WSU Cooperative Extension
Orchard Situation = 4th leaf Fuji on M26, 6×15; microsprinklers each 12 feet down each row, soil is a fine sand (1.2″ usable water holding capacity):
1. Sprinkler Heads Per Acre?
Divide square feet per head into 43560.
2. Gallons of Water Applied Per Acre Per Set?
Multiply gallons per minute by 60
Gross gallons per hour multiplied by the number of hours per set (12 hours in this example)
3. Net Gallons Per Acre Per Set?
Water loss due to evaporation and uneven application reduces the amount of usable water by 10 to 50%, depending on time of year and the application system. For instance, the wider the head spacing, and the less the potential overlap of the sprinkler patterns, the lower the efficiency. A good undertree system will be 70 to 85% efficient. A wide spaced overtree system may deliver only 50% of the gross gallonage into the trees root zone on a hot day in July.
Gross gallons per acre must be multiplied by .5 to .85, depending on your system estimated efficiency.
4. Net Acre Inches Applied Per Set?
Divide net gallons applied per acre per set by 27,000.
How well does this amount of applied water compare to your orchard soils water holding capacity? Is it equal or less? If your irrigation system applies much more water than your soil can hold, you are wasting water and nutrients, and need to reduce your hours of irrigation.
(in this example, 1.2 inches applied per set, 1.2 inches usable water holding ability. A good match. Now, use the blank worksheet below for your own blocks of orchard, then move on to the third step, “How Often do I Apply Water?)
ESTIMATING THE NET AMOUNT OF WATER YOUR IRRIGATION SYSTEM APPLIES
Blank Worksheet
by Tim Smith, WSU Cooperative Extension
1. Sprinkler Heads Per Acre?
number of feet between heads down the row? —–> ____________
number of feet between lateral lines (not heads)? X ____________
equals square feet per head ——————> = ____________
divide square feet per head into 43560.
_________ sprinkler heads per acre
_______/ 43560
2. Gallons of Water Applied Per Acre Per Set?
multiply the Gallons per minute output per head —> ____________
(measure this in several places)
by the number of heads per acre ————> X ____________
equals gross gallons per minute per acre —> = ____________
(multiply this by 60) ———————–> X 60
equals gross gallons per hour ————–> = ____________
gross gallons per hour multiplied by the
number of hours per set ——————> X ____________
equals the gross gallons per acre per set —> = ____________
3. Net Gallons Per Acre Per Set?
Water loss due to evaporation and uneven application reduces the amount of usable water by 10 to 50%, depending on time of year and the application system. For instances, the wider the head spacing, the lower the efficiency. A good undertree system will be 70 to 85% efficient. A wide spaced overtree system may deliver only 50% of the gross gallonage into the trees root zone on a hot day in July.
Gross gallons per acre must be multiplied by .5 to .85, depending on your system estimated efficiency.
gross gallons per acre per set ———–> ____________
multiplied by the estimated efficiency —> X ____________
equals the net gallons/acre/set ———> = ____________
4. Net Acre Inches Applied Per Set?
divide net gallons applied per acre per set by 27,000.
net gallons per acre  

= net acre inches per set 
27,000 
well does this amount of applied water compare to your orchard soils water holding ability? Is it equal or less? If your irrigation system applies more water than your soil can hold, you are wasting water and nutrients. You must either reduce the number of hours you irrigate each set or reduce the nozzle size, so less water is applied per hour. Consult your irrigation advisor prior the nozzle size adjustment.