Irrigation 101

If the precipitation rate varies significantly over the area being irrigated, uniformity is poor; a precipitation rate that is nearly equal throughout the area provides good uniformity. For good uniformity, you need to match the flow of the sprinkler to the area it is watering. This is known as "matched precipitation."

When you use spray sprinklers, matched precipitation is automatic. The nozzles, which are fixed arcs, use an amount of water proportional to the area they cover. For example, if a full-circle (360 degrees) sprinkler throws 15 feet and uses 3.7 gpm, a half-circle sprinkler (180 degrees) still throwing 15 feet would be expected to use 1.85 gpm, and a quarter-circle 0.93 gpm. All the precipitation rates would be theoretically equal at 1.58 inches per hour.

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Rotary sprinklers are different. Nozzles usually are interchangeable, and you can choose a nozzle from those available from the manufacturer. All the sprinklers could have the same nozzle regardless of the area covered. For example, a sprinkler with a #5 nozzle throws 35 feet using 2.4 gpm. There are five sprinklers, all with the #5 nozzle. Two of the sprinklers are full-circle (360 degrees), one is a quarter-circle (90 degree) sprinkler and two are half-circle (180 degrees) sprinklers. The precipitation rate is different for each area-the amount of water discharged stays the same for each sprinkler even though the areas covered by each vary. The quarter-circle sprinkler applies four times the amount of water as the full-circle sprinkler.

To uniformly irrigate rotary sprinklers, you need to use different nozzles or use separate zones for arced sprinklers. For example, a rotary sprinkler's available nozzles might be 1.5, 3.0 and 6.0 gpm. You would use the 1.5 gpm nozzle for the quarter circles, 3.0 gpm for the half circles and 6.0 for the full circles to get matched precipitation.

Water in zones

Divide all irrigation systems into zones, because the amount of water you need to cover an entire area at one time is probably not available, especially at a residence. Zoning makes the system more economical-piping is smaller and installation is easier. The zone size is strictly dependent on the amount of safe flow available at the water source. The more water available, the larger the zones can be.

In establishing zones, consider factors other than just the available flow. You should zone similar sprinklers together so that precipitation rates are equal. Zone sunny and shaded areas separately to address the varying water requirements of plant material. Zone turf and landscaped areas separately. They need different watering schedules because of their varying root-zone depths and water requirements. Different soil types also require different zoning, due to their water infiltration rates.

Zones are turned on and off using low-voltage electric valves. The required size of a zone valve is based on the amount of water flowing through it and its friction loss. Many times, the electric valve is smaller than the piping on which it is installed. You can find charts for valve sizing in manufacturers' catalogs.

Electric zone valves, other than inexpensive residential valves, will have flow control. This feature enables you to use a handle to adjust the flow of water and, as a result, the pressure.

You can buy valves with a pressure-regulating feature. Sprinklers rarely deliver the exact pressure that their design calls for, and many sites have pressure that is too high. A pressure-regulator option will lower the pressure immediately after the valve to a set value. However, a valve cannot produce more pressure than it receives.

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