Essay on the Methods of Irrigation | Methods | Irrigation | Agronomy

In this essay we will discuss about the top two methods of irrigation. The methods are: 1. Surface Irrigation Systems 2. Micro-Irrigation.

Essay # 1. Surface Irrigation Systems:

Surface irrigation is defined as the group of application techniques where water is applied and distributed over the soil surface by gravity. It is by far the most common form of irrigation throughout the world and has been practiced in many areas virtually unchanged for thousands of years. Surface irrigation methods, in general, have some basic advantages over sprinkler and drip systems.

Advantages:

1. Local irrigators, generally, have at least minimal understanding of how to operate and maintain the system. In addition, surface systems are often more acceptable to agriculturalists

2. These systems can be developed at the farm level with minimal capital investment

3. They are less affected by climatic and water quality characteristics

4. They have relatively low energy requirements in routine operations

5. Certain fruits and vegetables which can be damaged by sprinkling because of scorch from salt residue of sprinkled water can be safely irrigated by surface systems

6. Surface systems can avoid wind drift and canopy interception losses common in sprinkler system

7. Mechanical simplicity.

Disadvantages:

1. Although they need not be, surface irrigation systems are typically less efficient in applying water than either sprinkler or trickle systems

2. Surface systems tend to be labour intensive

3. Waterlogging and water table raise

4. Soil salinisation.

Surface Irrigation Process:

A surface irrigation event is composed of four phases as illustrated graphically in Fig 8.2.

(i) Advance phase:

As water is applied to the top end of the field it will flow or advance over the field length. Advance phase refers to that length of time as water is applied to the top end of the field and flows or advances over the field length

(ii) Wetting, ponding or storage phase:

After water reaches end of the field, it will either runoff or start to pond. The period of time between end of advance phase and shut-off of the inflow is termed the wetting, ponding or storage phase

(iii) Depletion phase:

As the inflow ceases, water will continue to runoff and infiltrate until entire field is drained. Depletion phase is that short period of time after cutoff when the length of the field is still submerged

(iv) Recession phase:

It describes the time period while water front is retreating towards the downstream end of the field. Depth of water applied to any point in the field is a function of opportunity time, length of time for which water is present on the soil surface.

Surface irrigation is often referred to as flood irrigation, implying that the water distribution is uncontrolled and therefore, inherently inefficient. In reality, some of the irrigation practices grouped under this name involve a significant degree of management (for example surge irrigation).

Surface methods are classified by the slope, the size and shape of the field, the end conditions and how water flows into and over the field. Classification is, perhaps, somewhat arbitrary in technical literature.

This has been compounded by the fact that a single method is often referred to with different names:

(i) Wild flooding:

(ii) Controlled flooding:

1. Check flooding (flat bed irrigation, bed and channel irrigation)

2. Basin flooding (level basin, check basin irrigation)

3. Border (strip) flooding

4. Furrow flooding.

(i) Wild Flooding:

This is the primitive and least controlled of all the surface irrigation systems. Water from the ditch is diverted to the upper part of the plot and allowed to spread over the land in accordance with the natural topography (Fig 8.3).

After the water leaves the ditches, no attempt is made to control the flow. Hence, it is called as wild flooding. Distribution of water is highly uneven. Consequently, part of the area becomes waterlogged, while other part remains dry leading to uneven pattern of crop growth. This method is applicable to inundation irrigation system or for pastures or forage crops where water is not limiting or high value crops do not justify adoption of better methods.

Wild flooding can be divided into the following types, based on the method by which water is admitted through the inlet into the field:

Spate irrigation:

It may occur in hilly regions in dry zones where small rivers produce spate floods; ditches and bunds are built to guide the water to the fields to be irrigated; the number of fields irrigated at each flood event depends on the duration and intensity of the flood.

Flood-plain irrigation:

It may occur in dry zones in larger river plains, where the river has high discharges during a short season only; bunds are constructed to retain the river floods and the lands are being planted to crops when the floods recede (flood recession cropping).

