Water though precious, is one of the scarce resources required for food grain production in our country. Every care should be taken to utilize this resource efficiently. Selection of an appropriate method of irrigation based on crop and soil type is a prerequisite for efficient utilization of water. It also helps in enhancing and sustaining soil productivity in the long run. Various methods of irrigation have been comprehensively discussed in this booklet. Characters of an efficient method of irrigation, factors affecting the suitability of an irrigation method and cultural practices for increasing water use efficiency have also been briefly highlighted.
Dr. K. T. Chandy, Agricultural & Environmental Education,
Water is an important constituent of the plant body. Plants need water to carry on different physiological processes essential to their growth and development. A great deal of water from plant body is wasted through the process of transpiration. Therefore, to compensate this loss of water, absorption of water from soil is an important phenomenon. Soil gets water mainly from two sources viz. precipitation and irrigation.
Hence, irrigation is a process, other than natural precipitation, which supplies water to crops, orchards, grass, or any other cultivated . plants. In the wetter parts of the world where rainfed cultivation is practiced, the farmer’s activities consist of selecting suitable land, preparing the soil for cultivation; sowing, tending and harvesting the crops. Natural rainfall provides the water needed. But in many other places otherwise favourable for cultivation, natural rainfall does not provide all the water needed and, as such, irrigation of crops is required to make up this deficiency.
Techniques adopted for carrying water from its source to the crop are called methods or modes of application. These are:
1. flooding -wetting all the land surface;
2. furrows -wetting only certain part of ground level;
3. sprinkler -in which the soil is wetted in much the same way as rains; 4. sub-surface irrigation -in which surface is wetted very lightly, but the sub soil is fully saturated; and 5. localized irrigation -in which water is applied at each individual plant at. a near daily rate.
II. Characteristics of an Efficient Irrigation Method
An efficient method of irrigation should fulfill five major objectives viz. (1) distribution of water uniformly over the field according to crop need, (2) storage of maximum fraction of water in the root zone for plant use, (3) crop growth should not be adversely affected, (4) soil transport or loss is negligible, and (5) the technique used is economically sound and adoptable at the farm.
III. Factors Affecting Suitability of Irrigation Method
The selection of a suitable irrigation method for a particular farm location depends upon the following factors.
Textural, crusting, cracking and infiltration characteristics of surface soil; nature and depth of relatively impermeable layers in sub-soil, if any; water storage capacity of root zone; nature and extent of land slope; size of field; surface drainage; nature and extent of salts in surface and sub-soil are the salient soil factors influencing between of an irrigation method. 2. Water
Nature of water supply (continuous or rational). source (pump or canal), size of water delivery, quality of irrigation water, and quantity of water supply (adequate or limited) area few factors that must be taken into consideration while deciding the method of irrigation.
Nature of crops, area under different crops and their rooting behaviour, optimum depth and timing of irrigation, sensitivity of crops to excessive soil moisture, cultural operations required, etc. must be considered at the time of selection of irrigation method for a crop.
There are other factors influencing irrigation method like outlook, managerial efficiency and financial resources of the farmer; nature of the farm machinery used; availability and cost of labour; wear and tear maintenance facilities and costs of irrigation equipments; and availability of power supply. As far as possible, an irrigation method should not only provide a high level of water application efficiency, but also ensure its economic viability, sustained soil productivity and wide’ adaptability to prevalent feature of the farm. Generally, irrigation methods followed in India lack in an economic use of irrigation water.
IV. Methods of Irrigation
Methods of irrigation, generally adopted in India, can be represented in the flow chart.
Methods of Irrigation
A. Surface irrigation
In this method, water from an irrigation channel is allowed to reach a part or whole of the field and spread by the gravitational flow incidental to the slope of the land. Water may be distributed to the crops in border strips, check basins or furrows. The important requirements to obtain high efficiency in surface method are (1) properly constructed water distribution systems to provide adequate flow of water to the fields; and (2) proper grading and leveling of land to achieve uniform distribution of water.
1. Border strip method
The border method of irrigation makes use of parallel ridges to guide a sheet of flowing water as it moves down the slope. The land is divided into a number of long parallel strips called borders that are separated by low ridges. The border strip has little or no cross slope but has a uniform gentle slope in the direction of irrigation. The essential feature of border irrigation is to provide an even surface over which the water can flow down the slope with a nearly uniform depth. Each strip is irrigated independently by turning into a stream of water at the upper end. The water spreads and flows down the strip in a sheet confined by the border ridges. The irrigation stream must be large enough to spread over the entire width between the border ridges without over topping them. When the advancing water front either reaches the lower end, or a few minutes before or after that, the stream is turned off.
