Need for Irrigation > Quantum of Water Required by Plants > Stages of Crop when Irrigation is Required > Critical Stages of Irrigation Requirement > Sources of Irrigation > Methods of Irrigation > Problems of Under Irrigation > Problems of Excess Irrigation > Losses of Water > Water Use Efficiency > Water Related Issues > Economics of Water Use >

Need for Irrigation

Irrigation is an artificial application of water to the soil for the following purposes

• Irrigation is needed for normal growth and yield of the plant.
• It is needed for metabolic processes of the plant.
• To reduce the soil temperature.
• For easy germination of the seeds from the soil.
• Irrigation water acts as a medium for transport of nutrients and photosynthates in the plant system.
• To provide crop insurance against short duration drought.
• To washout dilute salts in the soil.
• To reduce the hazard of soil piping.
• To soften tillage pans.

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Quantum of Water Required by Plants

• Water requirement of a crop is the quantity of water needed for normal growth,development and yield and may be supplied by precipitation or by irrigation or by both. Water is needed mainly to meet the demands of evaporation (E), transpiration (T) and metabolic needs of the plants. The water requirement of any crop is dependent upon,
• Crop factors like variety, growth stage, duration, plant population and growing season.
• Soil factors like texture, structure, depth,and topography.
• Climatic factors like temperature, relative humidity and wind velocity.
• Crop management practices like tillage, fertilization, weeding etc.,

Quantum of Water Requirement (mm) of Different Crops

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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
• ripening phases.
• 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 last 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.

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Critical Stages of Irrigaton Requirement

• The water balance in ID crops is refered to the soil water storage in the root zone and not to the level of standing water in the field, as in case of paddy.
• The critical stages or otherwise known as sensitive stages of different crops for irrigation water requirement are as follows,

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Sources of Irrigation

Canals

• The practice of equating a hectare of canal irrigation area with a hectare of area served by ground water is not appropriate. A striking analysis carried out in four states, Punjab, Andhra Pradesh, Haryana and Tamil Nadu has shown that the yield of food grains under well irrigation is very much higher compared to the yield under canal irrigation. The reason for this difference in yield between sources of water supply is not so far to seek. The farmer who depends on canal irrigation is at the mercy of a system over which he has no control.
• The utility of irrigation is judged by the cropping intensity. In most parts of the country, the cropping intensity is 200 % in the tube well (or) dug well irrigated land as against 100 % or less in canal irrigated land. About 18 % of the tail end area in canal commands of South India are particularly vulnerable for erratic and insufficient supply of water, not only because of losses to the extent of 50 % due to seepage, percolation and evaporation in transit from the storage reservoir to the farmers field but also because farmers in the upper reaches of the systems often succeed in cornering more than their due entitlement of water.

Wells

• Awell is a hydraulic hole to the water strata. Water in the well stands at a height equal to the static water level. There are different types of wells namely open well, tube well, artesian well, and bore well.

Open Wells

• The dug out wells upto water bearing strata of the aquifer are open wells. They derive water from the formation hole to the ground surface. The large diameter of the open wells permits the storage of water.

Tube Wells

• These are sunk by inserting pipes below ground surface and passing through different geological formations of water bearing and non-water bearing strata.

Artesian Wells

• Due to pressure, water from well comes to the ground without pumping are generally known as artesian wells.

Bore Wells

• When ground water availability is at deeper layers exceeding 16 to 20 m with hard strata, bore wells are suggested.

Tanks

• Large tanks irrigating more than 2000 ha are classified under medium irrigation source. Small water reservoirs behind earthen dams are tanks. Though the primary purpose of tank is for irrigating crops, it also provides drinking water for humans and cattle in the villages. Monsoon rains fall erratically and confined only to a few months in the year.
• Irrigation tanks serve to store and regulate water for crop production. In drought prone areas, tanks are considered to be a useful life saving sources. But day by day the area irrigated by tanks decreases due to neglect of maintenance of tanks, environmental degradation, cultivation of foreshore areas and cultivation of tank beds.

Filter Points

• These are shallow tube wells consisting of a well and a short length of casing pipe. Filter points are generally bored in deltaic regions where aquifer formation are of coarse sand and gravel and are very near to the surface. In coastal sands open dug wells are to be lined with concrete rings which is costly and also the availability of water is dependent on seepage water and season.
• To tap this water filter pipes (slotted filter pipes or PVC pipes with a conical bottom point) is driven inside the soil to a depth of about 9 to 15 m and water is lifted by means of ordinary pumpset from this filter point.

Rainfall

• 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.

