Ollas and evotranspiration
Defining a plant's irrigation needs
Most of the water a plant needs corresponds to the volume of water it loses through evapotranspiration each day.
Indeed, the amount of water stored in a plant and that used by its metabolism are insignificant compared to the amount the plant must absorb due to transpiration losses.
Therefore, we can consider that the water requirements of plants correspond to the losses through evapotranspiration.
The actual evapotranspiration of a plant is defined based on the potential evapotranspiration of the area, the type of plant, and its stage of development.
Thus, potential evapotranspiration rates can vary enormously depending on climates and times of the year.
In dry areas, the potential evapotranspiration rate of a simple lawn varies between 6 and 9 mm of water per day and m², while in temperate areas, it is around 1 to 2 mm per day and m². (*)
For vegetable crops or fruit trees, which have higher water requirements, higher values must be considered.
(*) Ch. Riou and GJ Dubois, First measurements of potential evapotranspiration in the Republic of Chad, 1966
Calculation of evotranspiration
Various methods and software programs exist for estimating a plant's water requirements, with varying degrees of accuracy. However, a simplified way to estimate irrigation needs is by first measuring the evaporation rate of the area using an evaporation tray, Ebac.
Then a correction coefficient of 0.66 is applied to the evaporation rate obtained to obtain the potential evapotranspiration rate of a plant cover.
To best take into account the stage of development of the plant cover in question, the correction coefficient must be divided by 2 (i.e. 0.66 /2=0.33) and multiplied by (1+C), where C is the fraction of the soil covered by plants (0 if sowing, 1 if the soil is totally covered).
This gives the formula I = (0.33 x (1+C) x Ebac
I = Irrigation requirement;
Ebac = Evaporation rate measured in the tank over 24 hours;
C = Plant ground cover
