Backflow Prevention, Valves, Controllers, and MWELO Compliance
This chapter covers the three subjects that turn an irrigation system from pipe into a regulated installation: protecting the public drinking water supply from the landscape, the electrical and mechanical control system that operates it, and the state ordinance that governs how much water a new California landscape is allowed to use. Backflow protection is a public health matter, and the specific device required, the testing interval, and who may test it are set by the local water purveyor under state requirements, so verify the local rule rather than assuming.
Backsiphonage, Backpressure, and Choosing the Device
Backflow is water flowing the wrong direction through a cross-connection, and there are exactly two ways it happens. Backsiphonage occurs when the supply pressure drops below the pressure in the landscape system, so water is pulled backward toward the main; a fire hydrant flowed on the street is the classic cause, and the irrigation system briefly draws water backward. Backpressure occurs when the downstream pressure exceeds the supply pressure, which is what a booster pump at the bottom of a hill creates when the landscape sits 60 feet uphill of the meter. Which hazard exists, and how severe the contaminant is, determines the device. A reduced pressure principle assembly protects against both backsiphonage and backpressure and is used for the higher-hazard conditions, which includes any system that injects liquid fertilizer or chemicals into the mainline downstream of the potable point of connection. A double check valve assembly is an acceptable choice for a conventional turf system fed only from the potable main with no chemical injection and no pump. A pressure vacuum breaker protects against backsiphonage only, which is why adding a downstream booster pump to a hillside system already fitted with a PVB creates a real problem: the device does not protect against backpressure. An atmospheric vacuum breaker is the most limited device of all: it cannot be under continuous pressure, and it must have no control valve downstream of it, so a zone with a control valve located downstream is ruled out. An anti-siphon control valve, which combines a valve body with an atmospheric vacuum breaker, must be placed above the highest head it serves by the required clearance, which is why a bed stepping uphill from the manifold is a problem for that arrangement.
Installing and Testing Backflow Assemblies
Installation position is part of what makes these devices work. A pressure vacuum breaker must be installed at least 12 inches above the highest downstream sprinkler head or outlet it protects, because the device relies on atmospheric pressure entering above the water level in the system, and mounting it low defeats that. A reduced pressure principle assembly has a relief port that must be able to discharge freely to atmosphere above grade, which is why an inspector will reject one installed inside a below-grade valve box: if the box floods, the relief port is submerged and the assembly cannot dump contaminated water as designed, and the box itself becomes a cross-connection. California tightened its framework in this area. The State Water Resources Control Board adopted a Cross-Connection Control Policy Handbook, effective July 1, 2024, which sets statewide standards for cross-connection control programs, backflow prevention assembly testing, and the certification of testers and specialists, and public water systems implement it through their own local programs. In practice that means two things for a C-27 contractor. After a backflow assembly is installed on a new commercial landscape, the water purveyor normally requires a field test by a certified backflow tester before the system is placed in service. And assemblies are re-tested periodically, with the interval and the reporting requirements set by the purveyor's program under the state framework, so confirm the interval and the reporting procedure with the purveyor rather than assuming a figure. MWELO reinforces this from the landscape side by requiring backflow prevention devices to protect the water supply from the irrigation system and directing the applicant to the applicable local agency code for additional requirements.
Valves, Controllers, and Electrical Troubleshooting
An irrigation solenoid valve is operated by a low-voltage 24-volt alternating current signal from the controller, which energizes a solenoid that vents a small chamber above the valve diaphragm and lets line pressure open the valve. That mechanism explains the most common valve fault: a valve that keeps running after the controller has shut the station off usually has grit lodged under the diaphragm, holding it off its seat, and the cure is cleaning the diaphragm and seat rather than replacing the controller. Electrical faults are diagnosed by scope. If one station on a twelve-station controller will not energize while every other station runs, the problem is in what serves only that zone: that station's wire and its solenoid, not the transformer or the common wire, which are shared. If a station blows the controller fuse and its solenoid measures about 3 ohms, the solenoid is shorted and must be replaced, because a healthy irrigation solenoid reads considerably higher resistance. Field splices at a remote valve manifold require waterproof splice connectors made up inside a valve box, so the connection stays dry and can be found later. Line voltage is a different trade: a C-27 who needs 120-volt power at a new controller location on the side of a house has a licensed electrical contractor install that branch circuit. Two accessories are worth naming. A quick coupler valve in a large planting bed provides a temporary hose connection for hand watering or washdown. And a master valve paired with a flow sensor on a large commercial landscape means a broken mainline can be detected by unexpected flow and shut down automatically, which is the pair's main benefit; a flow sensor alone on a smart controller is most useful for detecting a broken lateral line or a valve stuck open. Where a valve must be located on a system with no as-built drawing, the appropriate first step is to energize the station and trace the valve wire with a locator.
