Sprinklers, Drip, Precipitation Rate, and Scheduling
This chapter covers what happens after the water leaves the valve: head selection and spacing, the precipitation rate calculation that connects hardware to a schedule, uniformity and auditing, and how a run time is actually derived rather than guessed. It is the second of three irrigation chapters. Every formula here is worked with numbers, because CSLB states that some examination questions require mathematical computation and a calculator is provided.
Heads, Nozzles, and Matched Precipitation
A gear-driven rotor throws a single rotating stream over a large radius, thirty feet or more, and is the usual choice for large turf areas. A fixed spray head throws a fan pattern over a much shorter radius at a much higher application rate. That difference in application rate is why the two must never share a valve: a remodel that connects several spray heads and two rotors to the same control valve will overwater one group while the other stays dry, because a single run time cannot suit two devices that apply water at different rates. The same principle applies within a single device family. A zone containing quarter-circle, half-circle, and full-circle spray heads on the same lateral waters evenly only if the nozzle flow scales with the arc, so that a quarter-circle nozzle delivers roughly one quarter the flow of a full-circle nozzle. That is what a matched precipitation rate nozzle set does: apply water at the same depth per hour regardless of arc. Rotary nozzles, which replace conventional spray nozzles with a set of rotating streams, apply water far more slowly than the sprays they replace, so substituting them on a sloped turf area requires longer run times, not the same run times. Spacing follows the throw. Head-to-head coverage means each head is spaced so that its throw reaches the base of the adjacent head, so spray heads with a 15-foot radius are set about 15 feet apart. On a site exposed to steady afternoon wind, the designer reduces spacing below the normal head-to-head distance, because wind shortens the effective throw and creates dry gaps at the design spacing.
Precipitation Rate and Run Time, Worked
Precipitation rate is the depth of water a zone applies per hour, and it is the bridge between hardware and scheduling. The formula is PR equals 96.3 times GPM divided by area in square feet, where 96.3 is the constant that converts gallons per minute over square feet into inches per hour. Work it twice. A zone with four heads at 3 GPM each has a total flow of 12 GPM over 400 square feet, so PR equals 96.3 times 12 divided by 400, which is 1,155.6 divided by 400, or about 2.9 inches per hour. A zone with eight heads at 2.5 GPM each is 20 GPM over 1,200 square feet, so PR equals 96.3 times 20 divided by 1,200, which is 1,926 divided by 1,200, or about 1.6 inches per hour. Once you have the precipitation rate, a run time is arithmetic rather than habit: run time in hours equals the depth you want to apply divided by the precipitation rate. A zone applying 1.6 inches per hour that must deliver 0.4 inches runs 0.4 divided by 1.6, which is 0.25 hours, or 15 minutes. Flow converts for scheduling and budgeting the same way: 12 GPM times 60 minutes is 720 gallons per hour. Two habits follow from these numbers. Cycles are commonly scheduled for the hours before sunrise, because wind and evaporation losses are lowest then and the foliage has time to dry. And a single program watering every zone every day for ten minutes is the schedule to argue against, because every hydrozone then receives the same depth regardless of its plants, its soil, or its exposure.
Soil Intake, Cycle and Soak, and Reference ET
A schedule that ignores the soil produces runoff no matter how well the hardware performs. Every soil has an infiltration rate, the depth of water per hour it can absorb, and a compacted silty clay on a graded pad has the lowest infiltration rate and therefore the highest runoff risk of the common landscape soils. When the precipitation rate of a zone exceeds the infiltration rate of its soil, water begins to run off partway through the cycle. That is exactly what has happened when water starts running off a clay slope about eight minutes into a 20-minute rotor cycle, and the correct schedule change is cycle-and-soak: run three cycles of about seven minutes with a soak period between them, so the same total depth is delivered but each application has time to move into the soil. A soil report giving an infiltration rate of 0.15 inches per hour is therefore governing the application rate the irrigation system may use without runoff, which may drive the choice of rotary nozzles or subsurface drip rather than conventional sprays. The demand side of the schedule comes from evapotranspiration. A California schedule is normally built around a reference evapotranspiration value from a CIMIS weather station, and that value describes the water use of a standard reference crop under the local weather conditions, which is then adjusted by a plant factor and by irrigation efficiency for the actual landscape. A weather-based or ET-based smart controller applies exactly that logic automatically, adjusting run times using local weather data instead of a fixed calendar.
Distribution Uniformity and Auditing
Distribution uniformity measures how evenly water is applied across an area, and it is the number that decides how much water a landscape actually needs. The logic is not obvious until you see it: irrigation is scheduled to satisfy the driest part of the zone, because that is what the owner sees. If the driest cups in a catch-can audit collect about half as much water as the average cup, uniformity is poor, and the dry spots will drive the run time up until they are green, which means everything else is receiving roughly twice what it needs. Improving uniformity is therefore the most direct water conservation measure available on an existing system. The audit itself places catch devices in a grid across the zone, runs the zone for a measured time, and compares the volume collected. Common causes of poor uniformity are mechanical and easily found: mismatched nozzle brands and worn nozzles on the same turf zone produce uneven precipitation directly; heads out of adjustment throw water across concrete instead of onto turf, which is corrected first by setting the arc and radius so the spray stops at the turf edge; heads tilted or sunk below grade lose their pattern. Two hardware details prevent chronic waste on sloped zones. Check valves or anti-drain valves installed in the low heads stop the zone from draining out through the two lowest heads every time it shuts off, which is what causes a wet spot at the bottom of a slope. And pressure regulation at every head stops misting. An irrigation plan calling for both is addressing water waste from low-head drainage and from misting, not appearance.
Drip, Subsurface, and Special Situations
A drip emitter rated at 1 GPH delivers approximately one gallon per hour, which is a very different unit from the gallons per minute a sprinkler uses and a common source of scheduling errors. Emitter spacing follows soil texture, because water moves through soil by gravity and by capillary action in different proportions depending on the pore structure: in sand, water moves down much more than out, so emitters are spaced closer together to produce a continuous wetted zone; in clay, water spreads laterally, so emitters can be spaced farther apart. Inline drip laterals need routine flushing at their ends, typically at the start of each season, because fines accumulate at the far end where velocity is lowest. Subsurface drip is especially valuable on slopes and in narrow strips prone to runoff, because there is no surface application to run off and no overspray, and MWELO effectively requires it or an equivalent method in narrow areas. A subsurface zone drawing soil into the tubing each time the valve closes is missing an air and vacuum relief valve at the zone high point, which admits air as the line drains instead of pulling a vacuum that sucks soil through the emitters. Several plant-specific situations round this out. A mature oak in the middle of a new lawn should be placed on its own valve so it can receive deep, infrequent water rather than the frequent shallow schedule turf wants. A bubbler specified for a newly planted 24-inch box tree only works if there is a watering basin or root ball area that can hold the flow, because a bubbler delivers water far faster than soil absorbs it. Retrofitting an older system from spray heads to inline drip in the shrub beds almost always requires adding a filter and a pressure regulator at the valve. And a newly seeded lawn on the same controller as an established shrub bed needs brief cycles several times a day during establishment to keep the seedbed damp, on its own program.
Last updated: September 2026