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Irrigation - A Basic Guide

The purpose of this outline is to systematically plan and subsequently install a fully automatic irrigation system. It will also touch on the various factors that affect an irrigation layout and how to use the system efficiently.

Getting Started

SCHEMATIC LAYOUT: The first task is to draw a diagrammatic representation of the site noting all structures, paths, driveways, significant landscape features including trees, hedges, garden beds and grassed areas. This should be scaled to a usable size generally either 1:100 or 1:200.
Using a graph paper depicting a grid in either of these 2 scales is a big advantage. It is important to draw in the features as accurately as possible using scale rules, set squares and compasses.
A useful base to work from is a site or drainage plan of the house and block which is readily available from the department of Planning and Land Management. These plans will accurately show any changes of angles and the exact size and position of the house on the block.

Choosing The Right Sprinkler

Once you have your detailed plan it's time to select which type of sprinkler best suits the dimensions of your turfed areas. The following table outlines the performance details of the most commonly used sprinklers. More detailed data is available from the manufacturers' catalogues.

 

SPRINKLER

RADIUS

LIT/MIN

TORO 570 – 12/p>

 

3.8

8.67

TORO 570 – 15

4.8

15.05

HUNTER PS 12A

4.4

11.6

HUNTER PS 15A

5

15.5

TORO 300-02

6.5

11.1

TORO 300-03

8.6

20.9

HUNTER PGJ #3

9.3

11

TORO MINI 8 #2

9.3

6

HUNTER PGP #7

11.1

10.8

TORO S 800 #3

11.8

15

*Figures based on 360dg performance data

This information will help you to decide which sprinkler is the most suitable for your area. The following example shows how to select the required head.

sprink

In this example and using information from the table above the most suitable sprinkler is the Toro 570- 12 as the dimensions of the area to be watered are multiples of 3.8m. (it is a critical fact that sprinklers should not be spaced farther apart than their published radius of throw) otherwise known as "head to head" coverage. Once you have selected the types of sprinklers and plotted them on your plan ensuring head to head coverage, it is time to "zone "or "station" the system.

Zoning

Zoning is best described as the number of sprinkler heads you are able to run together based on the available water supply. To arrive at the answer as to how many heads you can operate at once, there are a number of steps required. Bucket test: Go to a water outlet or tap closest to either the water meter (if this is your chosen point of connection) or to the area being irrigated.

Bucket/Flow Test

Follow these steps:

  1. Remove any tap hardware from the tap and tum the tap on fully.
  2. Place a bucket or container of a known volume (normally 10 liters) under the tap.
  3. Using a watch or stopwatch, time how long it takes to fill the bucket to the point of overflow.
  4. Calculate your available flow in the following way: Divide 60 (seconds in a minute) by the time it took to fill the bucket (let's say 15 seconds) then multiply that figure by 10 (the number of liters in the bucket) or 60. 15 = 4 x 10 = 40 liters/minute.

Due to pressure losses in moving water through pipes and changes in elevation, it is prudent to calculate available flow at 80% of the bucket test results. In this case 40 liters/minute x 80% = 32 liters/minute. This is now the volume of water you have available to zone your system. Using the previous diagram (Fig 2) in conjunction with the performance data (Fig 1), it is possible to calculate the total volume required to run this area. It must be noted that the performance data is based on the demand of a 360dg sprinkler. Some types of sprinklers including the 570-12 are what we call "match precipitated". This simply means you are able to run sprinklers of various arcs on the same zone or station and that each head delivers a specific proportion of the volume of a 360dg sprinkler. For example if a 360dg sprinkler delivers 10 liters/minute, a 180dg head will deliver half this amount without affecting the distance it will throw.

This is not the case with all sprinklers, especially the single stream, oscillating gear drive models such as the Toro Mini-8 and Super 800; and the Hunter PGJ and PGP. With these types you have to change nozzle sizes so that a sprinkler with a 90dg arc has a nozzle putting out a quarter of the volume of a 360dg sprinkler. The other alternative is to zone all similar arcs on the same station and through adjusting run times on the controller thereby achieve matched precipitation.

Getting back to our example using figs 1 &2, we have 6 sprinklers with 180dg arcs and 4 sprinklers with 90dg arcs. From the table we can see that the Toro 570-12 requires 8.67 liters/minute as a 360dg sprinkler. The l 80dg sprinkler uses only half this amount or 4.33 liters/minute. The 90dg sprinkler uses half again or 2.16 liters/minute. The total demand therefore is 4.33 x 6 and 2 .16 x 4 = 34.62 liters/minute.

We have previously established that our available water supply is 32 liters/minute (80% of actual flow). With this in mind, it is probably safe to say that we could run the entire area on one zone, as the margin over 32 liters/minute is quite small. If however it was substantially more it would be necessary to create 2 zones. This procedure needs to be repeated for all areas to be irrigated.

Piping

Once you have established the number of zones required it is time to determine how to deliver water to each and the layout of the pipe work. There are basically 2 preferred options.

MANIFOLD SYSTEM: This involves grouping the valves that control water flow to each zone in a compact manifold and radiating distribution lines to the various areas of the garden. In most cases you would have a manifold in both front and rear gardens linked by the main or supply line. The principal advantage of this system is that it is easy to find & maintain the valves should the need arise.

BLOCK SYSTEM: This system relies on a single main line with valve take off points at various locations throughout the garden. Usually the take-off points are adjacent to the areas being irrigated. The main advantage is a saving in pipe, as there are no multiple runs as with a manifold system.

Using Fig 2, here is a basic pipe layout that hydraulically distributes water evenly throughout the zone.

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The preferred piping system is uPVC. It is easy to use and cost effective; another is Medium Density Polyethylene pipe, which requires screwed compression fittings. Low Density Polyethylene pipe is not sanctioned for use below ground and is subject to compaction or squashing by tree roots.

Solenoid Valves

 

In the case of automatic systems there needs to be a way of remotely opening and closing the station supply valves. Solenoid valves are electrically actuated normally closed hydraulic valves. Low voltage wires are connected in series from valves to controller. At a pre-determined time, power is supplied to a specific valve opening it. At the end of the programmed watering schedule the power is switched off and the

Controllers

There are a multitude of controllers on the market offering much the same in the way of programs, start times etc. With water restrictions now a fact of life it is important to consider controllers that provide for odds and evens programs, not just every second day as months varying in duration put that option out of sync. For the residential market the following controllers comply - Toro; Hunter and Rainbird. There are undoubtedly others but these 3 offer a good choice. Rain sensors are compatible with most controllers and Irritrol's new RS 1000 is radio remote- doing away with the need to hard wire between sensor and controller.

Regulations Governing Irrigation

 

Irrigation is classified as a plumbing activity and as such is governed by local and national codes of practice. It is a mandatory requirement that every irrigation system connected to the potable water supply is fitted with the appropriate Backflow Prevention Device.

In the case of residential dwellings or class 1 & 2 buildings a non-testable dual check valve is to be fitted at the irrigation take off point. If fertilizer is to be directly injected into the system the backflow device has to upgrade to a Reduced Pressure Zone Device (RPZD). This assembly must be able to freely vent to atmosphere and have a minimum ground clearance of 300mm. The minimum depth that pipes are to be laid at is 225mm and the minimum pressure rating of pipe upstream of the solenoid valves is PN12.

Only a licensed plumber is entitled to make the connection to a water service and install the backflow prevention device