Tidal irrigation:

It is the subsurface irrigation of levee soils in coastal plains with river water under tidal influence. It is applied in semiarid zones at the mouth of a large river estuary or delta where a considerable tidal range (2 m) is present. The river discharge must be large enough to guarantee a sufficient flow of fresh water into the sea so that no salt water intrusion occurs in the river mouth.

The irrigation is effectuated by digging tidal canals from the river shore into the main land that will guide the river water inland at high tide. For the irrigation to be effective the soil must have a high infiltration capacity to permit the entry of sufficient water in the soil to cover the evapotranspiration demand of the crop. At low tide, the canals and the soil drain out again, which promotes the aeration of the soil.

(ii) Controlled Flooding:

The land is leveled or graded and subdivided by means of channels and ridges. Water is guided to each of the subdivisions. Depending the manner in which the land is subdivided, controlled flooding methods are named differently.

1. Check Flooding:

It is the most widely practiced surface method for irrigating wheat, millets, groundnut, sunflower and pulse crops in India. It consists of applying water to nearly leveled plots (checks) enclosed by small bunds (Fig 8.4) at a rate sufficiently in excess of intake rate of the soil to rapidly cover the area.

The size of checks, generally, ranges from 3 × 2 to 10 × 6 m or even larger depending on porosity of the soil. This method is also known as check irrigation, flat bed irrigation, level border irrigation or bed and channel irrigation.

Merits:

1. Initial investment is minimal

2. High irrigation efficiency can be achieved with properly designed system

3. As there is no danger of soil erosion, unskilled labour can irrigate the crop.

Demerits:

1. High labour requirement

2. Levees restrict use of modern farm machinery

3. Restricted to relatively smooth lands because of expenditure involved in levelling the plots.

2. Basin Flooding:

This method is essentially a check flooding method for irrigating orchards. Basins are small level plots (usually circular or square) surrounded by low earth dikes, also called checks, within which water can be impounded to irrigate a single tree or a few trees in each basin of the orchard (Fig 8.5). This method is also called as basin method or check-basin method.

This method is ideal for soils with moderate to slow intake rates and to moderate to high available water holding capacity. On sloppy lands, basin flooding can be carried out in conjunction with terracing.

Suggested maximum basin areas (m²) for various soil types and available stream sizes are given in Table 8.2.

Size of the basin is also influenced by the depth (mm) of the irrigation application. If required irrigation depth is large, basin can be large. Similarly, if required irrigation depth is small, basin should be small to obtain good water distribution.

Basins should be small if the:

1. Slope of the land is steep

2. Soil is sandy

3. Stream size to the basin is small

4. Required depth of the irrigation application is small

5. Field preparation is done by hand or animal traction.

Basins can be large if the:

1. Slope of the land is gentle or flat

2. Soil is clay

3. Stream size to the basin is large

4. Required depth of the irrigation application is large

5. Field preparation is mechanised.

3. Border/Border Strip/Bay Irrigation:

Borders are usually long, uniformly graded strips of land, separated by earth bunds. In contrast to basin irrigation, these bunds are not to contain the water for ponding but to guide it as it flows down the field (Fig 8.6).

Border irrigation is, generally, best suited to the larger mechanised farms as it is designed to produce long uninterrupted field lengths for ease of machine operations. Borders can be up to 800 m or more in length and 3-30 m wide depending on a variety of factors. It is less suited to small scale farms involving manual labour or animal powered cultivation methods.

Suitable slopes:

Border slopes should be uniform, with a minimum slope of 0.05 per cent to provide adequate drainage and a maximum slope of 2 per cent to limit problems of soil erosion.

Suitable soils and crops:

Deep homogenous loam or clay soils with medium infiltration rates are preferred. Heavy, clay soils can be difficult to irrigate with border irrigation because of the time needed to infiltrate sufficient water into the soil. Basin irrigation is preferable in such circumstances. Close growing crops such as pasture or alfalfa are preferred.

Border layout:

The dimensions and shape of borders are influenced in much the same way as basins and furrows by the soil type, stream size, slope, irrigation depth and other factors such as farming practices and field or farm size.

Many of the comments made about basins and furrows are, generally, applicable to borders also and so do not require repetition here. Table 8.3 provides a guideline to determine maximum border dimensions. It must, however, be stressed that this table is for general guidance only as the values are based on field experience and not on any scientific relationships.