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Methods of Irrigation

Flood Irrigation

• Flooding method of irrigation is exclusive for lowland rice though it is used for some other crops also. Water is allowed from the channel into the field without much control on either side of the flow. It covers the entire field and moves almost unguided.
• The ideal size of each plot or basin is 0.1 to 0.2 ha for economising water.Uneven distribution and low water application efficiency are the common drawbacks of this method.

Basin Irrigation

• Basin method is almost similar to check - basin method except that in the check-basin method entire field is irrigated while in basin method only the basin around the trees are irrigated.
• This method is suitable for fruit crops. Basins are generally round in shape, occasionally square in shape. The basins are small when the trees are young and their size is increased with age of the trees. Basins are connected by an irrigation channel.

Check-Basin Method

• Check-basin method of irrigation is the most common method among surface methods of irrigation. In this method the field is divided into small plots surrounded by small bunds on all the four sides.
• Water from head channel is supplied to the filed channels one after the other. Each field channel supplies water to two rows of check basins and water is applied to one basin after another. This method is adopted when the field is quite large and is not easy to level the entire field. In such situations, the field is divided into small strips and each strip into several plots by putting bunds and these plots are called check basins.
• The advantage of this method is that the water can be applied uniformly and effectively. It is suitable for close growing crops like groundnut, wheat, fingermillet, pearlmillet, paragrass etc.,. The disadvantages are more labour is required, more land is wasted under channels and bunds. Intercultivation is not possible due to bunds.

Border Strip Method

• The field is divided into number of stripes by forming bunds of around 15 cm height. These parallel earth ridges are called borders, and are formed to guide a sheet of flowing water across a field.
• The area between two borders is the border strip. Length of the strip ranges from 30 to 300 m and width from 3 to 15 m. However, the most common sizes are 60 to 90 m in length and 6 to 12 m in width.
• The size of border strips depend on stream size, soil structure and slope of the land. The borders are laid out along the general slope or on the contour. Water from the channel is allowed into each strip at a time. This method is suitable for close growing crops and medium to heavy textured soils, but not suitable for sandy soils.

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,
• No fertilizer nutrient loss due to localized application.
• High water distribution efficiency.
• Levelling of the field not necessary.
• Only root zone is saturated.
• Moisture always at field capacity in the root zone.
• Soil factor plays less important role in frequency of irrigation.
• No soil erosion.
• Highly uniform distribution of water i.e., controlled by each nozzle.
• Low labour cost.
• Variation in supply can be regulated by regulating the valves and drippers.
• Fertigation can be adopted with drip irrigation.
• The disadvantages of drip irrigation is expensive i,e., initial cost is more in installing drip method.

Sprinkler Irrigation

• 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 over head 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.
• Centre-pivot system is largest sprinkler system with a single machine can irrigate upto 100 ha. A centre - 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 centre 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 centre-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
• When the soil is too shallow eliminating the possibility of levelling of lands.
• When the land is too steep ( > 1% slope).
• When light (< 5 cm) and frequent irrigations are to be given.
• When soils are very sandy (rapidly permeable coarse textured soils) and
• When supplemental irrigation is to be given to dryland crops during prolonged dry spells, without any land preparation.

Disadvantages

• High winds ( > 12 km/hr) cause improper distribution of water.
• Evaporation losses are high from sprinkler irrigation especially under high temperature and low relative humidity conditions.
• The initial cost is high,
• Some sort of knowledge is needed for successful operation of sprinkler system.

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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 aminoacids 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.

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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 waterlogging 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. iv. 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.

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Losses of Water

• Generally water is last 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 leaches or evaporates.
• 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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Water Use Efficiency

• Water use efficiency is the yield of marketable crop produced per unit of water used in evapotranspiration.
• WUE = Y/ET
• Water use efficiency is also known as crop water use efficiency or consumptive water use efficiency (Ecm) if the water used for metabolic purpose of the crop (G) and is included with ET.
• ECU = Y/G+ET
• If yield is proportional to ET, water-use efficiency has to be a constant but it is not so. Actually, Y and ET are influenced independently or differently by crop management practices, while ET is mainly dependent on climate and soil moisture. Fertilization and other cultural practices for high crop yields usually increases WUE. The factors affecting WUE are nature of the plant, agronomic practices, climate, ET, irrigation, fertilization and plant population.
• There are considerable differences between plant species to produce a unit dry matter per unit amount of water used resulting in widely varying values of water use efficiency. The water use efficiency for few crops is listed below.

