Coffee irrigation: when to irrigate and how much to apply
Evapotranspiration, crop coefficient, water balance and soil moisture sensors: how to move off the calendar and irrigate coffee by what the plant needs, and how to calculate the irrigation depth.
Irrigating coffee by the calendar is easy and expensive: it uses water and energy while the soil is still moist, and it comes too late when the plant is already stressed. The two questions that matter, when to irrigate and how much to apply, have a technical answer, and the answer comes from the farm’s own data.
How much water coffee uses
The worldwide reference is the FAO method (Irrigation and Drainage Paper No. 56). It splits water use into two parts:
ETc = ETo × Kc
- ETo is reference evapotranspiration: how much water a well-watered grass would lose that day, calculated with the Penman-Monteith equation from temperature, air humidity, wind and solar radiation.
- Kc is the crop coefficient: how much coffee uses relative to that reference. It changes with the age of the planting, the spacing and the stage of the cycle.
ETo depends on the climate of that place. Data from the station in the nearest town is wrong on windy days, low-humidity days and when a cold front passes. That is why an on-farm weather station is the foundation for everything else.
When to irrigate: the water balance
The soil works like a water tank. Available water is what the soil holds between saturation and the point where the plant can no longer extract water, and it depends on soil texture and root depth. Every day, the calculation is:
soil water today = yesterday’s soil water + effective rainfall + irrigation − ETc
You irrigate when the soil has used up a defined fraction of the available water, before the plant goes into stress. That fraction is the agronomist’s decision and it changes with the coffee’s growth stage.
The soil sensor checks the calculation
The water balance is a model, and models are wrong sometimes. A soil moisture sensor, at two or three depths, shows what is actually happening in the root zone: whether the rain soaked in, whether the irrigation reached the deeper layers, whether the soil is drying faster than the calculation predicted. Water balance and sensor together give a safer decision than either one alone.
Flowering and controlled stress
In Brazil’s Cerrado region, research by Embrapa (the Brazilian agricultural research corporation) has studied withholding irrigation in a controlled way during the dry season to concentrate flowering and even out ripening. It is an agronomic decision, risky if done at the wrong time, and it depends precisely on soil and weather data to be done safely.
How much to apply: the irrigation depth
- Net depth: the water the soil has used since the last irrigation, up to the limit of the available water.
- Gross depth: the net depth divided by the system efficiency. Well-maintained drip irrigation wastes little; sprinklers and pivots lose more to wind and evaporation.
- Irrigation time: the gross depth divided by the system’s application rate.
Efficiency is not a catalog number: it drops with clogged emitters and uneven pressure. See drip irrigation in coffee.
Where IoTData comes in
The weather station, with solar radiation when it has a pyranometer, the hour meter and flow measurement already exist. Soil moisture and line pressure sensors are in development for pilots.
Sources: FAO Irrigation and Drainage Paper 56, Crop evapotranspiration (Allen, Pereira, Raes and Smith, 1998); Embrapa (Brazil), research on controlled water stress in irrigated coffee in the Cerrado.