In arid region water is limited, and land is vast. Hence water management should aim to maximise production per unit of water rather per unit of land. Some of the technologies like extensive, deficit and pressurized irrigation have been developed to maximize the production in the arid region.
Extensive irrigation
Extensive irrigation approach seeks to apply a small quantity of water over a large area rather large quantity of water over a small area. Production per unit land may decrease, but production per unit water may increase. Wheat and mustard required 840 and 250 mm water per hectare to produce maximum yield. When the same water applied optimally in 3, 1.5 and 4.0 ha land in wheat and mustard, respectively gave less production per unit of land but total productivity per unit of water was more by bringing the larger area under irrigation (Table 6). Singh (1997) observed that under given water supply the area brought under irrigation in pearl millet was more in sub normal rainfall years than low rainfall years. However, the production enhanced in both the situation bringing the larger area under irrigation in pearl millet (Table 7).
Table 6. Extensive irrigation maximize production for given water supply
Deficit irrigation
Deficit irrigation approach seeks to avoid irrigation at less critical stages of crop growth and apply less water at the end to eliminate water stored at harvest. One has to well verse with the crop growth stages less sensitive to moisture stress and proper balancing between water given and ET demand of crop.
Table 7. Increased area under S-IRR enhances pearl millet production
RAINWATER MANAGEMENT
In arid and semi-arid parts experiencing rainfall from 150-350 mm and 350-550 mm respectively, rainfall is not sufficient to ensure a good crop. Crop production is adversely affected due to low and erratic distribution. In dryland farming, the solution to soil moisture problem lies in the storage of rainfall in the potential root zone of the soil by the water harvesting methods. Therefore, water harvesting would be an alternative to leave land fallow for increasing the available water supply for plant growth. In last two decades researchers and planners have given much attention to dryland farming in such areas and for various regions specific agro-techniques have been evolved.
In-situ Rainwater Conservation: Water Stored in Soil Profile Itself
Inter-plot water harvesting: In many arid areas, water harvesting is used to increase the total water supply available to crops. It is practiced in a variety of ways depending on topography, soil, and rainfall. Inter-plot water harvesting that uses a portion of land with 5% slope as catchment to generate runoff and divert it to the adjacent area was found highly beneficial to improve the yield of many crops. CAZRI has developed water harvesting technique with 3 m cropped area and 1.5 m catchment area (catchment to crop area ratio = 0.5) with 5% slope on both sides (Fig. 4). The response of pearl millet to water harvesting system was studied at CAZRI.
Crop water supply, over the check, ranged from 23 to 60 mm in the extremely dry season and 108 to 286 mm in the season with a good -1 rainfall. As a result, in water harvesting plot a yield of 3717 kg ha was obtained with 69% of normal rainfall. This ratio (0.5) was found optimum for the type of soil, climate, and surface sealing pond sediment.
Integration of regular row (RR) and double row (DR) plant geometries into water harvesting system on yield and water use efficiency of pearl millet best suited to runoff farming. In the case of prolonged drought, 22% increase in yield with DR over RR was recorded (Singh, 1988). This increase was due to suppression of early plant growth, a mild crop canopy, micro-climate and conservation of water and nutrient by suppression of weed in DR system. Higher water storage in the 120 cm profile supported the pearl millet crop during ear emergence and grain development period thereby significantly increased the yield attributing characters.
In the seasons with above normal, well-distributed rainfall, yield was similar for both RR and DR plant arrangements. Water harvesting proved well, particularly in low rainfall years. The yield of pearl millet was almost 3 times with water harvesting compared to conventional sowing (Table 8). In low rainfall situation, in-situ water harvesting helped in making efficient use of -1 limited rainfall and N-fertilizer in pearl millet. Application of 40 kg N ha with in-situ water harvesting improved the pearl millet by 27% with 69 mm of additional runoff
rainwater (Table 9).
Table 8. Water Use, yield and WUE of pearl millet by water harvesting
Table 9. In-situ water harvesting makes efficient use of limited rainfall and N-fertilizer in pearl
millet in low rainfall season (143 mm)
Micro catchment technique: Micro-catchment is one of the major forms of direct water conservation systems. Micro-catchment systems are relatively effective for use in growing trees and shrubs. In micro-catchment based cropping, rainwater is concentrated in a small portion of the cultivated area. The tree crops are deep rooted and can utilize the moisture stored in the sub-stratum and hence form a better option for micro catchment based farming in sandy soil situations (Sharma et al., 1986).
