Contents preface (VII) introduction 1—37

Table 3.7 Data and solution for Example 3.5
	Month	Crop	Pan			Effective rain-		Consump-
	evaporation,			fall, Dp – Dpl		tive use,	FIR (mm)
			factor, K
			Ep (mm)		(mm)		Det (mm)
	November	0.20	118.0			6.0		23.60	29.33
	December	0.36	96.0			16.0		34.56	30.93
	January	0.75	90.0			20.0		67.50	79.17
	February	0.90	105.0			15.0		94.50	132.50
	March	0.80	140.0			2.0		112.00	183.33
	Solution:
	According to Eqs. (3.12) and (3.15)
			Det = KEp
	and		FIR =	Det − (D p − Dpl )
					Ea
	Given		Ea = 0.6

Field irrigation requirement calculated for each month of the crop-growing season has been tabulated in the last column of Table 3.7.
3.9. FREQUENCY OF IRRIGATION
Growing crops consume water continuously. However, the rate of consumption depends on the type of crop, its age, and the atmospheric conditions all of which are variable factors. The aim of each irrigation is to fulfil the needs of the crop for a period which may vary from few days to several weeks. The frequency of irrigation primarily depends on: (i) the water needs of the crop, (ii) the availability of water, and (iii) the capacity of the root-zone soil to store water. Shallow-rooted crops generally require more frequent irrigation than deep-rooted crops. The roots of a plant in moist soil extract more water than the roots of the same plant in drier soil.
A moderate quantity of soil moisture is beneficial for good crop growth. Both excessive and deficient amount of soil moisture retard the crop growth and thus the yield. Excessive flooding drives out air which is essential for satisfactory crop growth. In case of deficient moisture, the plant has to spend extra energy to extract the desired amount of water.
Many of the crops have an optimum soil moisture content at which the yield is maximum; if the moisture content is less or more than this amount, the yield reduces. Wheat has a well-defined optimum moisture content of around 40 cm. However, there are other crops in which the yield initially increases at a much faster rate with the increase in the soil moisture content and the rate of increase of the yield becomes very small at higher moisture content. In such cases, the soil moisture is kept up to a level beyond which the increase in production is not worth the cost of the additional water supplied.
It should be noted that, because of the capacity of a soil to store water, it is not necessary to apply water to the soil every day even though the consumptive use takes place continuously. The soil moisture can vary between the field capacity and the permanent wilting point. The average moisture content will thus depend on the frequency of irrigation and quantity of water applied. As can be seen from Fig. 3.5, frequent irrigation (even of smaller depths) keeps the average mositure content closer to the field capacity. On the other hand, less frequent irrigation of larger depths of water will keep the average moisture content on the lower side.

SOIL-WATER RELATIONS AND IRRIGATION METHODS

Soilmoisturecontent

Time

111

Field capacity

Average moisture content

PWP

(a) MORE FREQUENT IRRIGATION

Soilmoisturecontent

Field capacity

Average moisture content
PWP

Time

(b) LESS FREQUENT IRRIGATION

Fig. 3.5 Effect of frequency of irrigation on average moisute content
For most of the crops, the yield remains maximum if not more than 50 per cent of the available water is removed during the vegetative, flowering, and the intitial periods of the fruiting stage. During the final period of the fruiting stage, 75 per cent of the available moisture can be depleted without any adverse effect on the crop yield.
The frequency of irrigation (or irrigation interval) is so decided that the average moisture content is close to the optimum and at each irrigation the soil moisture content is brought to the field capacity. Alternatively, the frequency of irrigation can be decided so as to satisfy the daily consumptive use requirement which varies with stage of growth. Thus, frequency of irrigation is calculated by dividing the amount of soil moisture which may be depleted ( i.e., allowable depletion below field capacity and well above permanent wilting point) within the root-zone soil by the rate of consumptive use. Thus,

_{Frequency of irrigation =} Allowable soil moisture depletion Rate of consumptive use
The depth of watering at each irrigation to bring the moisture content capacity w_fc in a soil of depth d can be determined from the following relation:
Depth of water to be applied = ^{(w fc − w) d E}a

(3.16)
w to the field

(3.17)

Example 3.8 During a particular stage of the growth of a crop, consumptive use of water is 2.8 mm/day. Determine the interval in days between irrigations, and depth of water to be applied when the amount of water available in the soil is: (i) 25%, (ii) 50% (iii) 75%, and

112 IRRIGATION AND WATER RESOURCES ENGINEERING
(iv) 0% of the maximum depth of available water in the root zone which is 80 mm. Assume irrigation efficiency to be 65%.



Solution:
(i) Frequency of irrigation

(ii) Depth of water to be applied

₌ 80 × (1 − 0.25) 2.8

• 
  21.43 days
  

• 
  21 days (say)
  

• 
  80 × (1 − 0.25)
  

0.65

• 
  92.31 mm
  

• 
  93.00 mm (say).
  

Other calculations have been shown in the following table:

		Amount of soil moisture depleted to
		25%	50%	75%	0%
Frequency of irrigation (days)		21	14	7	28
Depth of water to be applied (mm)		93	62	31	124