The handbook of Technical irrigaTion informaTion - Hunter Industries

14Handbook of Technical Irrigation InformationMaximum System Capacity RequirementThe following formula is to be used to determine the gallons per minute (GPM) necessary for peak waterdemand periods. While an area might survive for short periods with less water by utilizing soil moisture reserves,extended periods of drought would require the maximum system capacity. The formulas below (#10, #11, #12) are aguide and it should be remembered the answer is no better than the factors that go into the formula (e.g. ReferenceEvapotranspiration Rate, Area, Distribution Uniformity, etc.).Maximum System Capacity Requirement – GPM per Hydrozone(s)This formula is used to determine the maximum water required, in GPM, from the source based on the hydrozone’scharacteristics.GPM = 0.0104 x Et o x Area x K cDU x Hrs. AvailableVariable Value Ranges:GPM = gallons per minute requiredEto = peak daily evapotranspiration for the worst case scenario in inchesArea = area to be irrigated in square feetKc = Crop Coefficient – use 1.0 if actual crop coefficient is not knownDU = distribution uniformity or irrigation efficiencyHrs. Available = hours available for irrigation each day in the worst case0.0104 = constant for conversion of area, flow and inches per day, etc. into common unitsExample:A park has been designed with 450,000 square feet of turf, and all areas must be irrigated between 10 p.m. and6 a.m. The crop coefficient is 60% (0.6) with an average peak evapotranspiration rate of 0.35 inches per day. The systemdistribution uniformity has been estimated to be 65%. What will be the maximum GPM required for the park?GPM = 0.0104 x 0.35 x 450,000 x 0.600.65 x 8GPM = 189Maximum System Capacity Requirement – Total Area For Given GPMThis formula may be used to to determine the area that can be irrigated if you know the gallons per minute of thewater supply.Area = GPM x DU x Hrs. Available0.0104 x Et o x K cWhere:Area = area to be irrigated in square feetGPM = gallons per minute available from the water supplyDU = Distribution Uniformity or Irrigation EfficiencyHrs. Available = hours available for irrigation on the “Worst Case” dayEto = peak daily evapotranspiration for the worst case scenarioKc = Crop Coefficient – use 1.0 if actual crop coefficient is not known0.0104 = constant for conversion of area, flow and inches per day etc. into common unitsExample:A developer has a well on the site of a proposed golf course. The well will produce 350 GPM. The course will be de-