How much water for vegetables? Calculating evapotranspiration

What the result represents

Evapotranspiration combines the water evaporated from the soil and that transpired by plants. It is expressed here in millimetres per day. A depth of 1 mm over 1 m² equals 1 litre: this conversion allows you to move from a depth of water to a volume over the chosen area. FAO, units

The method used is ETc = ETo × Kc. In the FAO manual, this ETc corresponds to crops in large plots, well-managed and without water limitations. The calculator performs this multiplication with your data; it does not verify that your garden meets these conditions. FAO, method

You can access the calculator after preparing the four values below.

Preparing the four inputs

• ETo, in mm per day: the evapotranspiration of a reference vegetated surface, which expresses climatic demand. Use documented data for the location and period studied. The tool does not calculate or retrieve weather data. FAO, ETo
• Kc, dimensionless: the coefficient you have chosen for your scenario. Keep its source, the crop, its stage, and the conditions to which it applies. The tool does not propose any automatically.
• Duration, in whole days: the number of days during which the two previous values are assumed to remain constant.
• Area, in m²: the area to which your scenario applies. The result concerns this entire area, not each plant.

Enter numbers only; decimal points and commas are accepted. ETo can be zero. Kc and the area must be strictly positive, and the duration must be a whole number of at least one day.

Why no universal Kc per vegetable is provided

The coefficients in the FAO table vary throughout development. Their use also depends on reference conditions, soil wetting, and management practices, particularly staking for certain crops. Copying a value opposite “tomato” or “cucumber” would ignore these conditions. FAO, coefficients

The surroundings of a small growing area can also affect its evapotranspiration; the FAO recommends locally verifying estimates for such cases. This tool contains no validated adaptation for a pot, greenhouse, or mixed bed. FAO, small areas

If you do not have a suitable and documented coefficient, use the example solely to understand the calculations.

Reading an example calculation

Let’s take ETo = 4 mm/day, Kc = 0.8, duration = 7 days, and area = 10 m². These values illustrate the arithmetic; they do not describe any recommended crop or weather conditions.

1. Daily ETc: 4 × 0.8 = 3.2 mm/day.
2. Cumulative over seven days: 3.2 × 7 = 22.4 mm.
3. Equivalent volume: 22.4 × 10 = 224 litres over the 10 m².

This does not mean that 224 litres should be applied. The result retains the initial assumptions; the display rounded to a maximum of two decimal places is not a measure of precision in the garden.

An ETo value already totalled over seven days should not be entered as a daily value and then multiplied again by seven. If ETo and Kc change daily, calculate each daily product and sum the resulting depths of water. Multiplying their two averages does not always yield the same result.

Returning to the soil to decide on watering

The root zone water balance accounts for initially present water, rainfall, runoff, infiltrated irrigation, capillary rise, and deep drainage, in addition to evapotranspiration. This calculator does not perform this balance and does not subtract any rainfall, forecast or observed. FAO, water balance

A zero result therefore does not prove that an already dry soil can do without water. To plan watering and check soil moisture, continue with the practical watering guide.