The water temporarily stored in the border moves down the strip and infiltrates into the soil, thus completing the irrigation. It is adapted to most soils where depth and topography permit, 1 the required land leveling at a reasonable cost and without any ! permanent reduction in soil fertility. It is, however, more suitable to soils having moderately low to moderately high infiltration rates. It is generally not used in coarse sandy soils that have very high infiltration rates. It is also not well suited to soils having a very low infiltration rate. This method is suitable to irrigate all close growing crops like wheat, barley, fodder crops and legumes. It is, however, not suitable for crops like rice which requires standing water during most part of its growing season.
i. It is easy to construct border ridges even with some simple farm implements like a bullock drawn A-frame ridger or tractor-drawn disc ridger. ii. Labour requirement in irrigation is greatly reduced as compared to the conventional check basin method of irrigation. iii. Uniform distribution and high water application efficiencies are possible if the system is properly designed. iv. Large irrigation streams can be efficiently used.
v. Operation of the system is simple and easy.
vi. Adequate surface drainage is provided if outlets are available.
i. It requires an extensive land grading which is too expensive. ii. It is mainly suitable for deep soils with the availability of large flow of water. iii. Drainage may be essential.
iv. Water wastage is frequently observed.
Straight and contour borders
If the borders are constructed along the general slope of the field, they are known as straight borders or slope borders, and if they are constructed across the general slope of the field they are called contour borders. When fields can be leveled to desirable land slopes economically and without affecting its productivity, graded borders are easier to construct and operate. In case where land slope exceeds safe limit, fields are undulating and leveling is not feasible, borders may be laid across the slope. The design of a contour border is the same as that of a straight border. Each contour border is level crosswise and has a uniform longitudinal gradient as in a straight border. The width and length of a contour border are identical to that of a straight border for a particular set of conditions.
In laying contour borders, the field is divided into a series of strips on the approximate contour, and each strip is leveled as an independent area. Thus, a series of strips are formed in successive elevations around the slope. The vertical interval between the adjacent benches should, as far as possible, be limited to 30 cm, but should not exceed 60 cm. The height of ridge should be sufficient to check both the normal irrigation stream and run-off.
2. Check basin irrigation
Check basin irrigation is the most common method of irrigation in India and in many other countries. This is the simplest in principle of all methods of irrigation. There are many systems in its use, but all involve dividing the field into smaller units so that each has a nearly level surface. Ridges-are-constructed around the areas forming basins within which the irrigation water can be controlled. The basins are filled to the desired depth and the water is retained until it infiltrates into the soil. The depth of water may be maintained for considerable periods of time by allowing the water to continue to flow into the basins.
The distinguishing features of various uses of check basin method of irrigation involve the size and shape of the basins and ” whether irrigation is accomplished by intermittent or continuous” collection of water in the basins. The ridges or bunds may be temporary for a single irrigation as in the pre-sowing irrigation of seasonal crops. They may be semi-permanently constructed for the repeated use in the case of paddy fields. The size of ridge will depend on the depth of water to be impounded as well as on the stability of the soil when wet.
Water is conveyed to the field by a system of supply channels and lateral field channels. The supply channel is aligned on the upper side of
the area and there is usually one lateral channel for every two rows of check basins. Water from the laterals is turned into the beds and is cut off when sufficient water has been administered into the basin. Water is retained in the basin until it soaks into the soil. The size of the irrigation stream is not critical as long as it is sufficient to provide a coverage of the entire strip in a relatively short time span required to apply the desired amount of water into the soil. As the infiltration rate of soil increases, stream size must be increased or the size of the basins reduced in order to cover the area within a short period of time. A large size irrigation stream will permit a comparatively larger size of the basin.
The size of check basin may vary from one square metre, used for growing vegetables and other intensive cultivation, to as large as two hectares or more, used for growing rice under wet land conditions. When the land can be graded economically into nearly level fields, the basins are rectangular in shape. In rolling topography the ridges follow the contours of the land surface. The contour ridges are connected by cross ridges at intervals. The vertical interval between contour ridges usually varies from 6 to 12 cm. in case of upland irrigated crops like wheat and 15 to 30 cm in case of low land irrigated crops.