Arid horticultural plants like pomegranate, ber, and several others can be successfully grown with appropriate micro-catchments in the water scarce regions. In 1. The various sizes of micro-catchments have been used for different purposes. The 2 sizes may be up to 5 ha for annual crops, 31 -144 m for jujube orchards in India (Yadav 2 et al., 1980), 0.35 ha for fuel wood plantation in Israel (Zohar et al., 1988), 144-289 m for fig, olive, pistachio plantation in Syria (Ibrahim, 1994).
A number of catchment cropped area ratios and degree of slopes have been tried at CAZRI, Jodhpur. For ber, 2 circular catchments of 1.5 m diameter and 5% inward slope with 54 m of catchment have been found appropriate for conservation and proper utilization of rainwater (Sharma et al., 1986). The circular micro-catchment and trenching techniques were compared for soil profile moisture storage in the arid region (Fig. 5). The study revealed that the 20% increase in soil moisture was observed due to circular catchment or trenching that with any soil treatment (Ojasvi et al., 1999). For further improvement in water use efficiency, these circular micro-catchments can be covered with the plastic sheet (LDPE).
Ex-situ Rainwater Conservation: Water Stored in Reservoir or Pond for Recycling
Ex-situ rainwater harvesting is a promising technology for enhancing the availability of water in arid areas. Ex-situ water harvesting involves collecting runoff originating from rainfall over a surface away from the field and storing it in surface storage systems for later use in the field. This type of rainwater harvesting provides supplemental or protective irrigation during dry periods of the cropping season.
PRESSURISED IRRIGATION
Irrigation is a major problem in the arid regions which are characterized as having low rainfall, highly permeable soils and poor quality of irrigation water. The conventional surface or aerial (sprinkler) irrigation system can solve these problems only partially. The most befitting technique for efficient utilization of costly irrigation water in these areas is the ‘Drip’ method of irrigation.
“Drip irrigation” is a method of watering plants at a rate equivalent to (and not more than) its consumptive use so that the plants would not experience any moisture stress throughout their life cycle. In this method of irrigation water is conveyed from the source (i.e., tube well, dug-well or pond) through a network of pipelines (known as drip system) and finally released near the plant base.The primary objective of this type of irrigation is to provide an optimum quantity of water to the crop for the maximum yield and simultaneously saving of valuable water from wastage; thereby increasing the water use efficiently.
Though flood method of irrigation has been followed predominantly all over the world for cultivating crops, it is no longer desirable for countries like India mainly due to limited availability of water resources and growing demand for water for irrigation and other purposes. Therefore, for achieving sustainable agricultural development it is essential to increase the existing water use efficiency. Further, in view of the limited water resources, water is to be efficiently and economically used in arid and semi-arid conditions, for which pressurized irrigation systems can be one of the viable options.
The pressurized irrigation system has the capability of applying desired quantity of water both accurately and uniformly. Drip and sprinkler are the two examples of pressurized irrigationchilliesand both are able to save water as water is applied under pressure through a network of closed pipes, sprinkler nozzles, and emitters. The energy required labour to the water by a pumping unit, which in turn receives energy from either an electric motor or internal combustion engine.
Advantages of Pressurized Irrigation
- They provide a high degree of water application uniformity, often the highest of all irrigation systems in use.
- They allow excellent control of the amount and timing of irrigation. Small and frequent irrigations can be applied to match the trees’ water needs. Runoff is minimized because of the low application rates, and deep percolation losses can also be minimized if the correct amount of water is applied. The frequent irrigation provides an excellent soil water condition for optimal tree performance.
- They can easily irrigate irregular terrain.
- Weed growth is minimized since only desired portion is wetted.
Disadvantages of Pressurized Irrigation system
- High initial cost of the systems.
- Excellent management is needed to maintain the system since clogging of the emitters by physical particles, organic materials, and chemical precipitates may occur.
