Irrigation and FAO-56: Understanding ETo, Kc, and ETc

What evapotranspiration means

Evapotranspiration, or ET, combines water evaporation from the soil surface and plant transpiration. Expressed in mm/day, it represents a depth of water lost each day over a given area. The conversion is straightforward: 1 mm over 1 m² equals 1 litre. This is an estimated water loss, not directly a volume to apply. FAO, processes and units

FAO-56 distinguishes a reference surface, the crop under standard conditions, and then the necessary adjustments when the situation deviates. Understanding these steps prevents treating a multiplication as an irrigation prescription.

ETo: A reference for climatic demand

ETo describes the evapotranspiration of a hypothetical well-watered vegetated surface, comparable to a uniform lawn fully covering the soil. In FAO-56, the FAO Penman-Monteith method calculates this reference using solar radiation, air temperature, humidity, and wind, along with the necessary location-specific data. It therefore describes neither a tomato plant’s water needs nor a simple response to heat. FAO, reference and data

Before using an ETo value, retain its origin, location, period, and unit. A daily average and a weekly total are not interpreted in the same way. Data quality matters: the manual emphasises that a meteorological equation does not perfectly predict evapotranspiration in all situations. FAO, limitations

Kc: A coefficient linked to growth stage and conditions

The crop coefficient Kc is dimensionless. It relates the crop to the reference, accounting for differences in vegetation and soil evaporation. In the single-coefficient approach, it encompasses both contributions. FAO, coefficient approach

The method distinguishes four phases: initial stage, development, mid-season, and late season. The timing depends on the crop, variety, and local conditions; published durations serve as guidelines, not a universal calendar. FAO, stages

To select a Kc, also read the table conditions and notes: wetting frequency at the start of the cycle, climate, and crop management may require adjustments. The manual notably distinguishes staked vegetables. A value associated solely with the name “tomato” is therefore insufficient to describe your situation. FAO, coefficients

ETc: The calculation and its units

The relationship is ETc = Kc × ETo. With ETo in mm/day and a dimensionless Kc, ETc is in mm/day. Here, it corresponds to the standard conditions of the manual: a crop in a large field, well-managed, without water limitations or other constraints affecting its growth. FAO, equation 56

Consider this example for learning how the calculation works, without assigning it to a particular vegetable: ETo = 4 mm/day and Kc = 0.8 for one day.

• ETc: 4 × 0.8 = 3.2 mm/day.
• Estimated loss for that day: 3.2 mm.
• Equivalent over a 10 m² area: 3.2 × 10 = 32 litres.

These 32 litres represent the ETc of the entire area over one day. They are neither a recommended amount per plant nor an instruction to irrigate. For several days, sum the daily losses; multiplying by the number of days assumes ETc remains constant.

Moving from ETc to an irrigation decision

It is necessary to monitor the water reserve in the root zone. The balance starts with the soil’s initial state and adds rainfall, infiltrated irrigation, and any capillary rise. It subtracts evapotranspiration, runoff, and water drained below the roots. The net irrigation used in this balance refers to water that infiltrates, not the volume delivered at the tap. FAO, water balance

This monitoring helps determine the timing and quantity of an application. The accessible reserve depends notably on the soil and rooting depth; water deficit can reduce transpiration. Simply replacing each day’s ETc or subtracting all the forecast rain is therefore not enough. FAO, reserve and irrigation

For practical advice on examining the soil and checking the water you apply, consult the practical irrigation guide.

Limitations in the vegetable garden

Mulching, intercropping, and small areas are addressed with adaptations in the manual. A bed’s surroundings can alter its evapotranspiration; estimates related to oasis and edge effects require local verification. It would therefore be unwise to adopt a large-field coefficient as a value already validated for any garden. FAO, practices and small areas

You can try the multiplication and conversion with the educational evapotranspiration calculator. It illustrates ETc based on your assumptions. To plan watering, also record the soil conditions, actual rainfall, observed growth stage and how your system operates.