In irrigating orchards, square to contour basins may be used as in other crops. When the plants are widely spaced the ring method of basin irrigation is adopted. The rings are circular basins formed around each tree. The ring basins are small when the plant is young. The size is increased as the plant grows.
Check basin irrigation is suited to smooth, gentle and uniform land slopes and for soils having moderate to slow infiltration rates. Steep slopes require complex layouts and heavy land leveling. Both row crops as well as close growing crops are adapted to basins as long as the crop is not affected by temporary inundation. The method is especially adopted for, irrigation of grain and fodder crops in heavy , soils where water is absorbed very slowly. It is also suitable in very permeable soils which must be covered with water rapidly to prevent i excessive deep percolation loss of water at the upstream end.
i. Since in this method the entire area is not flooded, it ensures -high water use efficiency. ii. Excessive seepage loss can be avoided by adopting this practice iii. Damage to plants and loss of soil nutrients do not occur in this practice:..
i. The major disadvantage of check basin method of irrigation is that the ridges interfere with the movement of animal drawn or tractor drawn implements for inter culture operations or harvesting of crops. ii. Considerable land is occupied by ridges and lateral field channels and crop yields are substantially reduced. iii. The method impedes surface drainage.
iv. Precise land grading and shaping are required.
v. Labour requirement in land preparation and irrigation are much higher.
3. Furrow irrigation
The furrow method of irrigation is used in irrigation of row crops with furrows developed between the crop rows in the planting and cultivating processes. The size and shape of the furrow depends on the crop grown, equipment used and spacing between crop rows. Water is applied by running small streams in furrows between the crop rows. Water infiltrates into the soil and spreads laterally to , irrigate the areas between the furrows. The length of time water takes to flow in the furrows depends on .the amount of water required to replenish root zone and the infiltration rate of the soil. Both large and small irrigation streams can be used by adjusting the number of furrows irrigated at anyone time to suit the available flow. In areas where surface drainage is necessary, the furrows can be used to dispose off the run-off from rainfall rapidly.
Furrow irrigation can be used to irrigate all cultivated crops planted in rows, including orchards and vegetables. Among the common cultivated crops of India, the method is suitable for irrigating maize, sorghum, sugarcane, cotton, tobacco, groundnut, potato and other vegetables. Furrows are particularly well adapted to irrigating crops which are subject to injury from accumulated surface water or susceptible to fungal root rot. Furrow irrigation is suitable to most soils except sands that have a very high infiltration rate and provide poor lateral distribution of water between furrows.
i. Water in the furrows contacts only one half to one fifth of the land surface, thereby reducing puddling and crusting of the soil, and evaporation losses. ii. Early sowing is possible which is a distinct advantage in heavy soils. iii. It can be safely adopted on the sloppy lands by opening the furrows across the slope. iv. This .method reduces labour requirement in land preparation and Irrigation. v. Compared to check basin method, there is no wastage of land in field ditches.
i.It requires skilled labourers to operate. .
ii.It may cause serious erosion, if excess water flows over the ridges. iii.Difficult to carry on mechanical operations.
Irrigation furrows may be classified into two general types is based on their alignment. They are (i) straight furrows, (ii) contour furrows. Based on their size and spacing furrows, may be classified as deep furrows and corrugation.
a. Deep furrows
As mentioned above, deep furrows are of two types i.e. straight furrows and contour furrows.
i. Straight furrows
Straight furrows, like borders, are laid down across the prevailing land slope. They are best suited to sites where the land slope does not exceed 0.75 per cent. In areas of intense rainfall, however, the furrow grade should not exceed 0.5 % so as to minimise the erosion hazard.
ii. Contour furrows
Contour furrow method is similar to the graded furrow method in that the irrigation water is applied in furrows, but the furrows carry water across the sloping field rather than down the slope. Contour furrows are curved to fit the topography of the land. The furrows are given gentle slope along its length as in the case of graded furrows. Field supply channels rundown the land slope to feed the individual furrows and are provided with erosion control structures.