Wetting patterns:

Borders are irrigated by diverting a stream of water from the channel to the upper end of the border. When the desired amount of water has been delivered to the border, the stream is turned off. This may occur before the water has reached the end of the border. There are no specific rules controlling this decision.

However, if the flow is stopped too soon there may not be enough water in the border to complete the irrigation at the far end. If it is left running for too long, then water may run off the end of the border and be lost in the drainage system.

As a guideline, the inflow to the border can be stopped as follows:

1. On clay soils, the inflow is stopped when the irrigation water covers 60 per cent of the border. If, for example, the border is 100 m long a stick is placed 60 m from the farm channel. When the water front reaches the stick, the inflow is stopped

2. On loamy soils it is stopped when 70 to 80 per cent of the border is covered with water

3. On sandy soils the irrigation water must cover the entire border before the flow is stopped.

However, these are only guidelines. Realistic rules can only be established locally when testing the system.

As with the other irrigation methods, it is important to ensure that adequate irrigation water is supplied to the borders so that it fills the root zone uniformly. However, there are many common problems which result in poor water distribution.

These include:

1. Poor land grading,

2. Wrong stream size, and 

3. Stopping the inflow at the wrong time.

If the land is not graded properly and there is a cross-slope, irrigation water will not spread evenly over the field. It will flow down the slope always seeking the lowest side of the border. This can be corrected by regarding the border to eliminate the cross-slope or by constructing guide bunds in the border to prevent the cross flow of water.

A stream size which is too small will result in deep percolation losses near the field channel, especially on sandy soils. If the stream size is too large the water will flow too quickly down the border and the point where the flow should be stopped is reached before sufficient water has been applied to fill the root zone. In this situation, the flow will need to be left running until the root zone has been adequately filled and these results in considerable losses from surface runoff. Large stream sizes may also cause soil erosion.

If the inflow is stopped too soon, the water may not even reach the end of the border. In contrast, if the flow is left running too long, water will run off the border at the downstream end and be lost in the drainage system.

Border (strip) or bay irrigation could be considered as a hybrid of level basin and furrow irrigation. Borders of the irrigated strip are longer and the strips are narrower than for basin irrigation and are orientated to align lengthwise with the slope of the field. Water is applied to the top end of the bay, which is usually constructed to facilitate free flowing conditions at the downstream end.

The concept is to flush a large volume of water over a relatively flat field surface in a short period of time. Borders are raised beds or levees constructed in the direction of the field’s slope. The idea is to release water into the area between the borders at the high end of the field. Borders guide the water down the slope as a shallow sheet that spreads out uniformly between the borders. Border irrigation can be viewed as an extension of basin irrigation to sloping, long rectangular or contoured field shapes, with free draining conditions at the lower end.

Merits:

1. Utilises large water streams safely

2. Provides uniform wetting of the soil profile

3. Higher irrigation efficiency relative to wild flooding

4. Requires less labour.

5. No loss of land for crops

Drawbacks:

1. Requires relatively large water streams for quick advance of water to minimise deep percolation at the upper part of the plot

2. Since the length of each plot is relatively longer, wastage of water through deep percolation is common in lighter soils.

3. Land leveling involves huge expenditure.

4. Furrow Flooding:

Furrows are small, parallel channels, made to carry water in order to irrigate the crop. The crop is usually grown on the ridges between the furrows (Fig 8.7). Furrow irrigation is suitable for a wide range of soil types, crops and land slopes.

Suitable crops:

Furrow irrigation is suitable for many crops, especially row crops. Crops that would be damaged if water covered their stem or crown should be irrigated by furrows.

In summary, the following crops can be irrigated by furrow irrigation:

1. Row crops such as maize, sunflower, sugarcane, soybean etc.

2. Crops that would be damaged by inundation, such as tomatoes, vegetables, potatoes, beans.

Suitable slopes:

Uniform flat or gentle slopes are preferred for furrow irrigation. These should not exceed 0.5 per cent. Usually a gentle furrow slope is provided up to 0.05 per cent to assist drainage following irrigation or excessive rainfall with high intensity. On undulating land, furrows should follow the land contours.

