1. Lecture 6
Crop water requirement - Crop coefficients for various crops. Estimation of Crop water requirement - field water balance

2. IRRIGATION REQUIREMENT
Irrigation requirement is the total amount of water applied to the land surface in supplement to the water supply through rainfall and soil profile, to meet the water needs of crops for optimum growth.
The water can be directly measured with the help of water measuring devices like flumes, notches, orifices and water meters.

3. NET IRRIGATION REQUIREMENT
The net irrigation requirement is the amount of irrigation water just required to bring the soil moisture content in the root zone depth of the crops to field capacity.
Where,
D = net amount of water to be applied during an irrigation, cm
Mfci = field capacity moisture content in the ith layer of the soil
Mbi = moisture content before irrigation in the ith layer of the soil
Ai = bulk density of the soil in the ith layer
Di = depth of the ith soil layer, cm, within the root zone and
N = number of soil layers in the root zone D.

4. IR = seasonal gross irrigation requirement at the field head
THE GROSS IRRIGATION REQUIREMENT (IR) AT THE HEAD,
Where,
IR = seasonal gross irrigation requirement at the field head
d = net amount of water to be applied at each irrigation
Ea = water application efficiency
n = number of irrigation in a season

5. ESTIMATING WATER REQUIREMENT OF CROPS
Water requirement, includes the losses due to ET (or Cu) plus the losses during the application of irrigation water (unavoidable losses) and the quantity of water required for special operations such as land preparation, transplanting, leaching, etc.
WR = ET or Cu + application losses + special needs

6. Water requirement is a ‘demand’ and the ‘supply’ which would consist of contributions from any of the sources of water, the major sources being the irrigation water (IR), effective rainfall (ER) and soil profile contribution (S)
IR = WR - (ER + S)

7. PET rarely occurs in most of the irrigated field crops, with an exception of low land rice and probably for two to three days immediately after irrigation or rain.
For converting PET values into ET suitable crop coefficients (kc) should be evolved for different crops, soils and climatic conditions and also for different stages of growth for the same crop.
ET (crop) = kc .PET

9. Flow continues

10. Lecture 7
Effective rainfall - factors affecting effective rainfall - Scheduling – Irrigation requirement- Irrigation frequency.

11. EFFECTIVE RAINFALL (Re)
Precipitation falling during the growing period of a crop that is available to meet the evapotranspiration needs of the crop is called effective rainfall.

12. From the point of view of the water requirement of crops FAO (Dastane, 1974) has defined the annual or seasonal effective rainfall as the part of the total annual or seasonal rainfall, which is useful directly and / or indirectly for the crop production at the site where it falls, but without pumping.

13. Ineffective rainfall is that portion which is lost by surface runoff, unnecessary deep percolation losses, the moisture remaining in the soil after the harvest of the crop and which is not useful for next season crop.

14. Rainfall characteristics Land slope Characteristics of the soil
FACTORS AFFECTING EFFECTIVE RAINFALL
Rainfall characteristics
Land slope
Characteristics of the soil
Ground water characteristics
Management practices
Crop characteristics
Carry-over soil moisture
Ground water distribution
Surface and sub-surface in-and out-flows
Deep percolation

15. Irrigation scheduling
The water content at field capacity (the upper limit of the regime) was considered as 100 percent available for crop growth and that at the permanent wilting point as 0 percent available. The safe limit of allowable soil water depletion (the lower limit of regime) for a crop was determined by experimentation and it was taken as a criterion for a scheduling irrigations.

16. Climatic parameters play a predominant role in governing the water needs of crops and the criterion of soil water availability for scheduling of irrigations cannot be considered in isolation from that of climatic factors.
The concepts of evapotranspiration and evaporation is used as the criteria for timing of irrigations.
According to this concept the plant was considered simply as an aqueduct pumping water retained in the soil and losing it to the atmosphere under the influence of atmospheric sink.
Some physiological stages of growth were found to be more critical in their demand for water than others. In this way, the soil, the plant and the climatic factors influence the water needs of crops.

17. Crop response to water at different stages growth
The water requirement of crops vary with the stage of its growth.
When water supply is limited, it is necessary to take into account the critical stages of crop growth with respect to moisture.
The term critical stage is commonly used to define the growth when plants are most sensitive to shortage of water causing drastic reduction in yield.

18. Growth stages of cereals in relation to irrigation
(Salter and goode, 1967);
Stage
Details
Germination
The appearance of the radicle
Tillering
The formation of tillers, i.e, branches produced from the base of the stem
Jointing
The stage when 2 nods can be seen, i.e., the beginning Of shooting
Shooting
The stage of elongation of internodes
Booting
The end of the shooting stage and just prior to the Emergence of ears
Heading (earing)
The emergence of the ear from the formed by the leaf Sheaths
Flowering
The opening of the flowers
Grain formation
The period of grain development from fertilization Until maturity

19. Grain formation subdivided
Milk stage : Grain contents have a milky consistency
Dough stage: Grain contents have a doughy consistency
Waxy-ripe : Grain contents have a waxy appearance
Full-ripe : Grain contents are hard
Dead-ripe : Grain ripe for harvesting.

20. Plan to achieve more crop per drop