b. Corrugation irrigation
Corrugation irrigation consists of running water in small I: furrows, called corrugatio’1s which direct the flow down the slope. It is commonly used for irrigating non-cultivated close growing crops such as small grains and for pasture growing on steep slopes. Corrugation may be used in conjunction with border irrigation on lands with relatively flat slopes in order to get uniform coverage with water. The water is applied to small furrows and the crop rows are I not necessarily related to the irrigation furrows. In this method the soil may be prepared and the crop plan without regard for irrigation layout After the seed is sown, but before the germination has taken place, a corrugation is used making small furrows or corrugation to aid in controlling irrigation water. The corrugation may be used with a simple bamboo corrugators or cultivators equipped with small furrowers or other similar implements. Corrugations are V -shaped or U shaped channels about 6 to 10 cm deep. They are spaced 40 to 75 cm apart. The entire soil surface is wetted slowly by the capillary movement of the water which flows in the corrugations.
This method of irrigation is the most suitable in loamy soils in which the lateral movement of water occurs readily. Clay soil having poor infiltration capacity are quite unsuitable for irrigation by corrugations. This method is also not suitable for irrigation in deep sandy soils due to excessive loss of water by deep percolation before the entire soil surface is wetted.
Saline or alkaline soils or irrigation water having salt content is not suitable for this method because of the danger of salt accumulation on surface soils due to capillary movement of water.
i. Corrugation irrigation minimizes the crusting effect on the surface soil which may occur when the entire surface is flooded.
ii. High water use efficiency is ensured.
iii. It can be used for germinating seeds which are drilled or broadcast in the soil.
i. Not suitable for a wide range of soils.
ii. This is a labour intensive method.
iii. If the corrugations are placed across the land slope, the over flow of water may move down into the lower corrugations and may cause severe soil erosion.
B. Sub-surface irrigation
In this method of irrigation water is applied below the ground surface by maintaining an artificial water table at some depth depending upon the soil texture and the depth of the plant roots. Water reaches the plant roots through capillary action. Water may be introduced through open ditches or under ground pipelines such as tile drains or mole drains. The depth of open ditches varies from 30 to l00cm and they are spaced about 15 to 30 metres apart. This water application system consists of field supply channels, ditches or trenches and drainage ditches for the disposal of excess water. The irrigation ditches should be suitably spaced to cover the whole field adequately.
This method is suited to soils having reasonably uniform texture and are permeable enough for water to move rapidly both vertically as well horizontally within and for some distance below the crop root zone. The soil profile must control a barrier against excessive loss through deep percolation. Topography must be smooth and nearly level or slight slopes very gentle and uniform.
1.In soils having low water capacity and a high infiltration rates.: where surface methods cannot be used and sprinkler system : is very expensive, sub-surface irrigation method can be used effectively. ji. Evaporation loss from ground surface are minimum.
ill. In this method, it is possible to maintain the water level at optimum dephs for crops required at different growth stages.
i. It is quite expensive and labour intensive in the beginning. ii. The method requires an unusual combination of natural conditions, therefore its scope is limited. iii. Frequent removal of accumulated soil and other materials from channels is necessary.
In India, this i1rigation is practiced to a limited extent for growing vegetable crops around Dal Lake in Kashmir and for irrigation of coconut palms in the organic soils of Kuttanad area in Kerala.
C. Sprinkler irrigation
In the sprinkler method of irrigation, water is applied above the ground surface as spray. The spray is developed by tlte flow of water under pressure through small orifices or nozzles. The pressure is obtained by pumping with careful selection of nozzle sizes, operating pressures and sprinkler spacing. High efficiency in water application/distribution can be obtained with sprinkler system. Sprinkler systems are of generally two major types viz. (i) rotating head system, and (ii) perforated pipe system.
In case of rotating head system small nozzles are placed on riser pipes and these riser pipes are fixed at an even interval along the length of lateral pipes which are placed on the ground surface. However, they can be mounted on posts exceeding the crop height and made rotating through 90 degree. In rotating sprinkler, the most, important device to rotate the sprinkler head is a small hammer activated by the trust of water striking the vane connected to it.
In case of perforated pipe system, holes are perforated in lateral irrigation pipes which is especially designed to distribute water with a good deal of uniformity. This system is usually designed for low operating pressures (i.e. 0.5 to 2.5 kg/sq cm). Due to this low pressure, the system is attached to an overhead tank to achieve the requisite pressure head. The sprays are directed on both sides of the pipe which cover a strip of land from 6 to 15 metres wide.