Fig 8.8 shows two typical furrow irrigated conditions:

Suitable soils:

Furrows can be used on most soil types. However, as with all surface irrigation methods, very coarse sands are not recommended as percolation losses can be high. Soils that crust easily are especially suited to furrow irrigation because the water does not flow over the ridge and so the soil in which the plants grow remains friable.

Generally, the shape, length and spacing are determined by the natural circumstances (slope, soil type and available stream size). However, other factors may influence the design of a furrow system, such as the irrigation depth, farming practice and the field length.

Furrow length:

Furrows must be on consonance with the slope, soil type, stream size, irrigation depth, cultivation practice and field length.

Furrow slope:

Although furrows can be longer when the land slope is steeper, maximum recommended furrow slope is 0.5 per cent to avoid soil erosion. Furrows can also be level and are thus very similar to long narrow basins. However, a minimum grade of 0.05 per cent is recommended so that effective drainage can occur following irrigation or excessive rainfall.

If the land slope is steeper than 0.5 per cent, then furrows can be set at an angle to the main slope or even along the contour to keep furrow slopes within the recommended limits. Furrows can be set in this way when the main land slope does not exceed 3 per cent. Beyond this, there is a major risk of soil erosion following a breach in the furrow system. On steep land, terraces can also be constructed and furrows cultivated along the terraces.

Soil type:

In sandy soils, water infiltrates rapidly. Furrows should be short, so that water will reach the downstream end without excessive percolation losses. In clay soils, the infiltration rate is much lower than in sandy soils. Furrows can be much longer on clayey than on sandy soils.

Stream size:

Normally stream sizes up to 0.5 1 s^-1 will provide an adequate irrigation provided the furrows are not too long. When larger stream sizes are available, water will move rapidly down the furrows and so generally furrows can be longer. Maximum stream size that will not cause erosion will obviously depend on the furrow slope; in any case, it is advised not to use stream sizes larger than 3.0 1 s^-1 (Table 8.4).

Irrigation depth:

Applying larger irrigation depths usually means that furrows can be longer as there is more time available for water to flow down the furrows and infiltrate.

Cultivation practice:

When the farming is mechanised, furrows should be made as long as possible to facilitate the work. Short furrows require lot of attention as the flow must be changed frequently from one furrow to the next. However, short furrows can usually be irrigated more efficiently than long ones as it is much easier to keep the percolation losses low.

Field length:

It may be more practical to make the furrow length equal to the length of the field, instead of the ideal length, when this would result in a small piece of land left over. Values shown in Table 8.3 are lower than those generally given in irrigation handbooks. These higher values are appropriate under larger scale, fully mechanised conditions.

Table only provides approximate information relating furrow slope, soil type, stream size and irrigation depth to furrow lengths. This should only be used as a guide as the data are based primarily on field experience and not on any scientific relationships. Maximum values of furrow length are given for reasonably efficient irrigation.

However, furrow lengths can be even shorter than those given in the table and in general this will help to improve irrigation efficiency. Only by installing a furrow system, following the guidelines and then evaluating its performance can an appropriate system be developed for a given locality.

Furrow shape:

Shape of furrows is influenced by soil type and stream size. In sandy soils, water moves faster vertically than sideways (lateral). Narrow, deep V-shaped furrows are desirable to reduce the soil area through which water percolates. However, sandy soils are less stable, and tend to collapse, which may reduce the irrigation efficiency.

In clay soils, there is much more lateral movement of water and the infiltration rate is much less than for sandy soils. Thus a wide, shallow furrow is desirable to obtain a large wetted area to encourage infiltration. In general, the larger the stream size the larger the furrow must be to contain the flow.

Furrow spacing:

Spacing of furrows is influenced by soil type and cultivation practice. As a rule, for sandy soils the spacing should be between 30 and 60 cm (30 cm for coarse sand and 60 cm for fine sand). On clay soils, the spacing between two adjacent furrows should be 75-150 cm. On clay soils, double-ridged furrows – sometimes called beds – can also be used. Their advantage is that more plant rows are possible on each ridge, facilitating manual weeding. Ridge can be slightly rounded at the top to drain off water that would otherwise tend to pond on the ridge surface during heavy rainfall.

