Irrigation frequency refers to the number of days between irrigation during periods without rainfall. It depends on consumptive use of rate of a crop and on the amount of available moisture in the crop root zone. It is function of crop, soil and climate. Sandy soils must be irrigated more often than fine texture deep soils. A moisture use ratio varies with the kind of crop and climate conditions and increases as crop grows larges and days become longer and hotter.
In general, irrigation should start when about 50 percent and not over 60 percent of the available moisture has been used from the root zone in which most of the roots are concentrated. The stage of crop growth with reference to critical periods of growth is also kept in view while designing irrigation frequency.
The interval that can be safely allowed between two successive irrigations is known as frequency of irrigation:
Allowable soil moisture depletion
Irrigation interval = —————————————
Daily water use
Problem: Calculate irrigation interval when F.C=20.0% dry weight basis
PWP = 8.0 dry weight basis, BD = 1.4 g/cc
Root depth = 60cm ET ratio = 0.5 cm/day
Allowable soil water depletion is equal to 25% of available soil water
(20.0 – 8.0) X 1.40
Available water = ——————————- X 60
25 X 10.08 2.52
Allowable soil water depletion = —————- = ——– = 5.04 = 5 days
Irrigation must be given at 5 days interval.