Friction loss in aluminum and plastic lateral lines ahould not exceed 20 percent of recommended sprinkler operating pressure. Following this rule will assure reasonably uniform water distribution. As sprinkler pressure is reduced, due to friction loss down the lateral line, the volume of water applied by the sprinkler and the diameter of coverage is reduced. Certainly, gun sprinklers will require higher pressures, increasing the power required. Many of the portable systems use gun sprinklers.
Some growers are willing to trade off the higher energy costs for the savings in labor costs.
There are some gun sprinklers that will operate satisfactorily at lower pressures and some growers are using these, but they have reduced radius of coverage and, therefore, higher application rates, assuming equal flow rates. There are some low pressure impact sprinklers that have been marketed for the last several years. They also have reduced radius of coverage and this may increase the application rate. Flow is the rate water is being delivered to the sprinklers. After computing TDH, a pump efficiency should be assumed so that horsepower can be computed using the above formula.