In mechanised farming a compromise is required between the machinery available to cut furrows and the ideal spacing for crops. Mechanical equipment will result in less work if a standard width between the furrows is maintained, even when the crops grown normally require a different planting distance.

This way, the spacing of the tool attachment does not need to be changed when the equipment is moved from one crop to another. However, care is needed to ensure that the standard spacing provide adequate lateral wetting on all soil types.

Wetting patterns:

In order to obtain uniformly wetted root zone, furrows should be properly spaced, have uniform slope and the irrigation water should be applied rapidly. As the root zone in the ridge must be wetted from the furrows, downward movement of water in the soil is less important than the lateral (or sideways) water movement. Both lateral and downward movement of water depends on soil type.

In an ideal situation (Fig 8.9) adjacent wetting patterns overlap each other and there is an upward movement of water (capillary rise) that wets the entire ridge, thus supplying the root zone with water.

To obtain uniform water distribution along the furrow length, it is very important to have uniform slope and large enough stream size so that water advances rapidly down the furrow. In this way, large percolation losses at the head of the furrow can be avoided. The quarter time rule is used to determine the time required for water to travel from the farm channel to the end of the furrow, in order to minimise percolation losses.

Poor wetting patterns can be caused by:

1. Unfavourable natural conditions (compacted layer, different soil types, uneven slope)

2. Poor layout (furrow spacing too wide)

3. Poor management – supplying a stream size that is too large or too small stopping the inflow too soon

4. Unfavourable natural conditions.

Compacted soil layers or different soil types have the same effect on furrow irrigation as they have on basin irrigation. Solution to the problem is also similar. An uneven slope can result in uneven wetting along the furrow. Water flows fast down the steep slopes and slowly down the flatter slopes. This affects the time available for infiltration and results in poor water distribution. The problem can be overcome by regarding the land to a uniform slope.

Planting Techniques:

Location of plants in a furrow system is not fixed but depends on the natural circumstances.

A few examples are indicated:

1. In areas with heavy rainfall, plants should stand on top of the ridge in order to prevent damage as a result of waterlogging (Fig 8.10)

2. If water is scarce, plants may be put in the furrow itself, to benefit more from the limited water (Fig 8.11)

3. As salts tend to accumulate in the highest point, crop on saline soils should be planted away from the top of the ridge. Usually, it is planted in two rows at the sides (Fig 8.12). However, it is important to make sure there is no danger of waterlogging.

4. For winter and early spring crops in colder areas, seeds may be planted on the sunny side of the ridge. In hotter areas, seeds may be planted on the shady side of the ridge, to protect them from the sun.

Merits:

1. Saving of irrigation water over other surface methods

2. As part of the land area is not wetted due to irrigation, evaporation losses will be less

3. Furrows serve as drainage channels in areas of heavy rainfall

4. Especially ideal for crops like maize that are injured by contact with water

5. Fairly high irrigation efficiency.

Drawbacks:

1. Not suitable for light soils with high infiltration rates

2. Possibility of soil erosion

3. Relatively labour intensive

4. Water accumulation at the end of furrows

For improving the efficiency of furrow flooding, several modifications have been suggested.

Three such modifications are discussed:

1. Surge Flow Irrigation:

Surge flow is a method of applying water in a series of surges to furrows, borders and basins. The system will automatically save labor, water and improve irrigation efficiency.

In surface methods of irrigation, the soil surface serves two distinct functions. First, it is an infiltration surface. Water must pass through the soil surface into the soil below. The longer water is on the surface the slower the rate at which it enters. This means that less water infiltrates per unit of time near the end of the irrigation than at the beginning. The second function of the soil surface is to convey the water from one end of the furrow to the other.

Thus, while the soil surface is performing the conveyance function, it also performs the infiltration function. The two cannot be separated. Therefore, since water is on the soil at the beginning of the furrow longer than it is on at the end, more water infiltrates at the beginning than at the end.

