Irrigation scheduling

• Process of determining when and how much to irrigate.
• Irrigation scheduling helps to save water and energy.
• Increases the crop yield and make efficient use of inputs and lower production cost.
• It is mainly influenced by 3 factors:

1. Water needs for crop
2. Capacity of the reservoir for irrigation and
3. Capacity of root zone to store the water

Terminologies

A. Limiting soil moisture condition:

• Allowable moisture depletion in the field for not to wilt and not to get stressed to the plant.
• It is not less than 50-60% of the FC.

B. Optimum moisture:

• It is defined as moisture content in the field when the growth of the crop is optimum.

Estimating depth and frequency of irrigation on the basis of the soil moisture regime

dw = £ⁿ _i=1  [ (MFci-Mbi)/ 100 ] x A_i . D_i

where, dw= depth of the water to be applied

            MFci-Mbi = moisture % age at field capacity and optimum moisture in the i^th layer.

Again,

Fw = dw/ CU days.

Where, Fw = Frequency of irrigation

             Dw = depth of water       [ V= dw x A and V = q x t]

            CU = Consumptive use

C. Allowable soil moisture depletion:

• Allowable Depletion represents the amount of soil moisture that can be removed by the crop from the soil before the crop begins to stress.
• To avoid moisture stress and wilting, safe limit of soil moisture depletion has to be saved.
• In general, field crops are irrigated when 50% and not more than 60% of the available soil moisture in the root zone is used up.

D. Irrigation frequency and /or Irrigation interval:

• It refers to the number of days between 2 irrigations during periods without rainfall.
• Depends on how fast soil moisture is extracted.
• It also depends on the consumptive use, rate of a crop and on the amount of available moisture in the crop root zone.
• The design irrigation frequency may be computed as :

Design frequency ( in days) =  (Field capacity of the soil in effective xrop root zone- moisture content in the same zone) / Peak period moisture use rate of crop

E. Irrigation period:

• It is the number of days that can be allowed for applying one irrigation to a given design area, during the peak consumptive use period of the crop being irrigated.

Note: Frequency of irrigation should be higher than irrigation period . i.e. I_F > I_p.

F. IW/ Epan ratio:

• The ratio between a fixed amount of irrigation water (IW) and cumulative pan evaporimeter as well as the effective rainfall since previous irrigation is used for scheduling irrigation to crop.