Watering a lawn, measured once rather than guessed weekly

The principle: deep and infrequent

Roots follow water. A lawn watered lightly every evening never has moisture below the top few centimetres, so the roots stay there — and a lawn rooted in the top few centimetres wilts the day you stop.

A thorough soak every few days puts water deeper and the roots follow it down. That lawn survives a dry fortnight without help.

As a rule of thumb, an established lawn in dry weather wants somewhere in the region of fifteen to twenty-five litres per square metre per week, applied in one or two goes rather than seven. Soil changes it enormously: a sand holds almost nothing, a clay holds a great deal.

Measurement one: what the tap gives you

Take a bucket of known volume to the end of the hose you actually use — not the tap. Time how long it takes to fill.

That gives you the flow you really have, at the place the sprinkler goes, through the hose and the connectors you own. It is more useful than any pressure figure, and it explains most disappointing sprinklers: length, bore and every quick coupling take their share before the water arrives.

Measurement two: what the sprinkler puts down

Set four identical straight-sided containers around the area the sprinkler covers — tins, jars, anything the same. Run it twenty minutes. Measure the depth in each.

This gives two things.

The rate: millimetres in twenty minutes converts directly to litres per square metre, so you now know how long to run it for a given application.

The evenness: usually the surprise. Most sprinklers deliver considerably more near the head than at the edge of their claimed radius, and the claimed radius was obtained at a pressure the listing often does not print.

Choosing a method, by pressure

Mains pressure runs anything: oscillating sprinklers, rotating and impulse heads, spray guns and lances.

A water butt on gravity runs a watering can, a soaker or porous hose, and a short drip line. It does not run a rotating sprinkler — the mechanism is driven by the jet, and below its working pressure the head simply points one way and dribbles.

A butt with a pump runs most things, and the sizing question is head rather than flow. A bar is roughly ten metres of head; add the height from the butt and the losses in a long hose.

A pump’s flow figure is measured with nothing to lift and its head figure with nothing flowing. Neither point is where a garden hose runs, and this site prints both with the convention named.

Rainwater, honestly

The grass does not care where the water came from. What limits a butt is volume: a thorough watering of a hundred square metres is one to two thousand litres, and a domestic butt holds a few hundred.

For a lawn, rainwater is a supplement or a supply for a small area, not a season’s irrigation. And in a drip system it needs a filter, because roof grit blocks emitters invisibly.

What this site prints

Sprinkler coverage with the pressure it was stated at — or the fact that no pressure was stated. Pump flow and head with the conventions named. Hose bore and stated working pressure rather than a layer count, which is a marketing number nobody defines.