If a furrow stream is allowed to run long enough to completely fill the root zone at the end of the furrow, too much water is applied at the beginning and losses due to deep percolation occur. Furthermore, excess water must run off at the end of the furrow, and runoff losses occur. These two phenomena are the cause of most of the inefficiency in surface irrigation under conventional practices.

Stringham and Keller (1979) reported a new approach for automating surface irrigation systems in which problems with slow advance and excessive surface runoff occur. The approach was called surge flow to describe the hydraulic regime of the flow over the field. Surge flow is simply a method of applying water to a field in a series of intermittent surges rather than as a continuous stream.

Under the surge flow regime, irrigation is accomplished through a series of individual pulses of water onto the field such that, instead of the typically found advance-wetting-depletion-recession trajectory in normal surface irrigation conditions (Fig 8.2), it looks like that in Fig. 8.13. Thus, instead of providing a continuous flow onto the field for say six h, a surge flow regime would apply six 1 h surges.

Each surge is characterised by a cycle time and a cycle ratio. The cycle time is comprised of an on-time and an off-time related by the cycle ratio which is the ratio of on-time to the cycle time. The cycle time can range from as little as one minute to as much as several hours. Cycle ratios typically range from 0.25 to 0.75. By regulating these two parameters, a wide range of surge flow regimes can be produced which can significantly improve irrigation efficiency and uniformity.

In the advance phase, the cycle time (the time for one complete on/off cycle for a particular furrow or block of furrows) is relatively long (30 minutes or more). In such an advance phase, water is in the furrow for 30 minutes and out for 30 minutes, then back in again and so on until it reaches the end of the furrow.

In the advance phase, it is important that the water be out of the furrows long enough that the furrow completely de-waters or that all of the water disappears from the furrow. Much of the advantage of surging in the advance phase is lost if this is not the case. During this phase the alternate filling and emptying of the furrow quickly lowers the soil’s infiltration capacity, reduces the percolation depth and smooth the furrow surface.

These changes in soil behavior and furrow condition make for more efficient water use in the second, cutback phase:

(i) The advance phase where the water advances from the beginning of the furrow to the end of the furrow

(ii) The cutback phase where the flow rate of the water in the furrow is reduced.

In the cutback phase, the cycle time is shortened to about 10 to 20 minutes. This means that there will be water in the furrow at all times, infiltrating uniformly, but the average flow rate will be about half that of a conventional continuous application. The objective of the cutback phase is to reduce the time-averaged flow rate in the furrow to less than the instantaneous flow rate entering the furrow.

There are basically two field systems commercially available for surge flow. The first is shown in Fig 8.14 is the dual line system. Water is supplied to the field generally through a buried pipeline which connects to surface gated pipe through a riser and valve. The valve, shown schematically in Fig 8.15, is automated to switch the flow between two sets.

Surging is accomplished by alternating the flow between the two sets. When these two are finished, the entire flow is directed to another riser and valve by the irrigator. The dual line system is in widespread use in the USA where irrigators already have a gated pipe furrow irrigation system in place.

As large surge flows encourage rapid advance along the furrows, it is an effective method of achieving faster advance of water down the furrows compared to continuous application. The flow rates are large, causing wetting to occur faster down the irrigation row but accompanied by reduced percolation at the upper end of the furrow (Fig 8.16).

Total volume of water required to complete advance face is less. Frequent light irrigations can be given at initial crop growth stages when rooting depths are shallow and infiltration rates are high to reduce deep percolation losses.

2. Cablegation Irrigation:

The cablegation system was developed by the Soil and Water Management Research Unit of the US Department of Agriculture’s laboratory at Kimberly, Idaho.

Cablegation (Fig. 8.17) is a semi-automated system for achieving furrow inflow cutback that capitalises on the natural reduction in infiltration rate as irrigation progresses to greatly reduce runoff. The system consists of gated pipe oriented such that the outlets are located near the top of the pipe laid on a uniform graded slope along the head of the field.

A moveable plug retained by a cable through the pipe inlet diverts water through the open outlets into the furrows. The other end of the cable is attached to a reel located at the gated pipe inlet structure. As irrigation progresses, the cable is unrolled from the reel and the plug travels slowly downstream.