Nearly all cultivable soils can be sprinkler irrigated. It is, however, not suitable in very high textured soils where the infiltration rates are very low (i.e. less than 4 mm per hour). Most crops excepting rice and jute can be sprinkler irrigated. The flexibility of sprinkler equipment and efficient control of its application make this method adaptable to most of the topographic conditions. However, extremely high temperature and wind velocity markedly reduce the uniformity of water distribution and irrigation efficiency. This I system of irrigation is especially useful to the soils that have steep slopes or irregular topography and soils which are too shallow to level.
i. This technique enables judicious utilization of even small water flows and permits efficient irrigation of undulated lands, and soils with shallow depths. ii. It saves 10to 16% land that is used in construction of channels and ridges in other methods. iii. Highly permeable as well as relatively less permeable soils can be easily irrigated by sprinkler method without any risk of run-off and erosion, inundation and seepage losses. iv. Fertilizers, pesticides and weedicides can be applied along with water spray, thus, saving extra labour.
i. High initial cost of equipments.
ii. Operating costs are generally higher than irrigation by surface methods. iii. Winds disturb the sprinkler pattern giving uneven distribution of the irrigation water. iv. Sprinkling with water containing an appreciable amount of salts may result in bum or death of the plants. v. Under certain climatic conditions diseases may be encouraged. The problem of fruit rotting in tomato and strawberry gets aggravated especially in moist soil condition.
D. Localized irrigation
1. Drip irrigation
As the name signifies, drip irrigation, also termed as trickle irrigation, involves the slow application of water to the root zone of a crop. The method was initiated in Israel and is now being tried in other countries. In this method, water can be used very economically, since loss due to deep percolation and surface evaporation are reduced to the minimum. This method, therefore, is highly suitable to arid regions and orchard crops. The successful raising of orchards even on saline soils has been made possible by the drip system of irrigation. The system can also be used for applying fertilizers in solutions.
In this system, water is applied more frequently, close to the stems of plants through suitably spaced drippers (emitters) attached to plastic or metallic pipes spread above or below soils along crop rows. The pipes are hooked to source of water supply through a storage tank or pressure device which provides necessary hydraulic head or pressure for movement of water to the drippers. A pumping unit creates a pressure of about 2.5kg/sq cm. In this case only a part of soil in the vicinity of plant roots is wetted and kept close to field capacity. The amount of water dripping from nozzles can be regulated as desired by varying the pressure at the nozzles and the size of the orifice of the nozzles. The initial high cost of the equipment and its maintenance are the major limitations in this system. It may, however, work out to be cheaper than the sprinkler system especially for the orchards and other widely spaced crops.
2. Earthen pot irrigation
This method has been recently developed by the Haryana Agriculture University, Hissar. It is very cheap and convenient method and can be easily adopted. In this method, first a pit (60 cm deep and 90 cm wide) is dug out and the earthen pot is embedded up to the neck level and then it is filled with water. The water is absorbed by the soil through capillary action. One pot is sufficient to moisten one square meter area. Care should be taken to cover the pot by lid and supply water from time to .time. The pots may be replaced by new ones after two or three seasons. .
This system is especially useful for irrigating vegetable crops ego cucurbitaceous crops grown in sandy loam or sandy soils.
i. It is the most suitable method for vegetable and orchard crops where plants are widely spaced.
ii. It can be mly used in sandy and undulating lands.
iii. Saline water can be freely used because salts are deposited at the bottom of the pot
iv. High water use efficiency.
v. A simple and comparatively cheap method.
3. Double walled pot “Jaltripti”
It is a useful irrigation device for the desert, developed by the Central Arid Zone Research Institute (CAZRI), Regional Station, Bikaner. It reduces the frequency and total amount of water needed and ensures a regulated constant supply of moisture to tree plants for survival and better growth.
This device consists of a double walled earthen pot called “Jaltripti” (water satisfaction). The diameter of the outer pot is kept , approximately 25 cm at the top and at base it is 18 cm. The diameter ; of inner pot is 15 cm on top and 12cm at base. The height is kept 30 cm. (Since it is prepared on potter’s wheel, approximate dimensions have been given. Moreover, some variations in the measurement may occur). The dimensions of the inner pot have been kept slightly bigger than the size of the polythene bags (25 cm long measuring 10 : cm across) used for raising plants in nursery. Both the pots are joined together at the base and the basal portion of inner pots is kept almost open. The external side of the outer pot is made impervious with the f help of paint, cement or coaltar.