- The irrigation water must be pressurized, resulting in energy costs.
- Cover crops cannot be grown throughout the year due to the localized nature of the water applications.
DRIP IRRIGATION SYSTEM
Irrigation efficiencies under different methods of irrigation have been shown in Table 10. Research suggests that drip method of irrigation (DMI) is not only suitable for those areas that are presently under cultivation but it can also be operated efficiently in undulating terrain, rolling topography, hilly areas, barren land and areas which have shallow soils. Reduction in water consumption due to drip method of irrigation over the surface method of irrigation varies from 30 to 70 percent for different crops. According to data available from research stations, productivity gain due to drip method of irrigation is estimated to be in the range of 20 to 90% for different crops.
Table 10. Irrigation efficiency (%) under different methods of irrigation
Components of Drip System
- Pump or an overhead tank to generate pressure,
- Main line to carry water from source (pump/tank) to the field,
- Filter to remove solids,
- Fertilizer tank to inject nutrients in the line,
- Sub-mains to carry water to the subplots, and
- Emitters to deliver water to the crop (Fig. 6).
Water Supply Manifold
Pipes:-In drip systems pipes of schedule 40 and 80 are conventionally used. Similarly, plastic tubing of different classes that can withstand different pressures is available. These may be used for mains and sub mains.
Filters: These are important components of the system as these remove suspended solids that may clog the emitters. Depending on water quality and emitter design, filters of different types, size and capacity may be selected. Mechanical filtrations include settling basins, sand separators or hydro cyclone filters based on centrifugal force or vortex motion, serene filters that are usually final filters in a series, sand or gravel filters having crushed granite or silica, and cartridge filters having paper fiber or fiber glass
Fertilizer applicators: Fertilizers can be mixed in the irrigation water by (a) pressure differential, (b) the venturi and (c) metering pumps. In pressure differential system pressure difference is created between inlet and outlet through a valve resulting in mixing of fertilizer. In venturi rapid change in velocity reduces the pressure that forces fertilizer in the line. In metering pumps, a rotary, gear or piston is used to inject fertilizer in the line.
Emitters: These are the main components of the system through which water is discharged. Essentially emitters are of two types: (a) line source type and (b) point source type. In line source type the discharge points or orifices are closely spaced, or there are continuous perforations, or it has the porous wall. It is suitable for closely spaced row crops. Point source types are either pressure compensating types (can withstand pressure variations, the extent may depend on emitter type) or non-compensating types.
Similarly, emitters may be in-line or on-line emitters. As the name indicates, in-line emitters are embedded inside the tubing. Their advantages are: tubing is one piece thus can be easily stalled, and rollup for reuse and since no joints are involved there is no leakage and friction losses. Disadvantages include if need be their numbers cannot be changed. On-line emitters, in contrast, can be added as per requirement.
Crops, which have been grown with pressurized irrigation system, include the following:
Orchard crops: Grapes, citrus, apples, pomegranate, pears, delicious fruits (peaches, apricots, plums, etc.), nuts (almonds, pistachios), bananas, dates, olives, mangoes, guavas, etc.
Vegetables: Tomato, green pepper, cucumber, lettuce, green pea, cauliflower, okra, etc
Row and field crops: Cotton, sugarcane, corn, groundnut, and onion.
Others: Berries, melons, alfalfa, flowers (carnations, gladioli, and roses) and other
ornamental plants.
Planting Technique
Planting of seed or seedlings is done along the length of the lateral on one side or either side of it, irrespective of the position of the emitters on it. The emitters placed at a distance of 50 cm apart on a lateral line discharge the water at such rate that a continuous strip of about 40 cm width of soil remains wet throughout the length of the lateral. Different planting configurations (i) a single row (rectangular planting), (ii) double rows (square or equilateral planting) or (iii) triple row (hexagonal planting) were tried at CAZRI (Fig. 7). The major objective of such different types of planting is to reduce the number of laterals, thereby reducing the cost of the system without reducing the plant population in the field.
