A fixed "30 minutes every day" schedule is a habit, not a calculation. It gives too much in April and sometimes too little in July. To calculate you need three numbers: the lawn's daily water demand (mm), the rainfall (mm) and the line's flow rate (litres per minute). First, the units.
What does 1 mm of water mean?
In irrigation, water is measured as depth: millimetres. 1 mm of water is 1 litre per square metre. Giving 100 m² of lawn 5 mm means giving it 500 litres. This rule carries the whole calculation:
| Depth | 1 m² | 100 m² | 1,000 m² |
|---|---|---|---|
| 1 mm | 1 L | 100 L | 1 m³ |
| 5 mm | 5 L | 500 L | 5 m³ |
| 10 mm | 10 L | 1 m³ | 10 m³ |
How many mm does a lawn need per day? (ETc)
A plant's daily water loss is called evapotranspiration: evaporation from the soil plus transpiration from the leaves. It changes with the weather; it rises on hot, dry, windy days and falls on cool, humid days.
The calculation has two parts. First the reference value (ET0) is found from the day's temperature, humidity, wind and sunshine; the common method worldwide is FAO-56. Then it is multiplied by a crop coefficient (Kc): ETc = Kc × ET0. For lawn the coefficient is close to 1; trees and shrubs need less.
For cool-season turf in Türkiye, mid-season daily demand runs between about 4 and 7 mm in most places and drops to 1–3 mm in spring and autumn. The figure varies by region and year; that is why the calculation is redone every day.
Rain and the previous watering are deducted
The lawn needs ETc; but irrigation does not have to supply all of it. Rain in the last 24 hours is deducted. If too much was given yesterday, the water left in the soil is deducted too. What remains is "today's water":
Water to apply (mm) = ETc − effective rainfall − water left in the soil
Light rain is not deducted in full; part of it evaporates. That is why it is called "effective rainfall". If rain was enough, the result is zero or negative: no watering that day.
Converting to minutes: flow rate
The line's flow rate is the total water the sprinklers deliver in one minute. Number of sprinklers × sprinkler flow; or the value read from a flow sensor. The time follows:
Time (min) = water to apply (mm) × area (m²) ÷ flow rate (L/min)
Then there is sprinkler efficiency. With sprinklers, part of the water drifts in the wind or evaporates; not all of it reaches the soil. In practice the time is extended by 10–25%. Drip irrigation loses little.
Example: 100 m² of lawn on a summer day
Assumptions: area 100 m²; ETc 5 mm; rain in the last 24 hours 0; water left in the soil 0; 6 sprinklers on the line at 3 L/min each, 18 L/min in total; 15% added for efficiency loss.
| Step | Calculation | Result |
|---|---|---|
| Water to apply | 5 mm − 0 − 0 | 5 mm |
| Convert to litres | 5 mm × 100 m² | 500 L |
| Convert to minutes | 500 L ÷ 18 L/min | 27.8 min |
| Efficiency loss | 27.8 × 1.15 | ≈ 32 min |
The same line on a cool April day: ETc 2 mm, 3 mm of rain yesterday. Water to apply is below zero; no watering. The same line on a dry July day with ETc 7 mm: the time rises to 45 minutes. A fixed 30-minute schedule gives too much in the first case and too little in the second.
Every day, or twice a week?
The calculation gives the daily need; how you spread it is your choice. The common rule for lawns is two or three deep waterings a week instead of a short daily one: roots go deeper, the surface dries, weeds and disease decrease. 10 mm every other day uses the same water better than 5 mm every day. In clay soil water infiltrates slowly and runs off the surface in a long run. There, the time is split in two with a pause between.
Overwatering is harmful too: water and nutrients leach below the roots and the roots are starved of air. The Ministry's xeriscape guidance warns of this specifically. Calculated watering is therefore a matter of plant health as well as savings.
How does ESTRON do this calculation?
The system runs the steps above every morning for every line. ET0 comes from the hourly weather forecast, rainfall from the last-24-hour data, plant condition from satellite. The result is a percentage: 70% of the reference time, for example. You enter the reference time once; the rest is a daily decision. The decision steps: how does the AI make the irrigation decision?
The percentage decision is applied together with the safety limits: wind limit, frost risk, rain probability. Even if the calculation says "45 minutes", watering is postponed when wind is above 6 m/s. How weather data is used: weather-based irrigation.
On a line with a flow sensor the loop closes: the water given is measured in litres and compared with what was expected. A difference points to a leak or a blockage. See catch water leaks without going to the field. The result is up to 40% less water than a fixed schedule.
Frequently asked
I do not know my line's flow rate. What do I do?
Multiply the number of sprinklers by each one's catalogue flow. Better: open the line for 5 minutes, read the litres on the water meter, divide by five. With a flow sensor it is ready in the panel.
Are spray and rotor sprinklers the same?
No. A spray head delivers water fast to a small area, a rotor slowly to a large one. For the same mm, a rotor line runs longer. That is why the two types are not mixed on one line.
Do I have to calculate by hand?
No. A reference time is entered in the panel; the daily percentage comes from the system. The manual calculation is done once, to choose the reference time well. Panel: landscape irrigation software. Other questions are on the FAQ page.