Crop water requirement calculator
A crop’s water need is the local reference evaporation (ETo) times the crop factor (Kc). The tool uses long-term ETo for eight Antalya districts and FAO crop factors, and allows for the lower evaporation in a greenhouse, drip losses and drain water. You get the need for each of the 12 months in mm and tonnes, and the hottest day’s need that sizes the drip system, the tank and the pump.
Loading the calculator…
A worked example
25 decares of soilless strawberry in a greenhouse in Serik. Greenhouse factor 0.7, losses and drain 25%.
- June ETo 5.99 mm/day in Serik
- June need 5.99 × 0.75 (Kc) × 0.7 ÷ (1 − 0.25) = 4.2 mm/day; 126 mm in the month, 3,145 tonnes on 25 decares
- Hottest day 4.2 × 1.2 = 5.0 mm; 126 tonnes on 25 decares
- January 43 mm, 1,076 tonnes; July and August empty
- Year about 673 mm, 16,826 tonnes
[The example’s numbers await Perge’s check.]
How it is worked out
ETo is the water used by short, well-watered grass; it comes from sun, heat, humidity and wind. The tool uses the monthly values Karaca and others published in the journal Derim in 2017, worked out by the FAO Penman-Monteith method from long-term MGM station data. Aksu has no station of its own, so it takes the Antalya centre values.
Kc shows how much less or more water a crop uses than grass, by its growth that month. The values come from the FAO-56 table, laid on Antalya’s planting and harvest calendar. A greenhouse cuts sun and wind, so evaporation inside is about 60–80% of outside; the tool multiplies by 0.7 unless you change it.
The water the crop uses is raised for drip losses and, in substrate, the drain: need ÷ (1 − share). A monthly mean hides the hottest days, so the peak month’s mean is multiplied by 1.2, and the system is sized on that number.
Questions
How many tonnes of water a day does a decare need?
In Antalya, on the hottest day, most open-field crops need 5 to 9 tonnes a decare, and greenhouse crops 3 to 6. 1 mm of water is 1 tonne a decare. The exact figure depends on the crop, the district and the month; the tool works it out month by month.
How is crop water use worked out?
Crop water use = ETo × Kc. ETo is the reference value set by the local weather, and Kc is the crop’s factor. In Serik in July, for example, ETo is 6.6 mm/day; open-field tomato with a Kc of 1.15 uses about 7.6 mm a day.
What is the ETo in Antalya?
By long-term means, daily ETo is 1 to 2 mm in January and 5 to 6.6 mm in July. The lowest value is in Elmalı in January (0.9 mm/day) and the highest in Serik in July (6.6 mm/day). The yearly total is about 1,200 mm in central Antalya and 1,400 mm in Serik.
Does a greenhouse crop need less water?
It needs less each day: the cover cuts some of the sun and stops the wind, so evaporation inside is about 60–80% of outside. But a greenhouse crop grows through the winter too, so its yearly total may be no lower than in the open.
Why does soilless growing take more water?
So that salt doesn’t build up in the pot or bag, 20 to 30 percent of the water given runs out as drain. The plant uses the same water, but that much more goes into the system. Set the drain share by the EC of the drain water.
Is rain taken into account?
No, the table takes no rain off. In Antalya most rain falls from November to March, so in the open the water you must give in those months is less than the table shows. From June to September there is very little rain, so the hottest day’s figure stays the same.