The results of the study revealed that equilateral planting with the side of 25 cm to 35 cm of a triangle performed the best in almost all the crops tried (Table- 11). Therefore, equilateral planting is advocated for vegetable crops like tomato, cabbage, cauliflower, turnip, etc. Paired row planting with a single lateral of drip combined with mulch resulted in 40-50% saving in water and higher production of tomatoes. Drip irrigation at ET 100 saved 35% water and produced 40% higher yield over conventional irrigation. Drip ET 50 though yielded at par with conventional irrigation but saved 50% water (Research Highlights, CAZRI, RRS, Pali, 1959-2009).
Irrigation Scheduling and Frequency
The main objective of the drip method of irrigation is to provide each plant with a continuous supply of soil moisture which is just sufficient to meet the evapotranspiration demand. Thus it is desirable to irrigate the field daily at the end of the day break. The unique feature in drip method of irrigation is that since the water loss of the
day is duly compensated every evening, the root soil always remaining nearly at the field capacity and consequently the crop never suffers from moisture stresses. This is the main reason for higher yield of crops as compared to another method of irrigations The potential evapotranspiration as obtained from USDA class A pan may be multiplied with the crop factor (approximately 0.7) to get daily consumptive use of the crop. For a layman, this approximate quantity or irrigation requirement may be translated into the time period (in minutes) for which the system may be kept open in a usual sunny or cloudy day of the rabi season (Shankarnarayan et al., 1984). However, it has been observed by the scientists that by increasing the quantity of water in irrigation by 30 above the consumptive use through drip system the subsequent irrigation can be delayed by 2 days without much adverse effect.
Drip Irrigation for Horticultural Crops
Drip method of irrigation is more economical for orchard crops than that for the vegetables. In orchards of ber, lemon, date-palm, pomegranate, etc. the plants, as well as rows, are kept almost 5 to 8 meter apart which necessitates where the laterals are spaced nearly 1 m to 1.5 m apart. The drippers for fruit trees are different from that of vegetable plants. Moreover, less number of drippers are used in each of the lateral lines. For example, in a lateral line of 30 m length laid out for tomato crops, nearly 60 drippers are fixed on each lateral while for the same length of lateral to be used in ber orchard of grown up trees only 25 to 30 drippers will be necessary depending upon the quantity of water required. On the other hand, it has been found that in arid areas these fruit trees when irrigated produce almost two times the fruit as compared to that in unirrigated
conditions.
Use of Drip Irrigation for Saline Water Irrigation
In the desert, over 60% of the total area has the problem of groundwater salinity. Conventional irrigation with saline water caused considerable reduction in yield and deteriorated the soil health. Drip method of irrigation has its adaptability to saline water irrigation. Since the frequency of irrigation in this method is quite high (i.e. every day) the plant base always remains wet which keeps the salt concentration in the plant root zone below the critical level that may be hazardous for the plant growth. The maximum salt concentration occurs around the periphery of the wet zone. Therefore, studies were conducted using drip irrigation to evaluate the utility of drip system in applying saline water.
The study revealed that yield of potatoes irrigated by drip irrigation with water with a -1 conductivity of 3 dS m was 31% higher than that of furrow irrigation using good quality water (Table 12). In the case of tomato, the yield obtaining using water of conductivity of 10 -1 dS m was nearly 14% less than that of sweet water. Daily drip irrigation provided gainful use of poor quality water in potato and tomato maintaining higher water content in the rooting volume and forcing the salts to the sides and below the root zone. Hence, drip system has shown the potential to use sub standard water in our desert (Singh et al., 1978). -1 Drip irrigation in pomegranate with saline water (EC 9.5 dS m ) resulted in better growth, canopy and fruit yield (2.6 kg per plant) of pomegranate over ring basin irrigation system (Research Highlights, CAZRI, RRS, Pali, 1959-2009).
Table 12. Gainful use of saline water in potato and tomato under drip irrigation system
Fertigation – Combining Fertilization with Drip Irrigation
During the irrigation process the plant roots uptake nutrients (nitrogen, -1 -1 potassium) at a rate of up to 4-6 kg ha day. As in drip irrigation, the volume of the root system is relatively small, and a number of nutrients available in the soil is quite limited. Furthermore, the water flowing through the zone of the root system washes away dissolved salts, including nutrients. Therefore, it is essential to resupply nutrients to the root system, particularly nitrogen, which is highly soluble in water and is rapidly depleted from the root zone. Although potassium and phosphorus are less soluble and will be washed downward at a slower rate, there is a need to replenish them, especially in sandy soil.
