Irrigation Theory

• The Nature of Water
• Irrigation Design Rules of Thumb

Irrigation design comes down to hydraulics – in this case, the study of water as it moves through pipes to feed a system of distribution to plants.

The system of hydraulics involves having:

• A water source that provides appropriate flow and pressure for your distribution needs.
• A mainline piped distribution system connected to individual valves.
• Pipes sized so the water flow velocity is at acceptable limits to reduce wear and lengthen service life.
• Appropriate sprinkler head or drip emitter selection to provide an even distribution of water and in an amount that allows you to apply enough water for your time limitations for watering.

The Nature of Water

Water is actually quite heavy. One cubic foot of water weighs 62.4 lbs. One cubic meter weighs about one ton (1016 kg).

In order to move water through pipes, you need to have pressure behind your source to force the water forward. In a city, this pressure is supplied by the city's water source, such as a tank. The source is generally in an area that is much higher in elevation in relation to the rest of the city, such as a hill or an elevated tank. You may need to augment the water pressure for your project, or supply your own pressure, with a booster pump.

As you move water through pipes to get to your sprinkler heads, friction causes the water to lose pressure. Friction loss is the loss generated through the length of pipe from the valve to the farthest head, per the inside diameters of the pipe class chosen, the velocity specified, and the results of the Hazen-Williams formula.

Pressure loss also occurs every time the water moves through a valve – including a control valve, water meter, or backflow device.

You start with a certain pressure at your water source, and you lose pressure along the way as the water travels through pipes and valves, until it finally reaches last sprinkler head controlled by a valve. Whatever this final pressure amount is, it must at least be the amount of pressure that you, the designer, have determined is necessary to make this sprinkler type operate correctly. This pressure is known as the "design pressure" for that sprinkler head type.

Pressure is one thing; flow is another. Flow is simply the amount of water traveling through the pipe. Therefore, if any one sprinkler head needs a certain amount of flow to operate correctly, you not only need to supply that amount of flow to that head, but also add to that the flow for all other heads attached to the control valve that will operate at one time.

You essentially need to ask yourself, “Which valve in my irrigation system design will require the most flow, and which valve will require the most pressure?” These amounts are known as the "critical design flow" and "critical design pressure," respectively. These two numbers will determine the necessary flow required from your source.

Thus, irrigation is all about water in motion: moving water with the correct flow and pressure that are necessary to allow your sprinkler heads to distribute water over a planted area correctly and uniformly.

Irrigation Design Rules of Thumb

The Irrigation Design Rules of Thumb, referenced throughout this documentation section, are intended for your understanding of the general constraints of an irrigation system . This level of understanding will help you make intelligent early decisions on the proper limits of a system. The Rules of Thumb are only intended for early evaluation of your system requirements. As you develop your design, Irrigation F/X will deal precisely with all of the data aspects of the system within the software.

The Irrigation Design Rules of Thumb are available in Imperial and Metric units, and are downloadable as PDF files from the following links: