Irrigation

[kheteshwar borawat]

Irrigation

Introduction

From the point of view of sustained agricultural production and all round development, water is the most precious resource. Presently about 92 per cent of water is used for agriculture, 2 per cent for industries, and 6 per cent for drinking and domestic purposes. For farmers natural rainfall is still the most important source of water for farms. About 75 per cent of the total cultivated area in the country depend on rainfall to sustain crop production. Success in dry farming depends on moisture conservation practices and judicious use of available water for irrigation.

Under this situation, it is imperative to utilize the available irrigation water most scientifically so that the water use efficiency can be increased manifold.

need for irrigation

Irrigation is an artificial application of water to the soil. Irrigation has become necessary because of the constraint of using natural rainfall as the reliable source of water for farms and the difficulties in storing rain waters due to lowering of sub-soil water levels. In India, monsoon is usually erratic, sometimes delayed or nearly creating drought like situations. Though the monsoons usually last for four months, the rains are received only in the first two or the last two months. Due to this, it is very difficult to decide the strategies of cropping patterns. The mid-season changes in the cropping patterns, though, are suggested but are very difficult to follow. The sufficient recharging of ground water is essential for sustainable fanning, but usually the purpose is defeated due to erratic rains.

sources of irrigation

Canals, Wells, Open Wells, Tube Wells, Artesian Wells, Bore Wells, Tanks, Filter Points

Rainfall

Natural rainfall has been the most important source of water for farming for the Indian farmer. Rainfall is dependent in different degrees, on the South-West monsoon, North-East monsoon, on shallow cyclonic depressions and disturbances and on violent local storms. India receives most of its rainfall from the South - West monsoon originating in the Indian Ocean. About 75 % of the rainfall is received in four months i.e., June to September. Unequal geographical distribution, unequal seasonal distribution and frequent departures from the normal rainfall characterize the rainfall of this country.

• South - West Monsoon Rainfall received during the months of June - July is critical and the fate of the Kharif crop depends very largely on distribution and amount of rain during these two months. South-West monsoon is responsible for 75-80% or more of the total annual rainfall in the country.
• North - East Monsoon During October - November cyclonic storms form in the Bay of Bengal and when they strike coastal Andhra Pradesh or coromandel coast they bring heavy rain to these areas. About 11 % of the total rainfall in the country is received during this season.

stages of crop when irrigation is required

During the growth span, the plant passes through various phases and the stages of growth. The growth rhythm of plant is slow during some stages and fast during some other stages. Accordingly plant demands variable supply of water. The growth period of irrigated dry (ID) crops can generally be divided into 3 phases namely vegetative, reproductive and

Each of these phases has different stages.

• Vegetative phase: The early vegetative phase consists of crop establishment or initial stage during the first 2 - 3 weeks after sowing. This is followed by crop development stage, which last for 2 - 6 weeks in different crops.
• Reproductive or flowering phase: The reproductive or flowering phase comprises the period from initiation of buds to 75 % flowering. This period in most of the seasonal ID crops last for 2 - 3 weeks and in two seasonal crops and perennial crops for 4 - 6 weeks or more.
• In yield formation stage otherwise known as ripening phase the end product is formed. The flowering and yield formation period together is known as mid-season stage. During the last part of the ripening phase the crops undergo yellowing and drying to mature. This period is called maturity stage or late season stage and it lasts for 2 - 4 weeks in most crops. The entire reproductive phase is highly sensitive growth period when the growth rhythm is fast. Therefore the soil water stress should be avoided during this period. Active vegetative phase and yield formation stage are moderate in sensitivity while initial establishment and maturity stages are least sensitive to water stress.

Some crops like Cotton, Groundnut and pulses even prefer stress during early vegetative growth to suppress excessive vegetative growth. In many crops the initial establishment and flowering stages are highly sensitive to excess water conditions resulting in poor performance of the root system and also shedding of flowers, in addition to lodging at maturity in some crops.

Need for Scientific methods of irrigation

Limitations in the availability of water for irrigation purposes has necessitated the use of scientific methods of irrigation. The following are the major limitations of water availability for the agricultural crops.

• Erratic monsoons

• Priority for drinking water during scarcity

• Priorities for industries

• Water for power generation

• High Lifting charges

• Lower Sub-soil water level

The present irrigation systems suffer from a number of constraints and disadvantages leading to improper utilization of water.