Several methods have been developed to provide the plants during the growing season with required nutrients as often as necessary via the irrigation system. In fertigation, the water discharged by the drippers constitutes a nutrient solution. The composition of the nutrients in the solution and the concentration of each component can be adjusted to the specific needs of each crop at various times during the season.
Each of the fertilizers used in fertigation needs to be highly soluble in water and free of undissolved residues. In the case of water rich in calcium (hard water), sedimentation may occur when phosphates are introduced to the water. To overcome this problem, it is recommended to use either phosphoric acid or a compounded acidic fertilizer. Several devices are available for delivery of fertilizers via including a metal container for delivering fertilizers, pumps operating a venturi tube, and a wide range of pumps with different mechanisms and control devices.
Investigations were carried out at CAZRI, Jodhpur to economize and improve nutrients efficiency with drip irrigation for tomato crop. Higher moisture content maintained with drip irrigation make the nutrients in available form to the crop (Singh et al., 1989; Singh et al., 1999). The fertilizer (one fourth of total fertilizer) applied on lanted area basis with drip provided the higher yield than broadcast application in check basin (Table 13). This shows a saving of 75% fertilizer with drip irrigation over conventional irrigation.
Table 13. Effect of fertility level on tomato crop
Drip irrigation system was demonstrated in a farmer’s field for tomato cultivation by CAZRI for six years in 0.4 ha land for ascertaining the advantages derived from drip irrigation system. In flood irrigation system only 0.24 ha of land was irrigated as compared to 0.4 ha of land under the drip irrigation system with the same quantity of water. As such an increase of 66.6% more land was brought under irrigation. The tomato -1 yield was obtained to the tune of 60 q ha (Chauhan et al., 1988). By saline water as it applies water directly in the rooting volume of crops. Studies conducted at CAZRI Jodhpur 30-50% water and provide 2-3 times higher yield than conventional irrigation (Singh and Saxena, 2001).
Studies conducted at CAZRI Jodhpur 30-50% water and provide 2-3 times higher yield than conventional irrigation (Singh and Saxena, 2001). An experiment conducted at CAZRI, Jodhpur with drip irrigation system and furrow irrigation system of surface irrigation method showed the significant amount of water saving (50%) for potato crop without losing much yield (Table14). Adoption of drip irrigation in chilies resulted in 71% increase in yields over surface furrow irrigation (Singh et al., 1999) at CAZRI Jodhpur.
Table 14. Saving of water in potato crop under drip irrigation system
Under an experiment conducted at CAZRI, Jodhpur for assessment of yield of muskmelon, lady finger, and tomato, drip system gave higher yields of 17.7, 37.8 and 22.6% compared to the yield under check-basin irrigation (Table 15). Higher water use efficiency and water productivity under drip irrigation over conventional method of irrigation (check basin) of crop sequences is presented in Under an experiment conducted at CAZRI, Jodhpur for assessment of yield of muskmelon, lady finger and tomato, drip system gave higher yields of 17.7, 37.8 and 22.6% compared to the yield under check-basin irrigation (Table 15).
Higher water use efficiency and water productivity under drip irrigation over the conventional method of irrigation (check basin) of crop sequences is presented in Table 13. Effect of fertility level on tomato crop Table 14. Saving of water in potato crop under drip irrigation system 28 Table 16. Lady finger-tomato-muskmelon crop sequence for a year with drip irrigation gave maximum water productivity
Table 15. Yield of crops in cropping system (t ha )
Application of micronutrients through drip irrigation in groundnut increased pod and haulm yields, shelling out-turn, seed mass and sound mature seeds (SMS) and nutrient use efficiency of their soil and foliar applications. The drip application of Fe, Znanalysingand B increased pod yield by 31-36, 21-23 and 15-21%, respectively over control
(Singh et al., 2001). Further, it was recommended to apply micronutrients through drip irrigation for better nutrient utilization and high groundnut yield in a semi-arid region.
