Water the Garden to a Number, Not a Habit

Why millimetres and square metres do all the work

The litre-per-square-metre-per-millimetre identity is the reason irrigation maths is gentle. No conversion tables, no inches of rain per week of growth. Measure the bed once, pick a depth the crops want, and the litres write themselves.

Depths come from the crop and the climate rather than the calendar. Leafy beds in mild weather manage on twenty millimetres a week. Fruiting tomatoes in a hot week can want forty or more, and a mature squash plant drinks like a small household.

The one trap is area honesty. A bed measured including its bare paths inflates the number. Watering only the planted strip is normal, so count the planted strip. On mulched soil, the same figure already carries a quiet discount for reduced evaporation.

The identity also makes overwatering visible, which is the more common sin. A bed that received forty millimetres from rain and ritual combined has been drowned by maths, not love. Roots need air as much as water. Soil that never dries between soaks selects for weak plants.

Worked example: a twenty square metre kitchen garden

The default plot: 20 m² of vegetables, 25 mm wanted weekly, 10 mm of typical rain, drip lines at 90% efficiency. Net depth after rain is 15 mm, so the soil needs 300 litres a week. Because the system wastes a tenth on the way, the tap must supply about 333.

A four-week dry spell removes the rain credit entirely, so the tap must cover the full 25 mm. That is about 555 litres weekly and roughly 2,200 over the spell. And at a 60 litre per hour drip zone, the weekly water is about five and a half hours of run time, best split into two or three shorter soaks.

Compare the sprinkled version at 65% efficiency: the same soil demand pushes the weekly draw above 460 litres. The soil received the same water. The household paid for the difference, which is the entire argument for drip over spray in a water-priced garden.

Where the wasted water actually goes

Evaporation is the loud one. Midday sprinkling on hot soil loses a meaningful share before the droplets land, which is why every extension service says water early or late. Wind turns sprinklers into weather machines spraying the fence more than the bed.

Runoff is the quiet one. Fine spray on crusted soil pools and runs to the path instead of soaking, and the fix is slower application rather than less water. Drip lines at soil level dodge both failures at once, which is why their efficiency sits near ninety.

Deep percolation is the polite waste: water passing below the roots into the subsoil, invisible and unrecoverable. Frequent light watering avoids pulling it off. Daily soaks at the wrong depth train shallow roots that then beg for water on the first hot day.

Wind is the cheapest thing to fix. A fence line breaks it, a morning window avoids the dry afternoon, and both changes cost nothing at the tap. Households obsess over application rates while the real losses live in timing and exposure.

Feeding a rainwater system from these numbers

Garden demand is the input the rainwater household fears most, because it can empty a tank in an afternoon. Take the weekly litres from this calculator and put them into the dry-spell total before sizing any tank. A garden that drinks 2,200 litres over a spell is not an afterthought on a 1,000 litre tank.

The pleasant surprises are structural. Mulch, shade cloth in heat waves, and watering evenings instead of noons each shave the demand figure honestly, without touching plant health. Re-run the calculator after any of those changes and the tank shrinks with it.

Priority logic matters in real droughts. Drinking supply never funds the garden. Perennial plantings outlast annuals when the tank dips. And a garden planned at its measured demand can negotiate; one watered by ritual cannot even see the bill.

Rain credit deserves the same bookkeeping. A wet week is a deposit into the household water account, and the rota should pause in response. Watering anyway is spending money you did not have, on soil that was already paid for.

Accuracy, assumptions and limitations

Crop water needs swing with heat, wind and growth stage more than this weekly average admits. A calm mild week can run at half the demand and a heat wave at double. Treat the answer as a planning centre and let a dry soil probe or a finger in the ground handle the daily verdict.

Efficiency figures are class averages, not measurements. A leaking fitting or a clogged dripper turns 90% into fiction quickly. Checking output by timing a bucket once a season keeps the honest number in the calculator.

The model also assumes the rain input soaks the beds rather than running off covers and paths. A gauge standing in the open overestimates what a greenhouse bench receives. Measure where the water lands, not where it falls.

The dry-spell figure treats the rain input as a season average, not this week's forecast. When a heat warning lands, double the weekly litres before trusting any tank maths. A spell inside a spell is how small reserves fail in a single afternoon.