Constraints in the proper utilisation of water using traditional irrigation methods:

Land levelling

Irrigation water cannot be used efficiently by farmers, if the land is not properly levelled. Often the farmers do not have the adequate knowledge about land levelling and shaping. For better irrigation efficiency, land levelling is a prerequisite though it is expensive also.

Dominant area

The main activities under irrigation projects are on farm works such as construction of field channels and drains, promoting small buns across small streams, organising supply facilities, etc. Canal water is easily and conveniently available to the fields which are situated at low level and at the head ends of the channels while it is scarcely available at the tail ends and higher fields. Wherever the lift irrigation from canal is permitted, it becomes very costly and uncertain.

Wastage of water

Due to construction of kucha channels a large quantity of water is lost in transport due to percolation, leakages etc. Thus overall water requirement increases many more times than actually required for physiological processes of crop growth. Sometimes accumulation of water in the field is also observed as waste due to lack of proper drainage.

Excess labour involvement

In the conventional irrigation system, from outlet (distributions) to the fields, channels are made in the field which usually are subjected to leakages, etc. This requires constant supervision by labourers so that irrigation water is not lost to other fields. For distribution and irrigation also labourers are required.

Disadvantages of conventional irrigation methods:

• An estimated level of 43 per cent efficiency in surface water utilisation and 70 per cent efficiency in ground water utilisation (at farm level) has been recorded. This shows that a large quantity of water is not properly utilised in the present irrigation system.
   

• The distribution of water in the field by main canal, sub canals and distributions up to the outlet points into the fields and subsequently in the far off drainage points, is uneven due to poor levelling. This sometimes creates problems for getting sufficient and adequate irrigation to the crops.
   

• Crops are usually subjected to cyclic changes of flooding and water stress situations, by providing heavy irrigation at one time and leaving the fields to dry up for about 10 to 15 days. The moisture availability to the crops fluctuates from saturation to stress and again to saturation. This results in poor yields of the crops.
   

• The fields situated in low areas always get excess water causing prolonged water logging due to lack of levelling of fields. Thus crops are subjected to water logging resulting in poor yields.
   

• In the fields about 10-15 per cent of land is utilised for preparing channels and distributions, etc. which decreases effective area of cultivation.
   

• Extensive areas of land in the arid and semi-arid regions of India have gone out of cultivation due to rise of water table and accumulation of salts. Excessive irrigation and poor water management are the chief reasons of water- logging and gradual build up of excessive salts. Progressive build up of soil salinity has made the soils unsuitable for cultivation.
   

• Sometimes a large quantity of irrigation water is subjected to deep percolation and seepage resulting in the use of higher amount of fertilisers.
   

• In the absence of planned proper combinations of spacing, length and slope of furrows and suitable size of irrigation streams and duration of the water application, it will be very difficult in regulating the flow of irrigation water in the fields.
   

• Slow and gradual changes in soil physical properties, particularly soil structures due to top soil erosion, takes place under present irrigation practices.
   

• Cyclic changes of wetting and drying of soil particles around the roots result in disturbances of root activity of the crop.
   

• In conventional irrigation, unmanageable undesirable weed growth is usually observed.
   

• Due to the improper irrigation methods in the conventional irrigation system, crops cannot be raised satisfactorily. The moisture in the soil fluctuates between too dry to too wet stages.

Scientific Methods of Irrigation

Drip Irrigation

It is defined as the precise, slow application of water in the form of discrete or continuous or tiny streams of miniature sprays through mechanical devices called emitters or applicators located at selected points along water delivery lines.

It is also called trickle irrigation. Drip irrigation is adopted extensively in areas of acute water scarcity and especially for crops such as Coconut, Grape, Banana, Ber, Citrus, Sugarcane, Cotton, Maize, Tomato, Brinjal and plantation crops. The advantages of drip irrigation are

1. Controlled application of water as per the needs of plants at low pressure to limited soil areas (root zones).
    

2. Water saving to the tune of 50 to 70 per cent by reducing the total evaporative surface, reduction in runoff and controlling deep percolation losses.
    

