Depending upon the type of soil, the slope of land, the kind of crop and the size of the stream (water supply available) there are many methods of irrigating crops such as flooding, flat bed, strip and border strip, ridge bed, furrow, drip, contour ditch and basin.
Methods # 1. Border Irrigation:
In this method, a thin sheet of water advances down the narrow strip between line ridges and water infilters into the soil as the sheet advances, Fig. 17.1. The border is usually adopted where topography permits precise land levelling at a reasonable cost and where relatively large irrigation streams are available. The width and length of each border strip varies from 4 m × 30 m for sandy soils to 15 m × 300 m for clayey soils.
The strips are separated by small bunds 15 cm high. The land is graded smoothly along the natural slope (0.1 to 0.6%) in the direction of irrigation with no cross slope. Border irrigation is good for gentle slopes, close growing crops like wheat, grain (barley, bajra and berseem) and row crops like cotton, maize, jower and sugarcane (furrow irrigated in borders).
The advancing sheet of water should be adjusted so that there is enough time for the water to soak into the soil to replenish the soil moisture reservoir. When the required depth of water has been applied, the stream is shut off and diverted to another strip. The time of irrigation is given by the formula-
Where,
A = area covered with water in time t
I = infiltration rate
q = size of stream
t = time necessary to cover the strip
y = average depth of water as it flows over the land.
[All the above should be in consistent units]
Where,
i = time of irrigation in minutes
d = depth of watering (irrigation) in cm I = final intake rate in cm/hr.
A unit stream, frequently used in border irrigation, is the size of the stream required to irrigate 100 m2 of border area, i.e., an area 1 m wide and 100 m long.
The border method of irrigation has been introduced in India in some of the new irrigation project areas, such as the Tungabhadra project in Karnataka. Since the land is sloping, wet land rice cannot the grown on borders. The Government hopes to save water that way, so that more farmers can make use of the water from the large dam projects, by growing crops other than rice.
Suitable dimensions for border strips on different soils are given in Table 17.1.
Methods # 2. Furrow Irrigation:
Furrow irrigation is suitable for cultivation of row crops like maize, sorghum (jowar) sugarcane, cotton, tobacco, groundnut, potatoes, chillies and also for orchards and vegetables. The water is applied in furrows between crop rows and water soaks into the root zone of crops, Fig. 17.2 (a).
The spacing of furrows is usually decided by the spacing of the crop rows and varies from 60 to 120 cm. Furrows should be close enough so that wet areas meet. The furrows may run down the slope when the slope is within reasonable limits (0.7 to 1%) and across the slope or on the approximate contour, to reduce grade and prevent erosion, on steeper slopes. The size of the stream depends on the type of soil, slope and the length of run. The length of run depends upon the infiltration capacity of the soil, the size of the stream, the root depth of crop and furrow gradient.
The water should run in the furrow till the desired penetration is reached. Furrow grades of 0.2 to 0.5% are found satisfactory. Furrow streams of 5 to 30 Ips are common. The land must be graded so that the water moves down the entire length of the furrow without ponding. Short tubes (2 to 5 cm diameter) of metal, rubber, wood or tile, and rubber or plastic syphon tubes may be used for letting in water into the furrows from the channels, Fig. 17.2 (b). Furrow sizes (depth × width) vary from 15 × 25 cm to 30 × 60 cm. A suitable combination of discharge and length of run should be made for a given soil of known infiltration rate and slope. Maximum efficiency can be achieved by selecting a suitable combination of hydraulic variables, Table 17.2.
With furrows, initial flow rate will be 2-3 times the indicated rate to fill the run as quickly as possible and then the flow cut back to the indicated rate. The furrow stream should reach the lower end of the field within one-fourth of the total time required for irrigation.
Where,
t = time of irrigation (min)
d = net irrigation application (cm)
w = furrow spacing (cm)
q = average furrow intake rate (1pm) or furrow stream
L = Length of furrow (run), m
The maximum non-erosive flow rate in furrows (1pm) is estimated by the empirical formula-
qmax = 36/S
Where,
S = slope of furrow, expressed as %
With level furrows, the initial stream is continued till the end of irrigation.
