An irrigation zone is a specific section of a landscape watering system controlled by a single valve, allowing it to operate independently. Determining the correct number of zones is the foundational step in designing an efficient system that delivers precise hydration while avoiding waste. The optimal number of zones is dictated by engineering limitations and horticultural requirements unique to your property. Understanding these constraints ensures every plant receives appropriate hydration without compromising system performance.
Determining System Capacity Based on Water Supply
The primary factor limiting the size of any single irrigation zone is the available water supply, specifically its pressure (pounds per square inch, or PSI) and flow rate (gallons per minute, or GPM). Every home has a finite amount of water that can be delivered at one time. The true limiting factor is the usable GPM, which is the dynamic flow rate available when the system is operating at its required pressure.
To establish maximum system capacity, you must determine the actual flow rate your main service line can sustain while maintaining the minimum operating pressure for the sprinklers. This usable GPM figure dictates the total water volume available for any single zone. If the combined flow requirement of all sprinkler heads or emitters in a proposed area exceeds this usable GPM, the area must be split into two or more separate zones.
These zones are scheduled to run sequentially, ensuring the system never attempts to draw more water than the supply can handle. Exceeding your usable GPM causes a significant drop in pressure, resulting in poor performance, uneven coverage, and sprinkler heads that cannot project water across their intended radius. This establishes the maximum size and minimum number of zones required before plant needs are considered.
Grouping Plants by Hydrozone Requirements
Once engineering limits are established, the number of zones is further defined by horticultural needs, a practice known as hydrozoning. Hydrozoning involves grouping plants with similar water requirements into the same irrigation zone to ensure they receive the correct amount of moisture. This prevents the problem of overwatering drought-tolerant species or underwatering high-demand plants.
A landscape is divided into high, moderate, and low water-use areas, with each category requiring its own dedicated zone. For example, a manicured turfgrass lawn (high water-use) must be placed on a separate zone from an established shrub bed or native plant garden (moderate or low water-use). Mixing these distinct needs in a single zone makes efficient watering impossible.
If a zone contains both a thirsty lawn and water-wise shrubs, running the system long enough for the lawn will drown the shrubs, promoting disease and root rot. Conversely, running the system for the shrubs will leave the lawn stressed and brown. Creating separate hydrozones allows you to program the controller to deliver the deeply penetrating, less frequent cycles needed by perennial beds, while providing the shorter, more frequent cycles required by turf. This approach often reduces landscape water use by 20 to 50 percent.
Separating Zones Based on Emission Device Type
The type of watering device used is another constraint that requires separate zoning, primarily due to differences in precipitation rate (PR). PR is the speed at which water is applied, measured in inches per hour. For example, spray heads covering small areas quickly may have a PR of 1.5 to 2.0 inches per hour, while rotary nozzles covering larger areas slowly often have a PR of 0.4 to 0.7 inches per hour.
Mixing a high-PR spray head with a low-PR rotor on the same zone leads to drastic unevenness in coverage. The area watered by the spray head will receive water much faster than the area watered by the rotor. Drip irrigation systems, which deliver water at a near-zero PR directly to the root zone, require a pressure regulator and filter, making them incompatible with any overhead sprinkler zone. Therefore, all fixed spray heads, rotary nozzles, and drip emitters must be placed on separate, dedicated zones.
This separation is necessary to achieve the matched precipitation rate required for uniform water distribution within a zone. If precipitation rates are not matched, some areas will be saturated while others remain dry. This forces the user to run the system longer to compensate for dry spots, which wastes water and damages saturated areas.
Accounting for Terrain and Microclimates
The final layer of complexity in determining the number of zones involves the site’s physical characteristics, including terrain and microclimates. Significant slopes, for instance, cannot be watered effectively on the same zone as flat ground. Water applied to a steep incline is subject to runoff before it can fully infiltrate the soil.
To address this, sloped areas require a dedicated zone that utilizes the cycle-and-soak method. This method runs the system for a short period, pauses for absorption, and then repeats the cycle multiple times. This shorter, repeated watering schedule is incompatible with the single, continuous run time used for flat areas, necessitating separate zoning for any substantial slope.
Microclimates, which are localized environmental variations, also influence zoning decisions. Areas in full, all-day sun, such as a south-facing exposure, experience much higher evaporation and water use than areas in heavy shade. These distinct sun exposures should be placed on separate zones to allow for different run times. Furthermore, significantly different soil types, such as fast-draining sandy soil and slow-draining clay soil, may require dedicated zones to accommodate different infiltration rates.