Calculating Representative Zone GPMs
The SAWS Irrigation System Analysis Form requires the calculation of
the Peak Season Estimated Total Gallons Per Month. The electronic
PDF of the form has embedded formulas that eliminate the need to do
calculations separately. The form will calculate the estimated monthly
peak season consumption based on representative zone GPMs and
total run times provided by the irrigator.
The irrigator will need to calculate the GPM for one representative
zone for each zone type (spray, rotor, multi-stream rotor, drip or
bubbler) included on the system. There are two ways to determine
the GPM for an irrigation zone:
1. 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 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 (cf). 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) or 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).
Example:
1. Zone 1 ran for three minutes. The 2” meter on site has a fixed zero, meaning
it is recording in 10 cubic feet increments (a revolution of the meter equals
74.8 gallons). In three minutes, the meter recorded 13 cubic feet (the meter
dial completed 1.3 revolutions).
2. 1.3 x 74.8 equals 97.24 gallons
3. Divided by three minutes, equals 32.4 gallons per minute.
4. The GPM for this zone is 32 (round to nearest whole number).
* Smaller meters 1” or less will record in single cubic feet, or 7.48 gallons.
2. Pressure Reading and Manufacturer Performance Charts
Irrigation systems that are fed off a shared meter or a looped meter system will require the flow rate be
calculated using the manufacturer specifications for flow rate based on head pressure.
ROTOR ZONES:
To calculate the flow rate for your representative rotor zone you will need to determine the pressure for an
individual head, reference the nozzle in the manufacturer’s catalog to find the GPM, and then add up all the
heads in the zone to determine the flow rate in GPMs.
Step 1: Determine the pressure at individual rotor using a pressure gauge attached to a pitot tube.
Step 2: Determine the approximate radius throw of the Rotor nozzles in the zone
Step 3: Reference Manufacturer’s Performance Chart for the specific make/model of rotor and find the GPM
per head based on the pressure reading radius.
GPM
Step 4: Multiply the GPM for individual rotor by the total number of rotors in the zone to determine the Total
for the representative rotor zone.
Example:
Step 1: Zone pressure reading at individual rotor is 40 PSI.
Step 2: Rotors in zone have approximately a 40’ radius
Step 3: Reference Manufacturer Performance Chart to identify GPM per rotor:
Yellow #4 Nozzle with 40 PSI, 40’ Radius = 3.8 GPM
Step 4: Multiply individual rotor GPM x number of rotors in zone
10 rotors in zone X rotor GPM of 3.8
Total GPM for Representative Zone = 38 GPM
SPRAY ZONES:
To calculate the flow rate for your Representative Spray Zone you will need to determine the pressure for an
individual head, reference the nozzles in the manufacturer’s Performance Chart to find the GPM, and then sum
up all the heads in the zone to determine the flow rate in GPMs.
Step 1: Determine the pressure at individual spray head using a pressure gauge.
Step 2: Identify type and quantity of spray heads in the zone
Step 3: Reference the manufacturer Performance Chart for EACH ARC of specific spray head in the zone and
find the GPM per each head type head based on the pressure reading.
Step 4: Add up the GPM for all the spray heads in the zone to determine the Total GPM for the representative
spray zone.
Example:
Step 1: Zone pressure reading at individual spray head is 30 PSI.
Step 2: Identify type and quantity of spray heads in the zone:
- 10 count of the 360’ Van nozzle
- 10 count of the 180’ Van nozzle
Step 3: Reference Manufacturer Performance Chart to identify GPM per nozzle type:
- 360’ Van nozzle: GPM = 2.60
- 180’ Van nozzle: GPM = 1.45
Step 4: Add up the GPM for all heads in zone to determine the total GPM’s for the representative zone
2.60 GPM x 10 heads = 26 GPM
1.45 GPM x 10 heads = 14.5 GPM
Total GPM for Representative Spray Zone = 40.5 GPM
DRIP/BUBBLER ZONES:
To calculate the flow rate for your representative drip zone you will need to know the following: the approximate total
length of drip tubing, the emitter spacing and emitter flow rate (GPH, located in the manufacturer catalog).
Example
Step 1: Determine the total length of drip tubing in the zone:
The drip zone has 5 lateral lines of tubing. Each lateral line is 75’in length
5 x 75’ = 375’
Step 2: Determine the total # of emitters in the zone:
The emitters are spaced 18” (1.5 feet) apart
375 feet ÷ 1.5 feet = 250 emitters
Step 3: Determine the Total Flow Rate (GPM) for the zone:
Per manufacturer catalog, the emitter rate is .9 GPH
532 emitters x .9 GPH = 479 GPH
479 GPH ÷ 60 = 8 GPM
Total GPM for the representative drip zone = 8 GPM
SAWS Irrigation System Analysis Form
Zone #
GPM
Total Weekly
Run Time
Total
Gallons
# Zones
Total
Weekly
Gallons/Zo
ne Type
Spray
7
40.5
20
607.5
12
7,290
Rotor
1
38
30
1140
6
6, 840
19
8
45
135
3
360
Representative Zone
MP-Multi-Stream Rotor
Drip
Bubbler
Peak Season Estimated Total Gallons/Week
Peak Season Estimated Total Gallons/Month
14,490
gallons
53,406
gallons