The “Jaltripti” functions on two simple principles:
1. Soil moisture tension and plant roots create a suction force which draws moisture towards it from the neighboring high moisture zone. ii. Earthen pots have many micro-pores in their wall which do not allow water to flow freely but allow its seepage in the direction where suction develops.
At the place where planting has to be done, the device is fixed in the soil in such a way that the brim of the outer pot is above soil surface. A tree sapling along with a soil ball is transplanted in the inner pot The water is filled in the space between two pots and the circular surface of water is caused by a polythene sheet to avoid direct evaporation of water. The paint on the external surface of the pot prevents outward movement of water through seepage. But the suction force created by the inner pot allows for the seepage of water steadily in that direction and keeps the soil sufficiently moist for the I growth of the plants. Water is filled weekly or fortnightly depending upon the season and size of the pot. The device has been named ‘Jaltripti’.
i. Economy of water (80 to 90 per cent).
ii. It saves more water than drip and sprinkler system.
iii. Low irrigation frequency.
iv. Labour saving.
v. No loss through percolation, evaporation and seepage.
vi. Regulated supply of moisture. Moisture is always available at field capacity.
vii. Promising in stabilization of sand dune. .
Tueboponics is a fairly recent development in the field of irrigation. This technique has been developed in Israel and practiced in the desert areas of that country in order to convert it into green lush forests.
In tueboponics, water is provided to plants/trees through injections. The needle is inserted into the plant stem and water is delivered into the phloem. The needle and injection used for this purpose are of special shape and design prepared only for this purpose.
In India, this technique has not yet arrived but it may prove very useful in Indian deserves where water is very scarce. Its use in crop plants seems to be impracticable. It is a very simple device.
V. Drip and Sprinkler Irrigation in India: Constraints
India ranks first in respect of total irrigated area existing in the world. It has got approximately 80 million hectares of irrigated land. But the methods of irrigations employed are still very primitive and inefficient. Recent achievements in the field of irrigation for instance drip and sprinkler irrigations are yet not sufficiently popular in India.
More than 10 million hectare is irrigated by sprinkler method and I million hectare by drip irrigation in the world. But in India, it is only about 0.7 m. ha under sprinkler irrigation and less than 20,000 ha with drip irrigation. Therefore, it is necessary to popularize these advanced methods of irrigation especially in those areas where water is a scarce resource.
India is blessed with abundant water resources. However, the available water, particularly for irrigation is tending to diminish and at the same time its demand is gravely felt due to population explosion. The emerging challenge is to tap all the available resources of water.
Technological innovations are to be exploited to achieve the twin objectives of higher productivity and better water use efficiency. For this, we will have to popularize drip and sprinkler irrigation methods. On account of certain financial, technical and institutional constraints, these methods have not got their due place in India and consequently, the area benefited is negligible. Therefore, the question arises as to what are the constraints and problems holding up progress.
The following are the major constraints faced by the farmers in adopting the drip and sprinkler systems of irrigation.
1. High initial cost.
2. Inadequate subsidy amount.
3. Difficulty in getting subsidy amount
4. Lack of availability of technical input and after sale services.
5. Clogging of dripper and cracking of laterals.
6. Damages due to rats and squirrels.
7. High cost of spares and components.
8. Discrimination in subsidy distribution among different categories of farmers.
To exploit the full potential of these two innovations, the constraints are to be overcome by appropriate policy instruments. financial support and technical guidance. This calls for an integrated approach and endeavor on the part of government. Implementing agencies, rnanufacturing cornpanies, voluntary organizations and the ultimate users of the systems i.e. the farmers.
VI. Adverse Effect of Improper Irrigation
As water is a limited resource with no substitute. its efficient and judicious utilization is of utmost importance in sustaining and increasing agricultural production. If irrigational water is used inefficiently and unscientifically, it may cause certain adverse effects, rather than being useful, to the crop and soil.
Seepage from main and branch canals, distributaries, and field channels along with the deep drainage loss from the base and cropped fields due to heavy rains and over irrigation add to the ground water and cause rise of water table. If not checked, the water table may rise close to the surface and cause water logging of soil. If here is a salty layer in the soil, the salts may get dissolved in the rising water table and come up on the surface soil thereby rendering the soil less productive due to salinity. Soil aeration is also badly affected.
