Contents preface (VII) introduction 1—37

117


3.10.5. Trickle Irrigation
Trickle irrigation (also known as drip irrigation) system comprises main line (37.5 mm to 70 mm diameter pipe), submains (25 mm to 37.5 mm diameter pipe), laterals (6 mm to 8 mm diameter pipe), valves (to control the flow), drippers or emitters (to supply water to the plants), pressure gauges, water meters, filters (to remove all debris, sand and clay to reduce clogging of the emitters), pumps, fertiliser tanks, vacuum breakers, and pressure regulators. The drippers are designed to supply water at the desired rate (1 to 10 litres per hour) directly to the soil. Low pressure heads at the emitters are considered adequate as the soil capillary forces cause the emitted water to spread laterally and vertically. Flow is controlled manually or set to automatically either (i) deliver desired amount of water for a predetermined time, or (ii) supply water whenever soil moisture decreases to a predetermined amount. A line sketch of a typical drip irrigation system is shown in Fig. 3.6. Drip irrigation has several advantages. It saves water, enhances plant growth and crop yield, saves labour and energy, controls weed growth, causes no erosion of soil, does not require land preparation, and also improves fertilizer application efficiency. However, this method of irrigation does have some economic and technical limitations as it requires high skill in design, installation, and subsequent operation.

Porous pipe

Multi-outlet distributors

Submain		Sublateral
		loop
Nutrient tank
Gate valve	Filter
Check valve
						Emitters
	with bypass				Lateral
			Gate valve
			Pressure
From pump or				control valve
pressure supply							Main line
Pressure regulator

Fig. 3.6 Line sketch of a typical drip irrigation system
Trickle irrigation enables efficient water application in the root zone of small trees and widely spaced plants without wetting the soil where no roots exist. In arid regions, the irrigation efficiency may be as high as 90 per cent and with very good management it may approach the ideal value of 100 per cent. The main reasons for the high efficiency of trickle irrigation are its capability to produce and maintain continuously high soil moisture content in the root zone and the reduction in the growth of weeds (due to limited wet surface area) competing with the crop for water and nutrients. Insect, disease, and fungus problems are also reduced by minimising the wetting of the soil surface.
Due to its ability to maintain a nearly constant soil moisture content in the root zone, Fig. 3.7, trickle irrigation results in better quality and greater crop yields. Fruits which contain considerable moisture at the time of harvesting (such as tomatoes, grapes, berries, etc.) respond very well to trickle irrigation. However, this method is not at all suitable (from practical as well as economic considerations) for closely planted crops such as wheat and other cereal grains.

118 IRRIGATION AND WATER RESOURCES ENGINEERING

Field				Drip
capacity				method
				Sprinkler
content				method
				Surface
				method
Moisture
	Wilting
point
0	5	10	15	20

Days

Fig. 3.7 Moisture availability for crops in different irrigation methods
One of the major problems of trickle irrigation is the clogging of small conduits and openings in the emitters due to sand and clay particles, debris, chemical precipitates, and organic growth. In trickle irrigation, only a part of the soil is wetted and, hence it must be ensured that the root growth is not restricted. Another problem of trickle irrigation is on account of the dissolved salt left in the soil as the water is used by the plants. If the rain water flushes the salts near the surface down into the root zone, severe damage to the crop may result. In such situations, application of water by sprinkler or surface irrigation may become necessary.
Because of the obvious advantages of water saving and increased crop yield associated with the drip irrigation, India has embarked on a massive programme for popularising this method. The area under drip irrigation in India is about 71000 hectares against a world total of about 1.8 million hectares (6). The area coverage is the highest in Maharashtra. (about 33000 hectares) followed by Andhra Pradesh and Karnataka. Cost of drip irrigation system in India varies from about Rs. 15000 to 40000 per hectare. The benefit-cost ratio (excluding the benefit of saving in water) for drip irrigation system varies between 1.3 to 2.6. However, for grapes this ratio is much higher and may be as high as 13.

3.11. QUALITY OF IRRIGATION WATER
Needs of a healthy human environment place high demands on the quality of irrigation water. Irrigation water must not have direct or indirect undesirable effects on the health of human beings, animals, and plants. The irrigation water must not damage the soil and not endanger the quality of surface and ground waters with which it comes into contact. The presence of toxic substances in irrigation water may threaten the vegetation besides degrading the suitability of soil for future cultivation. Surface water, ground water, and suitably treated waste waters are generally used for irrigation purposes. In examining the quality of irrigation water, attention is focussed on the physical, chemical, and biological properties of such water.
The effect of undissolved substances in irrigation water on the soil fertility depends on their size, quantity, and nutrient content as well as the type of soil. Fine-grained soil particles in irrigation water may improve the fertility of light soils, but may adversely affect the permeability and aeration characteristics of heavier soils. The undissolved substances may settle in the irrigation systems and thus result either in the reduction of their capacity or even

SOIL-WATER RELATIONS AND IRRIGATION METHODS

119

failure of some installations such as pumping plants. The use of water having upleasant odour for irrigation may unfavourably affect the farmers.