Table 16. Water use efficiency and water productivity of different sequences
Maintenance of Drip Irrigation System
Drip irrigation systems require regular maintenance. A maintenance plan and checklist has been presented in Table 17 and 18.
Table 17. Components of maintenance plan for drip irrigation systems
Table 18. Maintenance checklist for drip system during growing season
Drip method of irrigation is undoubtedly a superior water management practice as compared to other methods of irrigation for arid regions. The only limitation is that it requires high initial investment at the outset. About Rs. 50,000 is to be spent for vegetable crops and Rs. 25,000 for orchard crops at the beginning for a layout of the
system in one hectare of area. The recovery of the cost depends on the type of crops selected, and the period (rabi or Kharif or both) the system is used for different crops. It has been found experimentally that the cost can be realized by the farmers within 2 or at least two years if multiple cropping is done by the drip system.
SPRINKLER IRRIGATION SYSTEM
In sprinkler irrigation, water is sprayed into the air and allowed to fall on the ground surface similar to natural rainfall. The flow of water under pressure passes through small orifices or nozzles resulting in the spray. The pressure is usually obtained by pumping. It is then sprayed into the air through sprinklers so that it breaks up into tiny water drops which fall on the ground. Careful selection of nozzle sizes, operating pressures and sprinkler spacing helps in applying water uniformly at a rate to suit the infiltration rate of the soil, thereby obtaining efficient irrigation. An understanding of the irrigation will enable its optimum use.
Advantages of Sprinkler irrigation
- suit to complete range of topographies and field dimensions
- A wide range of irrigation intensity can be changed with the infiltration capacity of the soil.
- Easy to operate. Operators may be trained quickly. Even unskilled personnel can operate the irrigation systems reasonably well.
- High irrigation efficiency due to uniform distribution of water.
- Accurate and easy measurement of water applied.
- Application of fertilizers through the irrigation system.
- Mobility, enabling the exploitation of one irrigation unit for the irrigation of many plots Can maintain micro-climate protection against frost.
Limitations of Sprinkler irrigation
- Pressure is required for operation which means an investment in energy
- Difficulties of irrigation during wind conditions, poor water distribution, and drift outside area
- Loss of water due to evaporation from the area during irrigation
- Loss of water from the marginal areas especially in small and irregular plots
- Sprinkling water may aggravate incidence of disease and wash off of spray materials·
- The use of poor quality water may cause leaf burn and leaf fall
- Sprinkling at high intensities and high application rates that are not adjusted to the soil will cause run-off
Adaptability of Sprinkler Irrigation
Sprinkler irrigation can be employed for most of the crops and on most soils. It is, however, not usually suitable for heavy clay soils where the infiltration rates are less than about 4 mm per hour. The method is particularly suited to sandy soils that have a high infiltration rate. Too shallow soils to be leveled properly for surface irrigation methods; can be irrigated safely by sprinklers. It is especially suitable for steep slopes or irregular topography (dune area). Land
Land leveling is not essential for irrigation with sprinklers. Some smoothing or grading is advisable if surface drainage is a problem or to provide a more uniform surface for seeding, tillage, and harvesting. Land too steep for efficient irrigation by other methods streams of irrigation water can be used efficiently, and well-designed sprinklers distribute water better than in other methods. Surface runoff of irrigation water can be eliminated. The amount of water can be controlled to meet crop needs, and light application can be made efficiently on seedlings and young plants.
Soluble fertilizers, herbicides, and fungicides can be applied in the irrigation water economically and with little extra equipment. Penetration of fertilizers into the soil can be controlled by applying the fertilizer at selected times during the application of water. Sprinkler irrigation can be used to protect crops against frost and against high temperatures that reduce the quantity and quality of the harvest. Labour costs are usually less than for surface methods on soils having a high infiltration rate and on steep and rolling land. More land is available for cropping. Field supply channels and bunds or ridges are not required. The irrigation method does not interfere with the movement of farm machinery.
Classification of Sprinklers
Sprinklers may be classified by levels of optimal operating pressures – low, medium or high.
Low pressure (up to 20 m): Suitable for whirling sprinklers, turbo-hammer and propeller sprinklers
Medium pressure (up to 50 m): Suitable for hammer and propeller sprinklers
High pressure (above 50 m): Giant (“cannon”) sprinklers or large hammer sprinklers Levels of pressure i.e. low, optimum and high have been depicted in Discharge range may be classified similarly to pressure range-low, medium and high.