MWELO: The Water Budget
California's Model Water Efficient Landscape Ordinance is a state regulation at Title 23 of the California Code of Regulations, Division 2, Chapter 2.7, beginning at section 490. It is a model: local agencies adopt it or adopt an ordinance at least as effective, so the local ordinance is what governs on the job, and its section numbers may differ from the state text. The Department of Water Resources amended and reorganized MWELO in a rulemaking effective in early 2025, so confirm section references against the version your jurisdiction has adopted. The ordinance applies to new construction landscape projects with an aggregate landscape area equal to or greater than 500 square feet that require a building or landscape permit, plan check, or design review, and to rehabilitated landscape projects with an aggregate landscape area equal to or greater than 2,500 square feet under the same permitting trigger. A project with an aggregate landscape area of 2,500 square feet or less may comply either with the ordinance's performance requirements or with its prescriptive measures. The performance path is a water budget. The Maximum Applied Water Allowance, or MAWA, is the upper limit of annual applied water for the landscape, and it is calculated from three inputs: the site's reference evapotranspiration, the size of the landscape area, and the evapotranspiration adjustment factor. In the familiar form, MAWA equals ETo times 0.62 times the quantity ETAF times the landscape area plus one times the special landscape area, where 0.62 converts inches of water over an area into gallons and special landscape areas such as recreational turf, edible gardens, and areas irrigated with recycled water are allowed a higher factor. The maximum ETAF is 0.55 for residential regular landscape areas and 0.45 for non-residential regular landscape areas, with special landscape areas allowed up to 1.0. The designer must then show that the Estimated Total Water Use, calculated from the actual plants and irrigation methods proposed, does not exceed the MAWA. A site with a lower local reference evapotranspiration value gets a smaller budget, because ETo is a direct multiplier in the formula. For the purpose of estimating total water use, MWELO assumes an average irrigation efficiency of 0.75 for overhead spray devices and 0.81 for drip devices, which is one reason converting spray to drip helps a project meet its budget.
MWELO: Prescriptive Irrigation Requirements
Beyond the water budget, MWELO imposes hardware and layout requirements that a C-27 contractor installs directly. Automatic irrigation controllers using either evapotranspiration or soil moisture sensor data with non-volatile memory are required for scheduling on all irrigation systems, and sensors that suspend or alter irrigation during unfavorable weather, such as rain, freeze, or wind sensors, are required as appropriate for local conditions. Master shut-off valves are required on all projects with a narrow exception, and a manual shut-off valve is required as close as practical to the point of connection so the system can be isolated for a mainline break or routine repair. Flow sensors are required for all non-residential landscapes and for residential landscapes of 5,000 square feet or larger. Landscape water meters, either a dedicated service meter or a private submeter, are required for non-residential irrigated landscapes of 1,000 square feet up to the threshold at which the Water Code takes over, and for residential irrigated landscapes of 5,000 square feet or greater; their purpose is to measure landscape use separately so it can be compared against the budget. On layout, the ordinance is specific. The system must be designed to prevent runoff, low head drainage, and overspray onto non-targeted areas. Overhead irrigation is not permitted within 24 inches of any non-permeable surface, with limited alternatives such as drip or other low-flow non-spray technology permitted in that setback, and the setback surface may be planted or mulched. Areas less than ten feet wide in any direction must be irrigated with subsurface irrigation or another method producing no runoff or overspray, which is why a three-foot planter strip between a sidewalk and a curb cannot be sprayed. Slopes greater than 25 percent may not be irrigated with a system whose application rate exceeds 0.75 inches per hour unless an alternative design is documented. Sprinkler heads and other emission devices must have matched precipitation rates, check valves or anti-drain valves are required where low point drainage could occur, swing joints or other riser protection are required on risers adjacent to hardscape or in high traffic turf, pressure-regulating devices are required where pressure is outside the emission devices' range, and each valve must irrigate a hydrozone of similar site, slope, sun exposure, soil, and plant water use. Grouping plants onto valves by water need, exposure, and root depth is called hydrozoning, and it is the design practice the whole ordinance is built around.
Last updated: September 2026