It is important to regulate not only the amount of water delivered to the plants but also the timing of that delivery; there will be an optimal time of day for the plants to absorb water, and seasonal adjustments will be necessary to reflect plant growth stages. Weather patterns will augment or subtract from the total soil moisture, and must also be assessed. Once the water requirements are fully understood, they can inform the proper selection and operation of a timer-controller for the irrigation system.
Areas can be measured from an accurate site plan and/or determined by field measurements. Area calculations can be factored with the plant water requirements to determine a volume of water required, or a rate of water flow required for a specific time period. Once these water demands are figured, the next step will be to determine if the existing utility service can accommodate them.
Utility investigation involves two key issues: water pressure available in pounds per square inch (psi), and the volume of water flow available, often expressed in gallons per minute (gpm). Water pressure can be obtained by asking the water utility service provider or by installing a pressure gauge to an exterior faucet, which will indicate the pressure available in psi. Water flow rates can be determined quickly by what is called a “bucket test”. A container of known volume (such as a 5-gallon bucket) is filled with the faucet wide open. The elapsed time to fill is noted and basic math reveals the water flow rate in gpm.
Both of these parameters are critical to further system design, since the selected system size and components must be functional within these pressure and flow-rate constraints. Particularly, the length, size and type of buried piping systems, the type and number of spray heads (rotor, spray, or seepage), and system losses attributable to elevation shifts and friction must be assessed.
Most irrigation system manufacturers have charts and tables that will aid in the analysis of a planned installation, to ensure that the sum of its components do not exceed the pressure and flow limits of the utility service. Spray head coverage and service pressures are a key element of this analysis.
Most municipalities will require the installation of a backflow preventer to ensure that no chemicals or sediments are siphoned back into the municipal water supply if system pressure is lost.