Rising of water table beyond the threshold depth can be prevented by providing requisite sub-surface drainage. Alternatively, in areas with good quality ground water, radial drainage with shallow pumps and recycling the water for irrigation can be practiced with advantage to keep down the water tables and stretch the irrigation supplies.
Another aspect associated with injudicious irrigation is leaching of the mobile nutrients like nitrate below the root zone of crops, which decreases the nutrient use efficiency by crops. In order to minimize this loss through leaching, it becomes necessary to regulate irrigation and fertilizer applications.
Vii. Cultural Practices to Increase Water Use Efficiency in India
Water has become a precious commodity in modern agriculture. Therefore, adoption of certain practices is greatly beneficial in judicious harnessing of available water resources. These cultural practices are:
Tillage operations have a great bearing on irrigational requirements of the crops through their effect on sub-surface water storage and utilization by crops. Tillage increases infiltration rate, reduces evaporation and enhances penetration of roots into deeper layers. With an increased infiltration, higher water storage in the root zone of soil becomes possible. Greater availability of soil decreases too need of supplemental irrigation.
It is a well known fact that direct evaporation from soil is an unproductive water loss. Tillage for seed bed preparation opens up soil and increases evaporation from the tilled layer, but at the same time, it also breaks the continuity of upward flow of water (capillary water) from the moist layers below and as such, reduces water loss through evaporation.
Puddling (a tillage practice, especially useful for paddy cultivation) and compaction of soil with suitable implements is a useful measure to reduce percolation loss in coarse-textured soils.
2. Land grading
It is an essential process for bringing about uniform distribution of irrigation water (especially in surface irrigation) on undulating and sloping lands. It minimizes the danger of flooding of low lying areas and under-irrigation of raised locations.
3. Fertilizer use
Recent studies have proved that fertilization of crops enhances I water use efficiency. Besides, fertilization promotes deeper and more profuse root system which extracts more water stored in deeper layers of soil. Care should be taken not to use high doses of fertilizers where water supplies are limited.
4. Cropping patterns
Much attention has not been paid towards the development of appropriate cropping patterns which can suit the irrigation supplies available at the farm. The usual tendency is to match irrigation needs to crops on considerations other than availability of water. The role of crop planning in respect of efficient use of water has not been well recognized .Such planning is important because:
a. crops differ marked by in timings and amounts of their irrigation need; b. they exhibit a wide range of photosynthetic efficiency for similar water requirements; c. they exhibit differential sensitivity to water stress imposed at various growth stages.
The crops that develop their covers rapidly, permit more efficient use of water by reducing direct evaporation. Crops with deeper and more profuse root system utilize greater amount of profile-stored water and can stand drought better-than the shallow rooted crops. In some crops, better tolerance of water deficits arises from higher physiological adaptation.
Mulching is the practice of spreading an extraneous material on surface of soil to increase water retention, check evaporation, reduce soil erosion, improve edaphic environment and suppress weeds. Various types of mulches, viz. paper polythene film, crop residues, petroleum products, etc, have been used to realize these objectives. But unavailability and high costs of most of these materials discourage their large scale use as mulch. In those regions where paddy straw and wheat straw are available in plenty, they can be used for mulching.
Straw mulching in crops, sown during hot and dry summer has been found to increase crop yields and economize irrigation water and fertilizer use. Therefore, it can be an important practical aid in improving water use efficiency of crops.
6. Weed control
As weed compete with the crops for light, water and nutrients, their development along with the crop growth enhances rate of soil- water depletion. Further, weeds obstruct the movements of water in the irrigation channels and fields and thus promote percolation loss. These weed-induced effects necessitate more frequent watering. The adverse effect of weed on soil water use is more pronounced during early stages of growth when the crop cover is sparse and weeds significantly contribute to canopy and, hence, evapo transpiration loss is high. Thus, an efficient weed control in the irrigation channels as well as in fallow and cropped fields through mechanical operations and weedicides is essential to economize farm irrigation supplies and to increase water utilization efficiency for crop production.
7. Irrigation scheduling
In irrigated areas, optimum irrigation scheduling to crops constitutes a major cultural practice because it is one of the most important factors governing the yields of crops. Besides, irrigation scheduling has profound influence not only on the efficient management of an individual farmer’s irrigation supplies, but also on judicious planning in relation to canal, command development and optimal allocation of water resources among crops and regions. A farmer can easily adopt an optimum irrigation schedule provided it is simple and economical.