3. Soil erosion is minimal, due to no runoff water on surface.
    

4. Weed growth is minimum.
    

5. Water loss through transpiration is low.
    

6. Development of surface crust and determination of surface soil structure is avoided. Soil compaction is less.
    

7. Limited soil wetting permits undisturbed cultural practices.
    

8. It is possible to obtain better yield and quality of crops by controlling soil moisture-air -nutrients level.
    

9. We can save the fertilisers by monitoring the supply of nutrients as per the need of the crop.
    

10. Improvements in biological fertility can be achieved by avoiding pollution.
     

The initial expenditure in establishing a long term drip irrigation system usually costs around Rs.32,000/- per hectare. This includes the cost of all the components mentioned earlier except the source of water and pump set on the well or boring. This heavy initial expenditure usually discourages the cultivator to install drip systems in his fields, though subsidies are given on the purchase of these equipment by the central and state governments. On the long term basis, the drip irrigation system is more economical and paying for the farmers.

Sprinkler Irrigation

The main characteristics of sprinkler irrigation system are

• Sprinkler irrigation system conveys water from the source through pipes under pressure to the field and distributes over the field in the form of spray of 'rain like' droplets. It is also known as overhead irrigation.

• Different types of sprinkler systems namely portable, semi-portable, semi-permanent and permanent are in vogue. But due to increased labour costs and energy costs, different types of sprinklers are developed.

• Center-pivot system is largest sprinkler system with a single machine can irrigate upto 100 ha. A center - pivot sprinkler consists of a series of sprinklers mounted on a lateral pipe, 50 - 800 m long, mounted or carried by a row of five or more mobile towers.

• One end of the lateral is fixed on a pivot pad. The unit rotates around a center pivot where water is pumped into the pipe, and water is distributed through sprinkler fitted on lateral. The limitations of this system are,

• 10 - 20 % of area is not irrigated at the corners of square or rectangular plot.

• High energy requirement and huge cost of the equipment.

• Now lateral - move systems are developed to overcome the draw backs in pivot-pivot system for irrigating square or rectangular plots. This irrigation system consists of lateral - move systems, which move up and down the field.

Sprinkler irrigation can be advantageously chosen in the following situations

1. When the soil is too shallow eliminating the possibility of leveling of lands.

2. When the land is too steep (> 1% slope).

3. When light (< 5 cm) and frequent irrigations are to be given.

4. When soils are very sandy (rapidly permeable coarse textured soils) and

5. When supplemental irrigation is to be given to dry land crops during prolonged dry spells, without any land preparation.

Sprinkler irrigation can be a disadvantage in the following situations

1. High winds (> 12 km/hr) cause improper distribution of water.

2. Evaporation losses are high from sprinkler irrigation especially under high temperature and low relative humidity conditions.

3. The initial cost is high.

Some sort of knowledge is needed for successful operation of sprinkler system.

problems of under irrigation

• Under irrigation causes reduction in photosynthesis due to reduction in photosynthetic rate, chlorophyll content and leaf area.

• Due to under irrigation, water deficit occurs, as a result stomata are closed, so that reduction in transpiration takes place.

• Translocation of assimilates is also affected by water stress.

• Respiration rate decreases with increased moisture stress.

• Due to under irrigation enzymatic activity decreases. So that accumulation of sugars and amino acids takes place due to breakdown of carbohydrates and proteins.

• Due to under irrigation hormonal balance is altered.

• Due to under irrigation reduction in fixation, uptake and assimilation of nitrogen takes place.

Problems of Excess Irrigation  

• Excess irrigation causes several changes in the soil and plant resulting in reduced growth and in some cases death of plants. 

• Germinating seeds are sensitive to water logging since they are totally dependent on the surrounding soil space for oxygen supply.

• Yield of cereals depressed if the excess irrigation given at panicle development stage. 

• Excess water causes injury to the plant due to low oxygen supply to the root system and accumulation of toxic substances in soil and plant.

• Wilting of tobacco takes place when bright sunshine occurs after a prolonged wet spell.

• Leaching of nitrates and denitrification occurs resulting in nitrogen deficiency.

• Shoot elongation, senescence, abscission and production of adventitious roots takes place as a result of continuous excess irrigation.

• Respiration in the roots change from aerobic to anaerobic with the result, toxic substances accumulates in roots and damage the root system.

• Permeability of roots decreased due to shortage of O2. It results in decreases water and nutrient uptake. 
  

Losses of Water 

Generally water is lost through leaching, drainage, evapotranspiration and runoff. The following disadvantages will be observed due to water loss· 

• Soil becomes very hard. 

• The germination percentage will be decreased.

• The nutrients in the soil leach or evaporate.

• The root growth retards, so that plant becomes stunted as a result yields become reduced.

• Stomata becomes closed, so that the transpiration process caused as a result accumulation of gases or metabolic wastes increases, leads to death of the plant.

• The soil micro organism activity decreases. 

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