Example 1:
Furrows 80 m long and spaced at 75 cm apart are irrigated by an initial furrow stream of 100 1pm. The initial furrow stream reached the lower end of the field in 40 min. The size of the stream was then reduced to 30 1pm. The cutback stream continued for 1 hr. What is the average depth of irrigation?
The corrugation method is an adoption of furrow irrigation for heavy soils, small streams and close growing crops. Corrugations are shallow furrows and are close enough so that the moisture is got both by capillary action and gravity, i.e., the soil between them will be wetted by lateral movement of water and, at the same time, moisture will reach the bottom of the roots. The spacing varies from 40 cm for sandy soils to 60 cm for heavy soils. Corrugations are 10 cm deep and 12-15 cm wide. A stream of 5-20 1pm is applied to each corrugation. The length of corrugation varies from 100 to 200 m.
In this method, small ridges are made on the bed with openings at alternate ends, Fig. 17.3 and the crops are planted on the sides of the ridges. Water is admitted at the upper end and passes through the furrows in a zigzag way. After it reaches the lower end of the plot, the supply is cut off. Thus, enough time is provided for the water to percolate into the soil.
The field efficiency is 70% for slopes up to 0.5% and the efficiency is reduced for increased slopes.
This method is most suitable for almost level plots and raising crops like sweet potato and turmeric.
In this method, small basins and made round the plant to receive water from the channel which gradually percolates to the root zone, Fig. 17.4. A hill is formed around the stem of the plant by putting up earth to protect the tree from direct contact with water. Basins can be made larger as the plants grow. This method is good for orchards and steep lands, especially if the soil is heavy.
Methods # 3. Sprinkler Irrigation:
Sprinkler irrigation is suitable for uneven topography, steep slopes, easily erodible or shallow soils, soils too porous or heavy, black retentive soils, and for irrigation stream too small for efficient distribution by surface irrigation. Much land levelling and laying out of the field, etc. are not needed, which means a great saving in cost. Liquid fertilizers, fungicides and insecticides, etc. can also be sprayed. Water can be used economically with high water use efficiency in places of inadequate water supply. It is suitable for all crops (except rice) on any irrigable area and highly permeable soils except in hot windy areas because of high evaporation loss.
The initial investment, annual depreciation of equipment and labour cost on transportations are the main disadvantages. It is, therefore, usually recommended for cash crops which can pay for the investment. Irrigation by sprinkling of commercial crops helps in crop cooling, frost protection and application of soluble fertilisers. When water is already being pumped to the point of use, the additional horse power needed for sprinkling is to provide an extra head of about 40 to 45 m with a minimum of additional capital investment. The details of a sprinkler head are shown in Fig. 17.5 (a).
The layout of the sprinkler equipment is most important. It requires rather highly technical know-how. The source of water supply, amount available, water table, evaporation and natural precipitation rate, wind velocity and direction, contour map of the area, soil types and crops and discharge-drawdown relationship of the well, etc. will determine the size and type of the equipment.
The salient points in the design of a sprinkler irrigation system are:
(i) The depth of application will depend on the available moisture-holding capacity of the soil.
(ii) The precipitation rate of the sprinklers should not be greater than the infiltration capacity of the soil.
(iii) The whole area should be covered within the irrigation interval (= depth of irrigation/daily consumptive use).
(iv) Natural precipitation should be taken into account and sprinklers have to be designed only for providing supplemental irrigation.
(v) Sprinkler heads generally operate at pressures of 2.1 to 3.5 kg/cm2; the precipitation rates vary from 0.5 to 1.2 cm/hr for 12 to 16 hours of operation of sprinklers per day. Sprinkler spacing on the lateral should be 0.3 to 0.5 of the wetted diameter of the sprinklers, and the spacing of the laterals should 0.5 to 0.7 of the wetted diameter to get the necessary spray overlaps. The wetted diameter for most sprinklers varies from 15 to 45 m for moderate pressures, Fig. 17.5 (b).
