Watering & irrigation system design

30 March 2022

The creation of an efficient, state-of-the-art watering or irrigation system has to be designed accordingly. The most technologically advanced equipment and smart watering management devices cannot overcome poor system design, which can affect efficiency, and waste water, energy, and money. Good design proceeds through three steps, preliminary, final and executive design, progressively expanding on the details of the project, in order to encompass the most complete description of the work that needs to be carried out.

Here we will describe the main phases of the design process, from field surveys through to layout planning on paper and the positioning of the various elements needed to implement the system.

1. Inspection and planimetric and altimetric surveying

The goal of this phase is to obtain a scale plan of the irrigation area via a topographic survey. A topographic survey of areas of considerable size or complexity should be performed by a specialized topographer, who has the equipment and skills necessary to make a scale plan of the area with altimetric indications, which are fundamental for the management of inclined areas or areas at different elevations. However, in the case of surfaces of small dimensions or simple geometries, without any unevenness, surveying can also be done using simple measuring tape. 

During this phase, all the dimensions of the areas that need to be irrigated must be precisely measured. Any mistakes made in this phase will go on to affect all the other phases in turn. So precision and extra care are crucial here. 

In addition to the edges of green areas, the plan must also include buildings, trees, shrubs and all other objects in the area. The plan should also indicate the position of any water supply connections, places where storage tanks could be located, and any other sources of water apart from the mains supply.

2. Water data surveying

In this phase, the source or sources of water supply must be defined, such as taps for the connection of the irrigation distribution system.

The main sources of supply may be:

a) Inspection chambers receiving water from the mains supply

In the case of a connection to an existing pipe (a), the diameter of the pipe must be measured.

The pressure must be measured with the tap completely closed (zero flow), in order to determine the maximum static pressure available in the network.

The flow rate must then be measured together with the pressure (with a pressure gauge tee, gate valve and flow meter, or bucket and stopwatch), in order to know the different flow rates at pressures of 2.0, 2.5 and 3.0 bar. A fourth measurement should be made of the maximum zero pressure flow rate (valve fully opened).

b) Storage tank and pump pressurization

In the case of supply from an existing or planned accumulation tank, and pressurization by means of a pump, we should distinguish between two different cases.

– If there is an existing pump, the technical characteristics shown in the pump’s instruction manual should be checked to make sure the flow rate corresponds to the design pressure needed to operate the irrigation system. 

– If the pump has not been bought yet, the most suitable flow rate can be chosen and then a pump found to match it.

c) Well water and pump pressurization

This case is almost completely analogous to case b) above. However, the pump withdrawal depth must be measured (to add the difference in height to the pump head), and a dedicated water filtration system must be installed to prevent clogging of the system and irrigation heads.

d) Gravity system, with an accumulation tank at a certain height above the ground

This type of system accumulates water in containers placed at a certain height, similarly to a water tower, in order to maintain the pressure of the supply.

Water pressure increases proportionally from one height to another lower height as a function of the difference in height and in potential energy. In practical terms, 1 bar of pressure can be achieved with approximately 10 metres difference in height.

For sprinkler systems, a design pressure in the order of 3.0 – 3.5 bar is generally needed, which would correspond to a difference in height of 30 – 35 m. This makes the gravity system virtually unusable in the majority of residential applications.

3. Division of the plan into uniform macro-areas

The plan should be divided into uniform macro-areas, according to light exposure, and type of greenery to be irrigated (e.g. lawn, shrub, flower bed).

4. Positioning of all the elements on the plan

In this phase, sprinklers, driplines and micro-irrigation devices should be positioned on the plan according to the characteristics of each macro-area

The devices (e.g. sprinklers) must be connected with piping according to their consumption, in relation to the maximum design flow rate (determined in point 2 above). The positioning of solenoid valves, control units and sensors should also be marked.

All the sprinklers must be positioned at a distance from each other equal to or less than that of their range. For example, if you want to irrigate a rectangular area measuring 9 x 4.5 m, you could use 6 static sprinklers with a 15 ft range head (equal to a range of 4.5 m at 2 bar), so that the jet of each sprinkler wets the base of the neighbouring sprinkler.

5. Assigning dispensers (e.g. sprinklers, driplines) to sectors

In this phase, depending on the design flow rate, an adequate number of sprinklers should be assigned to each homogeneous macro-area and connected by a line to represent the sector’s piping.

Each device (sprinkler or dripline) needs a certain pressure to work, which in turn corresponds to a delivery flow rate. The sum of all flow rates delivered by devices connected to the same line must never exceed the design flow rate (as measured in the field).

Furthermore, devices with different dispensing characteristics should not be connected on the same line. In particular, the following should be separated:

a) static sprinklers,

b) static sprinklers with multi-jet heads,

c) dynamic sprinklers,

d) driplines

Once the devices have been added to the piping, the positioning of solenoid valves, control units and sensors should be marked.

6. Sizing of pipes

In this phase, the diameter of sector pipes (those connecting the sprinklers to the solenoid valve) should be selected according to the measured hydraulic characteristics (system capacity) and the distance the pipes cover. 

The internal speed of water inside the sector piping should never exceed an average speed of 2.0 m/s, in order to mitigate water hammer and pressure drop effects.

The internal speed of water in the main pipe (the one that connects solenoid valves to water supply points) should not exceed an average speed of 1.5 m/s.

The diameter of the pipes should, therefore, be adjusted to respect these speed limits, and also to guarantee a pressure drop between the first and last dispenser of less than 20% of the operating pressure (e.g. for an operating pressure of 2.0 bar, the maximum pressure difference should be 0.4 bar).

Solenoid valves and any electric cables must be chosen to meet design needs, according to manufacturers’ data.

7. Filter system

In this phase, depending on the characteristics of the water resource (and supply method), the type of filter, or filters, should be chosen to minimize the risk of clogging the system’s water dispensers.

8. Calculation of the materials needed for final realization of the system

In this phase, the quantity of materials needed to complete the system must be estimated, along with all the fittings and elements necessary for the realization of the irrigation system as per design.