As the plug passes an outlet, the new discharge begins to flow as flow from the last upstream outlet ends. The rate of outlet discharge steadily decreases as the distance from the plug increases due to decreasing pressure (increasing elevation relative to the plug).

The size of the outlet openings and rate of plug movement are adjusted to adequately irrigate the lower end of the furrow while minimising runoff. The operational advantage of cablegation is the ability to cut back furrow inflow while maintaining a constant delivery of water to the field along with inherent automation.

Hydraulically, a cablegation system operates in the free surface flow regime upstream of the travelling plug except immediately adjacent to it. In the region near the plug, the flow is slowed and expands to fill the pipe. Thus, in the uniform open channel flow region of the pipe, the water surface is below the outlets which are therefore shut off from the field.

Near the plug, the water level rises above the outlets to supply the field. The unique feature of the cablegation system is the high outlet flows nearer the plug. This feature gives the advance phase discharge needed to facilitate field coverage. As the plug moves downstream, the outlet flow is cutback to allow soaking time without causing excessive surface runoff.

Cablegation and surge flow are two examples of an alternative approach to managing surface irrigation. After years of trying to regulate discharges unsuccessfully, these two methods accomplish this end by managing time and equipment speed.

3. Corrugation:

This is a partial surface flooding method of irrigation. Irrigation water does not cover the entire surface of the field. Water flowing in the corrugations (rills) soak into the soil and spreads laterally to irrigate the area between corrugations (Fig. 8.18).

Corrugations should be spaced such that the lateral movement of water will provide an adequate irrigation between the corrugations by the time sufficient water has been applied to refill the soil profile. This method is well adopted for medium and heavy soils, if the soil tends to crust after irrigation.

This method is useful for small grain crops, especially wheat. Corrugations are about 6 to 8 cm deep, spaced 40 to 75 cm apart, depending on the physical properties of the soil and spacing for the crop. Simple bamboo frame (marker) can be used for making corrugations immediately after seeding (Fig 8.19).

Issues Associated with Surface Irrigation:

While surface irrigation can be practiced effectively using the right management under the right conditions, it is often associated with a number of issues undermining productivity and environmental sustainability.

Waterlogging – Can cause the plant to shut down delaying further growth until sufficient water drains from the rootzone. Waterlogging may be counteracted by drainage and watertable control.

Deep drainage – Over irrigation may cause water to move below the root zone resulting in rising water tables. In regions with naturally occurring saline soil or saline aquifers, these rising water tables may bring salt up into the root zone leading to problems of irrigation salinity.

Salinisation – Depending on water quality irrigation water may add significant volumes of salt to the soil profile. While this is a lesser issue for surface irrigation compared to other irrigation methods (due to the comparatively high leaching fraction), lack of subsurface drainage may restrict the leaching of salts from the soil. This can be remedied by drainage and soil salinity control.

Elements of Surface Irrigation Designs:

i. Check Flooding:

Design of check flooding system consist in determining the plot size suitable for a particular stream size and finding out the duration of irrigation to bring the root zone of the crop to field capacity.

Quantity of water required to irrigate a particular size of check basin can be obtained using the formula:

Qt = Ad

where, Q = Stream size

t = Time for which water has to be applied

A = Area of the check

D = Depth of irrigation.

The quantity of water calculated with the above formula will irrigate the entire check if the opportunity time is same all over the check. Generally, it will not be the same over the entire check area. At the tail end of the check, the opportunity time will be less to an extent of the time required for water to advance (spread) all over the check.

As such an additional opportunity time equal to the time of spread should be allowed at the end. It is assumed that after the water has spread all over the check initially, the rest of the water introduced into the check is uniformly available over the entire area.

ii. Border Flooding:

Graded slopes should be uniform throughout the length of the border except, the first 9 to 15 m should be level to ensure water distribution over entire width of the border. Border strip should be level crosswise. Under any circumstances, it should not exceed 3 m in the width of one strip. Greater cross slopes cause the water to concentrate along one side of the strip giving uneven irrigation.

Size of border strips:

Dimensions of border strips depend upon the irrigation stream size. Where possible, width should be a multiple of the width of the least flexible farm implement to be used in the farm. Length of the strip is limited by irrigation stream size that will not cause erosion. Larger streams that can be used safely on flatter slopes permit longer border strips. General recommended land slopes for different soil types and maximum length of borders are given in Table 8.5.

