The amount of water held in the root zone available to the plant,
SAW in (m), is
SAW ¼ ðFC PWPÞ Rz ð3Þ
where FC (volume %) is the field capacity, PWP (volume %) is the
permanent wilting point and Rz ðmÞ is the plant root zone. The
amount of water allowed to leave the root zone without causing
plant stress is the readily available water, RAW ðmÞ, [37]. Irrigation
should be applied when the water level drops by a percentage
known as the maximum allowable depletion, MAD ð%Þ, which is
the amount of water relatively easily extracted by plant without
causing plant stress [28]. The two quantities are related as follows:
RAW ¼ SAW MAD ð4Þ
This means that the amount of water that can be extracted from the
soil by the plant, S (m) is such that
ðFC RAWÞ 6 S 6 FC ð5Þ
The plant water extraction then leads to a water balance such that
S ¼ I þ P ETc Ro Dr ð6Þ
In this equation, all units are (m). Irrigation (I) aims to match the
crop evapotranspiration (ETc) losses and precipitation (PÞ added