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Irrigation System Design

The main objective of an irrigation system is to supply adequate and timely water to plants. This objective should be considered when designing an irrigation system. Two considerations that help achieve this objective are irrigation zones and irrigation system layout. The hydraulic engineering of the irrigation system is secondary to a good system layout. Proper hydraulic engineering is of little value if the system has not been properly zoned and planned.

Irrigation Zones

Irrigation zones should be selected based upon the plant’s irrigation water requirement and sized according to available water supply. Zone irrigation systems should consider the following:

  1. Plants from deep rooted zone & low water use plants separate from high water use. Separate and distinct watering requirements for turf, drought tolerant vs. non drought tolerant landscape plants and herbaceous garden plants (vegetables and flowers).
  2. Exposure to sun and wind.
  3. Different soil types.
  4. Different depths of rooting and thus different depths of wetting for trees (18-36 in.), shrubs (12-24 in.), herbaceous plants and turf (6 -12 in.) (Figure 1).

Figure1: Typical Plant Root Zones

Turf areas are better suited to irrigation by sprinklers, while landscape plants (especially in heavy soils with low water infiltration rates) are better irrigated with low-flow drip or micro-spray irrigation.

Some zones may require daily irrigation; others require weekly irrigation; and still others may reqire only bi-monthly or monthly irrigation. Some controllers do not have the capability of separating irrigation frequency for different zones. The more complicated electronic controllers have these programming capabilities. These controllers are more expensive and require more effort to program and understand than the simpler controllers. In some situations it is less expensive and more practical to have two or more separate, simpler controllers to accommodate the irrigation of different zones.

Irrigation System Layout

Controller (time clock) and valves boxes should be located in accessible locations. Controllers should be in a protected area away from the rain and direct sunlight unless they are in a protective enclosure. Before laying out irrigation lines, sprinklers or emitters a scale drawing should be made showing the entire property with the location of all plants indicated.

Sprinklers should be selected which fit the size of the area being watered. It is usually best to begin in the corners where a sprinkler is required when determining sprinkler location. A head-to-head sprinkler spacing usually applies water quite uniformly. Head-to-head coverage implies that water from one head -reaches the adjacent head. Locate sprinklers based on the mature landscape, considering size and location of any shrubs.

When laying out a drip system, begin by locating the emitters associated with individual plants or plant groupings. Spacing of emitters is based on the ultimate spread of the root system (root zone) and the soil type. Refer to the section on Water Scheduling for details on emitter placement and water application. After determining the number, output and placement of emitters, water supply lines (laterals) may be located, noting the length of tubing and connectors needed to bring the water from the source (hose bib) to the emitters.

Complicated and large irrigation systems (spray or drip), should be designed by professionals who select pipe sizes based upon friction losses and water velocities. Residential drip systems using less than 250 gallons per hour, with laterals less than 200 feet in length, using pressure-compensating emitters, and using a 25 or 30 p.s.i. pressure regulator need no technical design considerations. Residential landscapes with only a few sprinklers on each zone can use 1 or 3/4-inch pipe through the entire system. The wire to control the valves should be sized considering the amperage requirement of the valve and the length of run.

With the help of information supplied by many irrigation supply retailers, drip systems can be sucessfully designed and installed by the home gardener. One of the most important aspects of the design is to determine the system=s capacity. Most home systems use less water than the hose bib is capable of delivering. If, however, the system needs more water than the hose bib can deliver at one time, it can be divided into additional zones as necessary. To determine the hose bib capacity, run the water full force into a measured bucket. If, for example, a three gallon bucket takes 30 seconds to fill, then the capacity is 6 gallons a minute or 360 (6 gal. x 60 min.) gallons per hour. Total the gallonage output of the emitters in the system. You can increase the capacity of the system by reducing emitter sizes and running the system longer. To maintain a safe margin, the system should be designed so that it doesn't use more than 75% of the bib's capacity.

The following are basic components of an irrigation system.
  • Water source (municipal, effluent, well, etc.).
  • Meter (may be required by water supplier).
  • Backflow prevention and vacuum breaker device protect water supply. This is required by law and is also a good safety feature.
  • Valves (manual or automatic) and wire.
  • Pressure regulator (drip irrigation).
  • Filtration system (drip irrigation).
  • Controller and related hardware if using automatic valves. Most automatic valves are electric and require wires connecting the controller and valves.
  • Distribution pipe and pipe fittings.
  • Sprinklers, drip emitters, drip tubing or bubblers.

A landscape should be designed considering the irrigation system, and the irrigation system should be designed considering the landscape plan. Automatic irrigation systems should be zoned considering irrigation type (drip or sprinkler), plant types, exposure (north or south) and soils. The factors that influence plant water use should be considered in determining irrigation zones. An irrigation zone is the area irrigated by opening a single valve. In selecting an irrigation system, a combination of several system types can be used but not in the same zone. Each zone should have only one type of nozzle or emitter. Spray nozzles should not be mixed with impact (rotating) sprinklers, bubblers, spider sprayer or other dissimilar types because of their varying application rates (figure 3). All sprinklers in a zone should have matched application rates. For example, a half circle sprinkler should have half the flow rate as a full circle sprinkler.