Principles of Operation
Most sprinklers used in field crops are activated by the hammer mechanism. Other types include
Whirling Sprinkler: Water jet emitted from the end of the arm results in a reverse rotary movement at relatively high speed. The whirling sprinkler may have one, two or three nozzles and operates under low pressure. Coverage area is small and mainly used in orchards and gardens
Turbo-hammer: Water jet operates a wheel which activates the hammer and causes the sprinkler to turn. The turbo-hammer is manufactured from plastic materials. It is used for irrigation of orchards and gardens and has low discharge
Propeller: Water jet strikes propeller which rotates at high speed around its shaft and causes a circular motion of the sprinkler. The propeller breaks the jet into very fine drops, and consequently, irrigation intensity is low. It is made of plastic material and used in solid sets for field crops
Mini-sprinkler: Water jet strikes a bearing possessing one or two channels resulting the mini-sprinkler to rotate quickly and distribute the water. It is manufactured from plastic materials and is used in solid sets in orchards and gardens. It is small in size and gives low discharge.
Usage
Sprinklers with low angle jets: Under canopy sprinklers
Giant (cannon) sprinklers: Irrigation of cereals and fodder crops, wide spacing covering large areas
Part circle sprinklers: For part circle irrigation of marginal areas to prevent wastage of water and wetting of roads
Pop-up sprinklers: For lawns and gardens. Inserted underground with cover, pop-up when valve is opened and drop when valve is closed
Regulated sprinklers: These may be either discharge or pressure regulated
Selection of Sprinklers
Selection of sprinkler depends on the crop, soil, and quality of irrigation water, irrigation schedules, water supply conditions (pressure, discharge, availability) and labour availability, etc. Apart from this, sprinkler characteristics must be taken into account, viz., quality of water application, pressure and discharge range, and sensitivity to the wind. The final decision of choosing the right sprinkler is guided by the limitations – low pressure, wind conditions, infiltration rate of soil and availability of water
Irrigation Planning
An irrigation system is planned so that the correct amount of water will be applied efficiently at the right time. Some of the planning considerations are crop requirements (irrigation schedules), soil type (available water capacity, infiltration rate), precipitation, wind, evaporation, water quality (physical and chemical), water supply conditions (discharge, pressure, time), topography and shape of the field, labor and economic considerations. The irrigation system will be selected after a complete evaluation considering all factors.
Specific limitations will affect decisions. The basis of good planning depends upon the provision of exact and reliable data by the farmer to the planner. The data will include: a topographical scale map with details of borders, paths, direction of tillage and rows, existing network, ditches, electricity lines and the like, crop irrigation schedules, including special requirements (day or night irrigation, above or below canopy), water supply conditions and water source, soil data (soil analyses), agrotechnical considerations, and any other relevant data.
The basis of good planning depends upon the provision of exact and reliable data by the farmer to the planner. The data will include: a topographical scale map with details of borders, paths, direction of tillage and rows, existing network, ditches, electricity lines and the like, crop irrigation schedules, including special requirements (day or night irrigation, above or below canopy), water supply conditions and water source, soil data (soil analyses), agro-technical considerations, and any other relevant data.The planning of a specific field (i.e., the specific area which represents a unit of cultivation, treatment, and irrigation) will be carried out in stages.
After collecting and analyzing the data, the planner will select the sprinkler (type and nozzle size), determine layout and duration of the cycle and thus determine required discharge. Pipe diameters will be chosen and calculated by the basic principles allowing for maximum differences of discharge of up to 10% of the average discharge of sprinklers operated simultaneously. The pressure required at the head of the field will also be calculated. At this stage, the considerations will be hourly discharge, an organization of labor.
The uniformity of application achieved under field conditions depends on (i) the type of sprinkler pattern, (ii) the spacing of sprinklers, and (iii) the effect of such factors like the wind, variation in rotation of sprinklers, tilting of sprinkler risers, etc.
