There are many technologies available today that support the maintenance of beautiful landscapes, while maximizing water efficiency. A popular application recently discussed by conservation specialists involves lowering water pressure to increase watering efficiency.

Many sprinklers operate at water pressures exceeding those recommended by the manufacturer. Higher operating pressures create system inefficiencies due to excessive flow rates, misting, uneven coverage and watering areas outside the landscape. Lower water pressures generate appropriate water droplet size from the spray heads, provide a more uniform distribution of water and increase performance.

Drip irrigation generally employs point of use emitters to deliver low volumes of water to specific locations at or near plant root zones. Drip irrigation requires low inlet pressures to operate correctly and is very water efficient.

The most straightforward method of lowering water pressure to an irrigation system is adding a single water pressure reducing valve (PRV) at the point of connection (POC) to the irrigation system. The single PRV installed along the POC line will regulate variable high-inlet water pressures to a constant lower optimal water pressure for the entire irrigation system. This allows spray heads and drip emitters to operate in their recommended high efficiency pressure range. Increased water efficiency will be immediately realized with this one change. Any irrigation system that experiences high inlet pressures would realize significant water savings when designed or retrofitted with a PRV installed along the POC to the mainline. This technique is a common practice for irrigation designers and landscape contractors.

The increased efficiency when focusing only on the water savings from the reduction in flow can be calculated by determining the difference in flow rate before and after pressure regulation. Using Bernoulli's equation, we find the relation in flow rate is proportional to the relation in pressure.

Flow/Flow_PRV = (P_main/P_PRV)^-2

Flow = flow rate without pressure reducing valve

Flow_PRV = flow rate with pressure reducing valve installed

P_main = system pressure without pressure reducing valve (mainline pressure)

P_PRV = system pressure with pressure reducing valve installed

Water savings = Flow – Flow_PRV = 1 – (P_PRV/P_main)^-2

Most spray head manufacturers recommended 30 psi to 45 psi. If the mainline pressure is 50 psi and we lower it to 30 psi, we see water savings of 23%. If the mainline pressure is 70 psi and we lower it to 30 psi, we see water savings of 35%.

Lower water pressure provides other benefits to the irrigation system. The components that operate under lower pressures will have a longer lifespan, as higher water pressures increase component failure rates. Increased lifespan of components decreases operating costs over time, as less replacement will be required. In the event of an open line failure from a typical spray head break, the lower water pressure will reduce the open line flow rate of water being expelled through the break. This flow rate reduction will help to mitigate water waste and potential flood damage to the landscaping.

Pressure reducing valves (PRVs) may be installed at several different locations along the point of connection to the main water line. If code allows, the PRV may be installed in line between the backflow preventer and the mainline, allowing the backflow preventer to experience the benefits of the constant lower pressure. A typical installation will have the PRV installed on the upstream riser leading up to the backflow preventer. If the local code does not allow components installed in line before the backflow preventer, the PRV can be installed in line after the backflow preventer. A benefit of this installation is the backflow preventer is isolated from any water hammer effects created by the irrigation control valves. Typical installations will have the PRV installed on the downstream riser after the backflow preventer, as seen in the installation drawing below.