Calculating Total Gallons Per Cycle, Per Zone The SAWS Irrigation Checkup Form now requires that the licensed irrigator provide total gallons used by the irrigation system for both summer and winter schedules. Determining the total gallons a system uses during each cycle of operation is the first step to evaluating the efficiency of the irrigation system. Water Meter Method To calculate irrigation water used by an irrigation zone using the irrigation meter, follow these directions for each irrigation zone: 1. For a set interval of time (for large zones we recommend 3‐5 minutes) record the consumption on the irrigation meter while the zone is operating 2. Take the consumption recorded on the meter and convert it to gallons 3. 1.5” or larger meters record in tens or hundreds of cubic feet. The register on the meter will indicate this by one or two “fixed” zeroes. One full revolution of meter dial is therefore either 10 cf (74.8 gallons) 100 cf (748 gallons). * 4. Multiply the volume recorded by the correct gallon conversion to get gallons consumed during the interval of time. 5. Divide gallons by the number of minutes the zone operated during the test to find gallons per minute (GPM) 6. Multiply the GPM by the run time to give you total gallons Example: 1. Zone 1 ran for five minutes. The 2” meter on site records in hundred cubic feet. The meter registered 120 cf. (The meter dial completed 1.2 revolutions. 1.2 x 100 equals 120) 2. 120 x 7.48 equals 897.6 gallons. 3. Divided by five minutes equals 179.52 gallons per minute. 4. The GPM for this zone is 180 (round to nearest whole number). 5. If the zone is scheduled to run for 20 minutes, the total gallons for Zone 1 is 3,600 gallons Zone # GPM Total Run Time Minutes Total Gallons 1 180 20 3,600 * Smaller meters, 1” or less will record in single cubic feet, or 7.48 gallons. Determining Precipitation Rate with Area/Flow Method
Calculating average precipitation rate requires measurement of zone coverage
area (in square feet) and flow rate in GPM.
Precipitation Rate (inches per hour) =
PR =
96.25 x GPM
A (ft²)
PR -- Precipitation Rate (inches per hour)
96.25 -- Conversion Factor - Converts GPM to inches per hour
GPM -- Flow rate for the zone; refer to meter method on worksheet for
Calculating Total Gallons Per Cycle, Per Zone
A -- Area of coverage (square feet)
Precipitation Rate for Drip:
231.1 x Emitter Rate
Spacing between emitters (inches) x Spacing between tubes (inches)
2800 U.S. Hwy. 281 North
P.O. Box 2449 San Antonio, TX 78298-2449 www/saws.org