Pump discharge (flow)

Back in Step 5. Calculate your irrigation requirement, we looked at peak flow requirement. This is the measurement that you use to calculate your pump flow requirement.

High level formula to determine flow:

Metric calculation

Step Example
Obtain your peak water requirement: mm/day 8.5 mm/day
Convert mm to m3/ha  
Convert mm to m by dividing mm by 1,000. 8.5 mm = 0.0085 m
Multiply the m x 10,000. 0.0085 x 10,000
= m3/day/hectare 85 m3/day/hectare
Calculate how much water is required for the entire irrigation system  
Multiply the m3/day/hectare by the total cropping area that the pump has to service 85 x 130 hectares
= Total pump output required m3/day 11,050 m3/ha
Calculate the flow requirement per hour, adjusted for how many hours the pump will be used  
Divide the pump output m3/day by the number of hours the pump will operate 11,050/20 hours
= Total flow (discharge) per hour that the pump needs to deliver 552.5 m3/hour
   

US measurement calculation

Step Example
Obtain your peak water requirement:
in/day
0.33 in/day
Convert inches per day to gal per acre per day 0.33 in/day x 27,154 = 8,961 gal/acre/day
Calculate how much water is required for the entire irrigation system  
Multiply the gal/acre/day by the total cropping area that the pump has to service 8,961 gal/acre/day x 320 acres = 2,867,520 gal/day
= Total pump output required gal/day 2,867,520 gal/day
Calculate the flow requirement per hour, adjusted for how many hours the pump will be used  
Divide the pump output gal/day by the number of hours the pump will operate 2,867,520 gal/day / 20 hours/day = 143,376 gal/hour
Divide the total flow (discharge) per hour by 60 to determine the flow rate of the pump in gpm 143,376 gal/hr / 60 min/hr = 2,390 gpm
   

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