The economics of sprinkler irrigation from borewells drilled in hard rock formations of Karnataka is given in the following example:
Example 2:
Project – Nuggehalli village, Hassan district, Karnataka,
Economic status or farmers – Poor with individual holdings 1/4 to 4 ha
Average annual rainfall (a.a.r.) – 59 cm with 30 rainy days; 58 cm during the months of April to November
Soil – Sandy loam with water holding capacity of 3 cm per 30 cm depth
Crops recommended – Maize, Jowar, ragi, groundnut, chillies, sun flower, soyabeans, etc.
Location of borewell sites – Hydrogeological survey with electrical resistivity method is conducted on 91 ha of project area and borewell sites are located in the low resistivity areas, Fig. 17.6 and the vertical resistivity curves for six borewell sites are shown in Fig. 17.7. 15 cm borewells are drilled by the Ingersoll Rand (DHD) Rig and their details are given in Table 17.3. Yield from six borewells = 96.4 m3/hr.
Since the farmers have small or marginal holdings, the community irrigation project under a cooperative farming registered society is the only solution for the farmers to avail the loans advanced from the cooperative land development banks.
Design of Sprinkler Irrigation System:
For the types of crops grown in the area, a peak consumptive use of 4 mm/day and effective depth of root zone of 90 cm are assumed.
Since the moisture holding capacity for the soil is 3 cm per 30 cm depth, the depth of irrigation required to bring the soil moisture from 50% depletion level-
= 30 × (90/30) × 0.5 = 4.5 cm
Irrigation interval in days = depth of irrigation/daily consumption use = 4.5/0.4 = 11 days
Since the rainfall during the 8 months of April to November (i.e., 244 days) is 58 cm, the average rainfall available per cycle of irrigation (or 11 days)
= 58 × (11/244) = 2.62 cm
Assuming 50% supplemental irrigation from natural rainfall, depth of irrigation to be applied-
= 4.5 × 1/2 = 2.25 cm
Assuming a spacing of sprinklers as 12 × 18 m, i.e., spacing of sprinklers on each lateral as 12 m and the spacing of laterals as 18 m, hours of operation of sprinklers as 12 hours per day in 3 shifts, i.e., 4 hours per shift and an application efficiency of 70%, the precipitation rate of the sprinklers is-
r = 2.25 cm/(4 hr × 0.7) = 0.8 cm, or 8 mm/hr.
Due to undulating topography site selected on promising geological features:
(i) Average depth of drilling = 359.5/7 = 51.3 m
(ii) Average depth of casing = 157/7 = 22.4 m
(iii) Average yield from borewells = 96.4/6 = 16 m3/hr
(iv) Cost of sprinkler irrigation in hard rock areas (from borewells alone) = Rs. 28.50 per ha-cm
(v) Success rate of borewells—sites selected from Ground Water Survey = (6/7) × 100 = 86%
Data for 50% supplementary sprinkler irrigation:
(i) Area irrigated per borewell (with 12 hr pumping in a day) = 39.6/6 = 6.6 ha (for the crop types and soil)
(ii) Power for lifting water (to the common sump) = 45.5/39.6 = 1.15 hp/ha or 0.85 kw/ha
(iii) Power for operating the sprinklers = (2 × 15)/39.6 = 0.76 hp/ha or 0.56 kw/ha
(iv) Total power for sprinkler irrigation from borewells = 1.91 hp/ha or 1.41 kw/ha
(v) Electrical consumption per day = 17 kwh/ha; power cost = Rs. 220 per crop-ha
(vi) Precipitation rate of sprinklers = 8 mm/hr (3 shifts per day, 4 hr per shift).
Which is well within the intake rate for the soil type, i.e., <3 cm/hr. The capacity of the sprinkler-
A suitable model of the sprinkler to have a pressure of around 3 kg/cm2 at the base of the nozzle and to give a discharge of 1.73 m3/hr for the given spacing and precipitation rate is selected from the models available from the prospective sprinkler manufacturers in the country like, LAEC, Voltas, Premier and Jindal Sprinklers.
Assuming the hours of pumping from six borewells (one borewell located is dry) to be the same as the hours of operation of the sprinklers, the area covered per shift-
With 14 sprinklers on each lateral, 4 laterals are used as shown in Fig. 17.8. By shifting the laterals thrice in a day, the whole area of 39.6 ha is covered in 11 days.
