While making a DIY irrigation system for your garden does not require any special knowledge, a basic understanding of hydrodynamics – fluids in motion – will be useful.
Of the properties of water in hydrodynamics, two are essential to irrigation. Consider a garden hose with an adjustable nozzle. With the nozzle open slightly, a small amount of water will shoot out at high pressure in a jet. Open the nozzle fully, and the water escapes faster, but at a lower pressure. This shows the relationship between flow and pressure, and how both come in handy when watering your garden.
By measuring the velocity of water over a known area, one can calculate flow. While there are a number of instruments used to determine the volumetric flow rate of water, for home use, a vessel of known volume (eg. a 10L bucket) and a stop watch will suffice. Divide the volume by time to get a reckoning of flow rate in liters per hour.
Generally speaking, irrigation systems use pressurised water. Water from a closed system such as a water main will be pressurised, either by gravity or a pumping station. Water pressure is measured in bar, kPa or psi relative to air pressure. To design a large scale irrigation system correctly, one must correctly measure water pressure using a pressure gauge.
For home irrigation, many pressure-regulated fittings are available and certain fittings are effective at low pressures. For example, we supply many fittings that are pressure-compensated – these will have a certain flow rate regardless of pressure. This feature is most useful in drippers and driplines – for example, a dripline may have a pressure-compensated flow of 4 Lph. This means each hole in the dripline will have 4 litres of flow each hour regardless of pressure – provided of course there is sufficient flow to fill the entire dripline. Adjustable drippers can be used to great effect in small irrigation systems for pot plants and hanging plants. See our How-to on Driplines or Micro-irrigation for more info.
Non pressure-compensated fittings include most sprinklers. When you turn on a sprinkler, it introduces a small aperture into your water system, from which a relatively low flow of water emerges at high pressure. This water will be directed by a nozzle or otherwise dispersed by some mechanism. Each sprinkler will have a certain flow rate at a given pressure. When you introduce a second sprinkler to a water line, this is the equivalent of opening the hose nozzle at the beginning of the article – flow increases at the cost of pressure. If you already had your tap fully open and your first sprinkler already at at max flow, now you have two sprinklers at lower pressure, hence less range. We supply sprinklers designed for a variety of flow rates and pressures so you can ensure proper coverage in your garden. See our How-to on Sprinklers or Popups for more info.
Now you have an understanding of flow and pressure of water, follow on to our How-to on Designing a simple irrigation system