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How to measure your water flow and pressure for irrigation

Measure the tap before you buy a single fitting. A zone that asks for more water than the tap can give will have weak sprinklers that do not pop up, and drippers at the end of the line that hardly drip. Ten minutes with a bucket, a watch and a cheap pressure gauge tells you how much you can run at once.

Measure the flow: the bucket test

1. Use the tap you will irrigate from, with nothing else in the house using water.
2. Open the tap fully.
3. Time how long it takes to fill a bucket of known size.
4. Work out the flow:

flow (litres a minute) = litres in the bucket ÷ seconds to fill × 60

For example, 10 litres in 30 seconds is 20 litres a minute.

Do the test twice, at the time of day when you will water. Pressure in many areas is lower at peak times.

Measure the pressure

A pressure gauge that screws onto a garden tap is sold at irrigation shops. It reads in bar or kPa, and 1 bar is 100 kPa.

• Static pressure: close every other tap, screw on the gauge and open the garden tap.
• Working pressure: the pressure while water is flowing. It is always lower.

What each kind of emitter needs

• Drip: low pressure, often about 1 to 1.5 bar. Most drip zones on a mains tap need a pressure regulator.
• Micro-sprayers: low to medium.
• Pop-up sprays: medium, often about 2 bar. They mist and waste water if the pressure is too high.
• Rotors: medium to high, often 2.5 to 3.5 bar. They will not turn properly on low pressure.

Check the maker's figures for the parts you buy.

How much can one zone use?

Add up the flow of every emitter in the zone. Sixty drippers at 4 litres an hour use 240 litres an hour, which is 4 litres a minute.

The rule of thumb: keep each zone's total to about three-quarters of the flow you measured at the tap. If the tap gives 20 litres a minute, a zone may use about 15.

If the garden needs more than that, split it into more zones that run one after another.

If the supply is weak

• Use drip wherever you can. It needs little flow and pressure.
• Make smaller zones.
• Use larger pipe on long runs to reduce friction.
• Ask the municipality if the supply seems far below normal for your area.

Slopes

Water loses about 1 bar of pressure for every 10 metres it rises, and gains the same as it falls. On a steep garden, emitters at the top get less pressure and those at the bottom get more. Pressure-compensating drippers and sprinkler heads with check valves solve most of this.

Pressure changes

Pressure drops in the early morning when many households shower, in heat waves, and during water restrictions. Design for the lowest pressure you measured at watering time, not the best.

Keep a record

Write down the date, time, tap, static pressure and bucket-test result. When something stops working later, compare today's pressure with the record. A big drop points to the supply, not to your system.

Thinking of a booster pump?

Never connect a pump directly to the municipal supply pipe without the municipality's approval. It can draw water away from neighbours and cause backflow. The usual lawful arrangement is a tank filled from the supply, with the pump drawing from the tank. Often the better answer is a design that suits the supply.

This guide is based on Chapter 3 of Garden Irrigation DIY.