In designing the lateral, the frictional loss should not exceed 20% of the average pressure at the base of the nozzle. In the main pipe, the pressure loss should be limited to 10%, but 3/4 of this loss is considered in the actual design practice. Therefore, if a pressure of 3 kg/cm2 is required at the sprinkler head, the pressure at the junction between the laterals and mains should be 3.6 kg/cm2 and at the pump outlet 3.9 kg/cm2. For reasons of lightness and ease of handling, aluminimum pipes are common, but now PVC pipes are getting popular due to flexibility in laying in horizontal and vertical curves.
The total head (H) acting on the pump operating the sprinklers = difference in level from the lowest water level in the sump to the highest patch of land + the height of the riser + the frictional loss in the main pipe and lateral + the pressure head at the base of the nozzle + losses in the suction pipe and foot valve, bends and elbows. If the pumping rate is Q m3/sec, the power of the pump required for operating the sprinklers in kW (excluding the power of the pumps installed on the borewells for lifting water to the sump).
Thus, the poor farmers in the area holding about 1/2 ha each can have a net savings of about Rs. 360 p.m. apart from the labour provided for them from cultivation of their lands. They also have some money (64% of the capital investment) put aside towards the depreciation of the pumpsets, sprinkler equipment, etc. The scheme is envisaged as economically viable and helps to improve the economic status of the poor farmers in Nuggehalli village.
They can hope for prosperity after the complete repayment of loan, i.e. from the ninth year onwards, when they can have a net savings of Rs. 783 p.m. per ha, which can further be increased to Rs. 1000 p.m. by including crops like sunflower, soyabeans, chillies, etc. Crops like ragi and soyabeans are drought resistant and can withstand failure of rain for some time. Thus, optimum utilisation of water resources can be achieved by applying the required depth by sprinkler irrigation to supplement natural rainfall, so as to bring maximum area under production and improve the economic status of the poor farmers in the area.
During any crop season if the rains completely fail, the full depth of 4.5 cm has to be sprinkled at a precipitation rate of 8 mm/hr, which takes 8 hours to irrigate 1.2 ha in one shift. If the hours of operation of the sprinklers and pumping from the borewells could be increased to 16 hr/day, 2.4 ha can be irrigated per day in two shifts and the total area irrigated in an irrigation interval of 11 days = 2.4 × 11 = 26.4 ha, which is the maximum area that can be covered by assured irrigation from sprinklers with the available yield from borewells, i.e., 96.4 m3/hr.
The limitation of this cooperative sprinkler irrigation scheme from bore, wells is that due to the limited yield in the hard rock formations, the whole area covered by the borewells, which are mostly drilled in the sites selected by the Ground Water Investigation Team after detailed hydrogeological surveys, cannot come under irrigation. This may create misunderstanding among the farmers who own different portions of the land.
Methods # 4. Drip Irrigation:
Drip irrigation is a new system of irrigation developed in the deserts of southern Israel. The method had given great hopes of possibilities of agriculture under arid conditions with poor sand soils high evapotranspiration rates and water supply that is both limited and high in salts. From the water source, a booster pump pressurises about 25 to 50 m3/hr (per 1/2 – 1 ha to be irrigated) and delivers it to the control head, Fig. 17.9, where metering, filtering, pressure regulation and fertiliser injection take place.
From the control head the water travels through the conducting pipe (4-5 cm main or feeder line) and then through the distribution tubes (12- 16 mm plastic tubes, i.e., laterals laid along the crop rows spaced at 90 cm covering the entire field) on which the drip-nozzles are inserted at intervals of about 50 cm, and from the drippers, the steady drops of water are discharged at zero pressure. One dripper supplies water and fertiliser for each plant and the drip-nozzle discharge ranges of 2-10 lph.
