🍉 How much water does watermelon need?
There is no single answer, and a page that gives you one is guessing at where you live. Watermelon has a fixed thirst relative to the weather — its crop coefficient — and the weather is yours. Pick your climate and this works out the litres.
Watermelon roots to about 100 cm. In loam / medium soil that root zone holds roughly 140 mm of usable water, so watering when about half of it is gone means roughly 70 L per m² every 18 days rather than a little each day.
Deep and infrequent beats shallow and daily for almost everything. Light daily watering keeps the roots in the top few centimetres, which is the layer that dries first the moment you miss a day.
Set your plot and its crops once and the app costs the water you actually put on, warns when a critical stage is coming, and raises the job on the day rather than leaving it to the weather to remind you. Free while in early access.
Create your free accountThe weeks that decide the crop
Water stress is not evenly costly across a season. For watermelon the stage that matters is fruit sizing, before ripening — stress there does damage that watering afterwards cannot undo.
What it looks like when it goes wrong: Splitting, and sugar diluted by late watering. Ease off as the fruit finishes.
The corollary is that the rest of the season has more slack than most people assume. If water is short, protect that window and economise around it, rather than spreading the shortage evenly and damaging the crop at the one point it cannot recover from.
Why the number changes with where you are
A crop does not decide how much water it uses; the weather does, and the crop takes a fixed share of it. That share is the crop coefficient — watermelon sits at about 1 at mid-season — and the weather is reference evaporation, which runs near 2.5 mm a day in a cool maritime summer and near 9 in a desert.
That is the whole reason a figure copied from one country misleads in another. Same crop, same coefficient, three times the water. It is also why "an inch a week" survives as advice: it is about right for a temperate summer, and it is quietly wrong everywhere hotter or cooler.
Rain counts, and most people forget to subtract it
The figures above are total demand, not the amount to irrigate. Rainfall in the week comes off the top — and only rain that actually soaked in counts. A brief shower on dry ground mostly evaporates before it reaches the roots, and on heavy soil a downpour runs off. A rain gauge pays for itself in a season, mostly by stopping you watering when you did not need to.
Drip or sprinkler for watermelon
At this crop's demand, drip is usually the better fit. Drip puts water at the root and leaves the leaves dry, which matters for anything prone to foliar disease, and loses far less to evaporation in hot wind. Either way, water early in the morning: midday irrigation in a hot wind can lose a quarter of what you apply before it reaches the soil.
→ When to plant watermelon ← All farm calculators
Method: weekly need = Kc × ET0 × 7, where Kc is the FAO-56 mid-season crop coefficient and ET0 is reference evaporation for the climate band you picked. One millimetre of depth over one square metre is one litre. Rooting depths are FAO-56 maximum effective values. These are planning figures: your own soil, shelter and season will move them.