Concept: Depth of water Held in Root zone: For ease in calculation water present in voids of soil needs to be expressed as the depth of water.
Let root zone depth = ‘D’ in m
Specific wt. of soil (dry) =
γdThe cross-sectional area of soil considered = A
Equivalent depth of water stored in voids of soil = ‘d’.
We know,
∴
Fc= ∴
Fc= ⇒ d =
×D×Fc It is the depth of water stored in the root zone for
full field capacity. But this entire depth of water cannot be extracted by plants. Hence available moisture will be
davailable =
×D×[Fc−ϕ] Where ϕ = permanent wilting point.
Depth of Readily available moisture (d’): d=×D×(Fc−M0) Frequency of Irrigation: The frequency of irrigation is decided based on the maximum allowable deficit and rate of consumptive use.
fw= Where,
Cu = Consumptive use/day
Calculation: Fc = 29 % = 0..29, ϕ = 10% = 0.10, D = 0.8 m,
Relative density of soil
()=1.302 Frequency of irrigation = 10 days, Optimum moisture
(M0)=17% Depth of Readily available moisture (d′) =
×D×[Fc−M0] ∴d′=1.302×0.8×[0.29−0.17] ∴ d' = 0.12499 m = 124.99 mm
∴ Frequency of Irrigation =
=
= 10 days
∴ Cu = 12.49 mm = 1.25 cm.