   

  Crop	Water requirement (mm)	Grain yield (kg/ha)	WUE (kg/ha mm)
  Rice	2000	6000	3.0
  Sorghum	500	4500	9.0
  Pearlmillet	500	4000	8.0
  Maize	625	5000	8.0
  Groundnut	506	4680	9.2
  Wheat	280	3534	12.6
  Fingermillet	310	4137	13.4

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Water Related Issues

Water Ph

• pH is the negative logarithm of hydrogen ion concentration. If pH is 7.0, it is considered as neutral. If the pH is less than 7.0 and H+ concentration exceeds OH- it is referred as acidic and if pH ranges 7 - 14 it is considered as alkaline. The pH is a sort of voltage measurement to cover the entire range of 0-14. The pH is one of the parameters to assess the water whether it is suitable for irrigation or not based on pH values.
• Main cations present in irrigation water are calcium, magnesium, sodium and potassium. In effluents and sewage waste waters ammonium and heavy metal cations are also found. The important anions like chlorides, carbonates and bicarbonates, sulphates and nitrates are also present in irrigation water.
• For appraisal of irrigation water quality the water samples are mainly analyzed for total salts (EC) relative proportion of cations, anions and toxic substances such as excess boron and fluorine. For example, the pH of bicarbonate (HCo3) waters is usually more than 7.5 and its determination may reflect the degree of sodicity in the sample.
• Sulphate content will be more in saline water having higher E.C. If boron content is more than 2.0 mg/1(ppm) in irrigation water, it is harmful to most of the crops. Fluorine content beyond 10 ppm in irrigation water is harmful indirectly to animals who feed on plants irrigated with high fluoride waters. Sodium at higher levels in irrigation water exerts a toxic effect on crop growth.
• Good irrigation water should not have excessive amounts of any salt or toxic substances.

Water EC

• Natural water has E.C value of much less than one unit. These values are reported as milli mhos (EC x 10-3) or micro mhos (EC x 10-6) at 250C. Electrical conductivity serves as a guide to know the extent of soluble salts present in irrigation water. The criteria for judging the quality of irrigation water is the total salt concentration as measured by electrical conductivity. The harmful effects increases with increase in total salt concentration.
• Irrigation water may be classified based on EC are,

C1 - Low Salinity Water

• If electrical conductivity is less than 0.25 ds/m, the irrigation water is classified as low salinity water. It can be used for irrigation on all soils and on most crops but leaching is required in case of extremely low permeable soil.

C2 - Medium Salinity Water

• It has EC between 0.25 to 0.75 ds/m. This water can be safely used for crops with moderate salt tolerance. The soil should have moderate level of permeability and leaching to avoid accumulation of salts.

C3 - High Salinity Water

• Water with EC ranges of 0.75 to 2.25 ds/m is called high salinity water. This water can not be used on soils with poor drainage. This water can be used for salt tolerant crops by providing good drainage and also by practicing management practices for salinity control.

C4 - Very High Salinity Water

• If EC is more than 2.25 ds/m the water is classified as very high salinity water. It is not suitable for irrigation under ordinary conditions but may be used occasionally if the soil is permeable by providing adequate drainage.

Central Soil Salinity Research Institute (CSSRI) suggested another classification of Irrigation water based on EC as follows,

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Economics of Water Use

• Average yields of irrigated crops are below the economic optima because data on the best combination of fertilizer, plant population and irrigation regime are meagre to recommend to the farmers. Maximum WUE can not alone be the goal always. The economics of obtaining high yields dominate the scene. Yield increases from fertilizers, plant population, irrigation, etc., follows some kind of decreasing increment function after a stage, such that each successive unit of input produces less profit than its predecessor.
• The general tendency is to over-irrigate, especially if water is not brought on the basis of quantity used. This tendency can be avoided only if information is available on the most efficient way to use water, and if field service is organized to advice the farmer on when to irrigate and how much water to apply at each irrigation for a certain level of fertilization and plant population. The three important approaches listed below are to be taken into account for irrigation.

Soil Based Criteria

• Depletion of available soil moisture i.e., feel and appearance method.

Plant Based Criteria

• Critical stages approach, visual symptoms of the plant, water content, leaf temperature.

Climatological Criteria Iw/Cpe Ratio

• Surface irrigation methods are commonly used for various crops. But rice is irrigated by flooding. Crops like Potato, Maize, Sugarcane, Cotton are commonly irrigated with furrow method. Basin method of irrigation is adopted for fruit trees.
• The amount of water to be applied at each irrigation depends on the amount of moisture depleted in the effective root zone depth.
• The moisture extraction pattern from different depths of the soil within the crops root zone depth in deep uniform soils is about 40 % of the total moisture from first quarter of the root zone, 30 % from the second, 20 % from the third and 10 % from the last quarter. At early stages of crop growth, the depth of water applied should be less since the root system is shallow.
• Generally the amount of water applied at each irrigation is about 50 mm in red soils and 60 mm in black soils.

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