Drip emitter in a zone may or may not have matched flow rates. If zones are determined on the basis of rooting depth, such as if all trees are on the same zone, then emitters on that zone should all have the same flow rate or gallonage output. However, if zones are detemined on drought tolerance; and a mixture of plants with varying rooting depths such as trees, shrubs, and ground covers are used, then emitter output can be altered to reflect this variation. For example, trees may be placed on higher gallonage output emitters than shrubs, and shrubs on higher output emitters than ground covers. In this way, although the watering duration may be the same, the depth of wetting will be determined by the gallonage output.

The water requirements and pressure of irrigation zones (spray or drip) should by less than the available water supply and pressure.

1. Water source

See the section on Water Testing.

2. Water Meter

Some water suppliers require a water meter to be installed. Residential water meters are normally 5/8 or 3/4 inch. Flows through residential water meters should be limited to about 15 gpm; otherwise there is excessive pressure loss through the meter and other water fittings and valves. Private and commercial recreation areas with large irrigated areas require larger meters to supply the necessary flow of water without high pressure losses. Where meters are installed on the irrigation system, they can provide a management tool to help determine irrigation scheduling and evaluate watering efficeincy.

3. Backflow Prevention and Vacuum Breaker Devices

Backflow prevention and vacuum breakers are required to protect drinking water supply (figure 4).

Contaminated irrigation water can enter a municipal system if the municipal system loses pressure and there are no backflow prevention devices. The pressure loss across a back-flow prevention device needs to be considered in the design of an irrigation system. A 3/4-inch backflow prevention device may have a 5 to 10 psi pressure drop at 10 gpm.

Many communities require a specific backflow prevention device for a specific location or use. Backflow occurs through either back siphonage (temporary lower pressure occurring upstream from the point of contamination) or back pressure (temporary higher pressure occurring downstream from the point of contamination). Either can result in contaminated water entering a potable water delivery system.

4. Booster Pump or Pressure Regulators

The pressure of the water supply may not be the pressure required by the irrigation system. Municipal water system pressure may be too high for drip and spray irrigation systems. To reduce the system pressure, pressure regulators can be used. Municipal water system pressure can to be too low for an irrigation system using large rotating sprinklers. To increase the pressure, a booster pump can be used.

5. Filtration System

Drip emitters and drip tubing have small orifices which, without filtered water, may clog. Filters are a good precaution, even for high-quality irrigation water, when using drip irrigation.

6. Chemigation Equipment

Chemigation can be a cost-effective way to apply chemicals such as pesticides and fertilizers. Safety and backflow protection devices are essential for chemigation. Most chemical injector systems inject the chemicals at a higher pressure than the irrigation system’s pressure. This could potentially contaminate a pressurized municipal water system. The uniformity of the irrigation application will also be the uniformity of chemical application. High uniformity is a must. Excess deep percolation during an irrigation in which chemigation is being used will result in chemical movement past the root zone and possible ground water contamination.

7. Valves and Wire

Irrigation systems require valves to control the water being applied. Most automatic valves are electric (24 Volt) and require wires connecting the controller and valves. Wires must be sized to account for the length of the wire and the current requirement of the valve. Undersized wires result in a voltage drop and inadequate voltage at the valve.

Most valves will have a "manual bleed valve" or "bleed screw" located on the valve body or on top of the flow control handle. The bleed screw allows for the opening of the valve (turning on the sprinklers) without electrically actuating the valve. This is an aid when troubleshooting a system (figure 5). Some valves have a "flow control" handle. This handle controls the rate of water flow through the valve and is especially useful for drip irrigation systems and low-pressure sprinklers.

All electric valves have a solenoid. The valve solenoid is an electromagnet that, when electrically operated, allows water in the line to push a diaphragm in the valve open which allows water to flow through the system. Some low-pressure drip irrigation systems may require valves that open under very low pressures to operate effectively.

8. Irrigation Controllers

An irrigation controller can control pumps and electric solenoid valves in an irrigation system. Some irrigation controllers can be coupled with electronic weather stations or soil moisture sensors, such as tensiometers, to assist in efficient irrigation scheduling.

9. Distribution Pipe and Pipe Fitting

Distribution piping includes the mainline pipes and the lateral pipes downstream of the valves. In sprinkler irrigation systems polyvinyl chlorine (PVC) pipe is usually used. This pipe is usually white or blue and is semi-rigid. Irrigation tubing is used for drip irrigation. This pipe is made of polyethylene (PE) and is black and flexible. Irrigation tubing (PE) is constructed to resist breakdown by sunlight and can be laid on top of the ground.

10. Sprinklers, Drip Emitters, Drip Tubing or Bubblers

Sprinklers, drip emitters, drip tubing and bubblers are used to apply the water to the soil. Water application devices should be selected to apply the water as uniformly as possible to the area being irrigated.

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