The quality of irrigation water depends mainly on the type and content of dissolved salts. The problem arises when the total salt content of irrigation water is so high that the salts accumulate in the root zone. The plant then draws water and nutrients from the saline soil with great difficulty which affects the plant growth. The presence of toxic salts is harmful to the plants.
The suitability of irrigation water from biological considerations is usually decided by the type and extent of biological animation.
Special care should be taken when waste water is to be used for irrigation. Waste waters for irrigation can be classified as municipal wastes, industrial wastes, and agricultural wastes. Waste waters selected for irrigation must be suitable and their use must be permissible from the sanitary and agricultural considerations as well as from the point of view of smooth operation of the irrigation system. Fields should be located in the immediate vicinity of the waste water resources. The requirement of a minimum distance of about 200 m between the irrigated area and residential buildings must be observed. Also, at a wind velocity of more than 3.5 m/s, sprinkler irrigation, using waste water, should not be used.


EXCERCISES

1. 
   Describe important physical and chemical properties of soil which are important from considera-tions of irrigation.
   

2. 
   What is the meaning of consumptive use? On what factors does it depend? How would one calcu-late the consumptive use for a given crop?
   

3. 
   The field capacity and permanent wilting point for a given soil are 35 and 15 per cent, respec-tively. Determine the storage capacity of soil within the root zone of the soil which may be taken as 80 cm. At a given time the soil moisture in the field is 20 per cent and a farmer applies 25.0 cm of water. What part of this water would be wasted? Assume porosity of soil as 40 per cent and relative density as 2.65.
   

4. 
   Determine the frequency of irrigation for the following data:
   

Field capacity of soil	30%
Permanent wilting point	10%
Management allowed deficit	45%
Effective root zone depth	0.75 m
Consumptive use	12 mm/day
Apparent specific gravity of soil
(including the effect of porosity)	1.6

5. 
   The consumptive use for a given crop is 90 mm. Determine the field irrigation requirement if the effective rainfall and the irrigation efficiency in the area are 15 mm and 60 per cent, respec-tively.
   

5. 
   The following data were obtained in determining the soil moisture content at successive depths in the root zone prior to applying irrigation water:
   

Depth of sampling	Weight of moist soil	Weight of oven-dried
	sample	soil sample
0-25 cm	1.35 N	1.27 N
25-50 cm	1.36 N	1.26 N
50-75 cm	1.23 N	1.15 N
75-100 cm	1.11 N	1.02 N

120 IRRIGATION AND WATER RESOURCES ENGINEERING
The bulk specific gravity of the soil in the root zone is 1.5. The available moisture-holding capac-ity of the soil is 17.8 cm/m depth of soil.
Determine (i) the moisture content at different depths in the root zone, (ii) the moisture content in the entire root zone prior to irrigation, and (iii) depth of water to be applied to bring the moisture content to the field capacity.

7. 
   For the following data pertaining to a cultivated land, determine irrigation interval and amount of irrigation water needed at each irrigation so that the moisture content at any stage does not fall below 40 per cent of the maximum available moistures.
   

Field capacity of soil	= 35%
Permanent wilting point	= 12%
Porosity of soil	= 0.42
Depth of root-zone soil	= 1.20 m
Consumptive use	= 12 mm per day
Application efficiency	= 60%

8. 
   Describe various methods of irrigation mentioning their advantages, disadvantages and appli-cability to different field conditions.
   

REFERENCES

1. 
   Hansen, VE, OW Israelsen, and GE Stringham, Irrigation Principles and Practices, 4th ed., John Wiley & Sons, 1979.
   

2. 
   Walker, WR and GV Skogerboe, Surface Irrigation: Theory and Practice, Prentice-Hall Inc., USA, 1987.
   

3. ...... Irrigation/Drainage and Salinity, FAO/UNESCO Publication, 1973.

4. 
   Bodman, GB and EA Coleman, Moisture and Energy Conditions during Downward Entry of Water into Soils, Proc. of American Society of Soil Science, 1943.
   

5. ...... Training Manual on Irrigation Water Needs, FAO Publication, 1982. 6. ...... Drip Irrigation in India, INCID, New Delhi, 1994.