All the components of the system, except the head and fertiliser apparatus are generally of plastic construction. The laterals and drip-nozzles are laid on the soil surface or buried not deeper than 5-10 cm. Water trickles to crop roots and low rates of water are applied frequently. The crop is irrigated daily in many cases. Soil samplings have shown accumulation of salts on the soil surface and at the edges of the wetted areas due to saline irrigation water and high evapotranspiration, but the region in which the plant roots are concentrated has actually the minimum possible salinity content, Fig. 17.9 (c), with considerable increase in the number and density of feeder roots than when other irrigation methods are employed.
Evaporation losses are eliminated and use of water of higher salinity than normally tolerated by the plant is made possible by the nature of the wetting pattern. Drip irrigation is best suited for row crops and orchards such as tomatoes, corn, grapes, citrus fruits, melons, etc. It has been proved that drip irrigation gives better crop yields than other irrigation methods like sprinkler and furrow, under desert conditions of high evapotranspiration and coarse soil of poor water holding capacity.
The capital cost per hectare works out to Rs. 35,000 for the imported equipment (control head, drippers and plastic) and Rs. 25,000 for indigenous equipment (booster pump, delivery pipe and fencing). India has a very highly developed plastic industry and this item forms the bulk of the investment. The net annual profit per hectare works out to Rs. 25,000. Assuming a daily consumptive use of 0.4 cm for the types of crops grown under this system, and an application efficiency of 75%, the daily water requirement per hectare will be 10,000 × (0.4/100) × 0.75 = 53 m3 through one hour of operating the system daily.
Some of the advantages and disadvantages of the drip irrigation are as follows:
(i) Does not wet the foliage and aisler.
(ii) 30-50% saving in water.
(iii) Salts accumulate at the outer edge of the wetted pattern.
(iv) Enables spraying, dusting, picking, cultivating, etc.
(v) The dry aisler prevents weed growth.
(vi) No compaction (due to floor irrigation) and erosion, (on hill slopes).
(vii) Increase in yield ranging from 20-30%.
(viii) Roots stay within the moist zone.
(ix) Better grade of tomatoes, firmer strawberries, cotton, sugarcane and potatoes have been reported.
(x) Can be used on hill sides (ideal system) and with all types of mulches.
(i) Does not offer frost protection as sprinklers do?
(ii) Plastic drip-lines and sub-mains may be attacked by rodents and small animals. However, this is not such a serious problem.
(iii) Requires regular flushing (to clear off the dirt collected near the ends of the drip- lines) and supervision.
Principles of Soil and Water Conservation:
1. Contour farming should be adopted on all the sloping lands.
2. Crops are grown in strips or bands at right angles to the slope of the land (strip cropping), Fig. 17.11.
3. On rolling lands, strips of row and cover crops are sown alternatively to control wind and water erosion.
4. On eroded lands mixed cropping is practised.
5. Leguminous crops and fodder grasses are included in the crop rotations. They add nitrogen and organic matter to the soil.
6. Contour bunds are constructed on sloping lands (< 6%) where the rainfall is less than 60 cm for storing surface runoff to improve soil moisture status for increased crop production, Fig. 17.12.
The bunds should be so spaced that the seepage zone of the upper bund should extend up to the zone of saturation of the next lower bund. The spacings shown in Table 17.5 are recommended in Maharashtra for contour bunds on cultivated lands.
7. Graded bunds are constructed in areas where the rainfall is over 60 cm and the soils are heavy. Bunds are aligned with a gentle gradient of 0.1 to 0.5% towards the outlet.
8. When the land slope is steeper than 6% and the rainfall is over 100 cm, terracing can be practised by having narrow strips with ‘cut and fill’ method, Fig. 17.13 (a), (b).
For graded terraces the spacing is given by-
Similarly, staggered trenching can be usually adopted for plantations for horticultural crops such as cashew-nuts.
9. Bench terracing and paddy terracing are practised on mountainous and sub- mountainous areas. Bench terraces may be level, sloping outward or slopping inward depending upon local conditions. Terrace spacing is given by Fig. 17.14.
Conditions of the depth of soil, slope, rainfall climate, and farming practices influence the terrace design. A vertical of 1.8 m is considered satisfactory. The back slope of the rise may be 1/2 to 1 or 1 to 1, a flatter back slope being at the expense of the bench.