The sprinkler pattern is the shape of the volume of water falling on the area wetted and can be represented by a vertical section through the sprinkler location or by contours showing the depth of water applied. Under ideal conditions, this pattern is symmetrical around the sprinkler, but under field conditions, it is seldom so. The Wind distorts and offsets the pattern. Lack of uniformity of rotation and tilting of sprinkler risers also distorts the pattern. Pressures below that for which the sprinkler is designed to produce low applications near the sprinklers and excessive applications in a ring around the sprinklers.
The triangular shaped pattern with the maximum application at the sprinkler and gradual reduction in application to the edges of the area covered to produce the most uniform application when sprinklers are spaced not more than about 55% of the diameter wetted by the sprinkler. For rectangular sprinkler arrangements with closer spacing on the laterals, the spacing between laterals can be increased slightly. For square arrangements of sprinklers, it is theoretically possible to obtain high uniformity coefficients with the spacing between laterals up to about 70% of the diameter covered by the sprinkler; for an equilateral triangle arrangement of sprinklers, up to 75% of the diameter.
The ideal pattern for wide spacing have a uniform application to about 50% of the radius covered then the application reduces uniformly. Such patterns are very sensitive to correct spacing, and the uniformity obtainable is lower for spacing both less and greater than the optimum because of excessive or insufficient overlap. For triangular patterns, the uniformity remains high for all spacing up to about 55% of the diameter covered. For securing a greater seasonal uniformity of application the laterals may be alternated, i.e. for successive applications to place the lateral midway between the positions occupied during the previous irrigation.
Spacing is determined according to the uniformity of water application and type of equipment (viz. available lengths of aluminium or plastic pipes). The recommendations define the type of sprinkler, nozzles, wind speeds, pressure range and spacing. And b depicts the water uniformity under overlap condition of sprinklers. If the overlap is less than 50% of the wetted diameter, it will lead to the no uniform application of water resulting in poor irrigation efficiency.
The application rate affects the water distribution under varying wind conditions. The higher the application rate, the greater the resistance to wind. Irrigation in windy conditions will lead to the dry zone between two laterals (Fig. 17a & b). To reduce the wind effects, it is desirable to irrigate under zero wind conditions. When it is not feasible, decrease the spacing and increase the nozzle sizes according to the infiltration capacity of the soil. Maximum spacing of sprinklers under different wind conditions is given in
Table efficiency the
. Maximum spacing of sprinklers under the wind
Fertigation (Fertilizer + Irrigation) Devices
Fertilizers may be combined with sprinkler systems. The fertilizer will be introduced at the head of the plot or the head of a large block. Precautions must be taken to ensure that irrigation water containing fertilizer is not used as a source of drinking. There are some different methods and fertigation devices each operating under different principles:
- Fertilizer tank with bypass flow
- Venturi
- Injection pump operated by a combustion engine electricity or water
Fertigation may be continuous or part of the irrigation. The allocation of the fertilizer may be quantitative or proportional.
The effect of drip, sprinkler and furrow irrigation on the long gourd, ridge gourd, round gourd, and watermelon was studied at CAZRI, Jodhpur. The almost same amount of water was applied through conduit pipes in conventional, sprinkler and drip systems. The water applied was 69 cm in drip and 84 cm in sprinkler and furrow irrigation. The benefits of drip irrigation were not the same for all the crops. The increase in yield from drip irrigation over sprinkler and furrow irrigation was 44-47% in the long gourd, 21-37% in round gourd, 9-22 % in watermelon and practically nil in ridge gourd (Table 20). Further, water -1 3 use efficiencies were 8.1, 6.5 and 11.0 kg ha m in a long gourd, round gourd, and watermelon, respectively. Sprinkler system also showed superiority over conventional method (furrow) regarding yield.
Table 20. Yield (t ha ) of various crops under different methods of irrigation
Under a demonstration of sprinkler irrigation system at farmer’s field, the yield of pearl millet increased by 64% and of wheat by 153% as compared to flood irrigation. -1 -1 Similarly the yield of mustard increased from 770 kg ha to 1657 kg ha compared to flood irrigation. It also saved 31 man days per hectare for the preparation of plots which is to be done under the flood system of irrigation (Chauhan et al., 1988.
Source-
- Central Arid Zone Research Institute
