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77 Principle 6 Irrigation Planning Graham Hunter, Molly Waters, and Heidi A. Kratsch Irrigation Systems Irrigation of a water-efficient landscape starts with designing a system that will most effectively deliver water to plants with the least amount of waste. The following are some key concepts to keep in mind during the design process: • The design of the landscape, including types and locations of plants, should drive decisions about the style and layout of the irrigation system. • The irrigation system should be designed to incorporate hydrozones. A hydrozone is a distinct area of the landscape defined by the water, drainage , and exposure needs of the plants within. Landscape plants should be grouped (hydrozoned) according to similar needs. Each hydrozone should be irrigated using a separate valve wired either to separate automatic controllers, or to a single multistation controller. • Parts of the system may be temporary, constructed only for the purpose of establishing very low-water-use plants during their first season or two. • The system should be timed to irrigate during periods of low evaporation. The best time to irrigate is early in the day. Two types of landscape irrigation systems exist, and they are uniquely designed to meet the special water needs of different plant types (Table 6-1). Low-volume (drip or microsprinkler) irrigation systems are designed to work at low output pressures, 10 to 30 pounds per square inch (psi) and deliver water directly to the root zones of drought-adapted plants, either individually or in 78 Water-Efficient Landscaping in the Intermountain West groups. Automatic sprinkler systems work at higher pressures (above 40 psi) and volumes, and are effective for irrigation of lawns, some ground covers, and annual flower beds (high-water-use plants). A typical water-efficient landscape may include a combination of sprinkler and low-volume irrigation systems, and each system needs to be zoned separately. Table 6-1. GALLONS OF WATER NEEDED TO WET THE ROOT ZONE* Plant water use Plant canopy diameter (feet) 1 2 3 4 5 6 8 10 12 14 Tree medium low 21 15 37 30 58 46 83 66 113 90 Shrub medium low 2.5 2 5 4 9 7 14 12 21 15 31 25 Perennial medium low 1 0.5 3 2 7 3.5 * Water requirements were estimated by multiplying the average crown area of a single plant, or the area of a homogeneous planting, by the average weekly reference evapotranspiration (ETo) for north-central Utah during the growing season, and by a plant factor. The plant factor is a dimensionless number expressed as a fraction of ETo and derived based on studies (below) showing that many conventional landscape species maintain their health and appearance when irrigated within a range of 20% to 80% of ETo. For hotter, drier sites in the southern part of the region, multiply these numbers by a factor of 1.4. Very low-water-use plants will need very little to no supplemental irrigation once established. Pittenger, D. R., D. A. Shaw, D.R. Hodel, and D. B. Holt. 2001. Responses of landscape groundcovers to minimum irrigation. J. Environ. Hort. 19(2):78–84. Shaw, D. A. and D. R. Pittenger. 2004. Performance of landscape ornamentals given irrigation treatments based on reference evapotranspiration. Acta Hort. 664:607–613. Staats, D. and J. E. Klett. 1995. Water conservation potential and quality of non-turf groundcovers versus Kentucky bluegrass under increasing levels of drought stress. J. Environ. Hort. 13:181–185. Most irrigation systems, whether sprinkler or low-volume, share the same basic components that make up the “backbone” of the system, including underground water supply pipes, valves, and controllers. The main differences between conventional sprinkler systems and low-volume systems are the type of emitter used and the pressures (and therefore the volume) of the water flowing through the system. The means of operation of the two systems is essentially the same, and conventional sprinkler systems can be retrofitted to be low-volume, with A hydrozone is a distinct area of the landscape defined by the water, drainage, and exposure needs of the plants within. Irrigation Planning   79 some minor adjustments to regulate the pressure and prevent clogging of the system. Low-volume systems are no more complicated than conventional sprinkler systems, and have the potential to save water by reducing evaporation and providing water directly to plant roots. Basic Irrigation System Components • Control valves – mechanical devices that either allow or prevent the flow of water to each zone...

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