                                                                                                                                       TABLE 8.5: Optimum grades and maximum length of borders for different soils

Border ridges:

They are necessary for controlling water flow. Ridges must be higher for flatter slopes than for steeper ones. Base of the ridge should be broad for the convenience of farming operations. Sides of ridges should be no steeper than two to one, horizontal to vertical. Thus, on a steep slope where a ridge with minimum settled height of 10 cm is needed, the base should be at least 0.60 m wide. On flatter lands, where a 20 cm ridge is needed, the minimum base width should be about 1.2 m.

Irrigation streams:

Coarse textured soils with high infiltration rate will require high discharge rates to spread water over entire strip rapidly in order to avoid excessive deep percolation losses at upper reach of the strip. On the other hand, fine textured soils with low infiltration rates, require smaller discharges to avoid excessive losses due to runoff at the lower reaches. The flow rates into borders are usually large (50 to 200 lps) to ensure quick spreading of water to all parts of the strip.

The rates of advance and recession of irrigation stream depends on intake rates of soils (Fig. 8.20). When these are approximately parallel, the intake opportunity time is about the same in all parts of the border leading to uniform application.

Relationship between the stream size (Q), average depth of water flowing over the strip (d), rate of infiltration (f), area irrigated (A) and time required to irrigate the area (t) is given by:

The following equation enables determination of maximum area that can be irrigated with discharge Q and soil having infiltration rate, f:

It can also be inferred from this equation that the discharge per unit area of border strip (Q/A) should be varied according to infiltration capacity of the soil (f), otherwise there will be loss of irrigation water.

iii. Furrow Flooding:

Efficiency of furrow flooding depends on selection of proper combination of spacing, length and slope of furrows, appropriate irrigation stream size and duration on water application.

Furrow spacing:

Furrow spacing depends on the row spacing of the crops to be grown and the implements used for their cultivation. Crops like maize, cotton, potato and many vegetables with row spacing of 60 to 90 cm have furrows between all rows. Crops such as carrots and onion with a row spacing of 30 to 40 cm have two rows between furrows. The spacing should be such that the lateral movement of water should wet the ridges by the time irrigation is completed. As such sandy soils that have vertical wetted pattern (Fig 8.21) should have closer furrow spacing than heavy soils.

For steeper slopes (> 0.5%), broad based furrows are recommended as steeper slopes cause larger flow velocities and less depth of flow. Conversely, shallower slopes, steep sided furrows are useful as the furrow capacities will be less.

Furrow length:

If the furrow length is too long, water soaks too deep at the head of the furrow by the time the stream reaches the lower end. This results in over irrigation at the upper end or under irrigation at the lower end. Short furrows require field supply channels to be spaced too close with consequent loss of land and increase in labour requirement.

Proper furrow length largely depends on hydraulic conductivity of the soil. Furrows must be shorter on sandy soil than clay soil. Furrow length may be as short as 45 m on soils which take up water rapidly or as much as 300 m or longer on soils with low infiltration rates. Table 8.6 may serve as a guide in deciding length of furrows for different soils, slopes and depths of irrigation.

                                                                                                                          TABLE 8.6: Recommended length of furrows for different soils, slopes and depths of irrigation

Furrow slope:

For planning purposes, maximum slopes if erosion is to be avoided are 2 per cent on sandy soils and 3 per cent on clayey soils. With careful management, good soils and small streams, slopes up to 15 per cent are possible.

On smooth, uniformly sloping fields, crops are sometimes planted across the slope to reduce furrow grade. Furrows are also used on graded benches across the slope. The slope of the land at right angles to the direction of irrigation (cross slopes) should not exceed 2 per cent. Where furrows are less than 15 cm deep, the cross slope should be less than 2 per cent and it may be necessary to use shorter runs to prevent accumulation of water and over topping of furrows.

Furrow stream:

At the beginning of irrigation, the largest stream of water that will not cause erosion is used in each furrow.

Maximum non erosive furrow stream is estimated with the formula:

where, Q = Flow (lps)

S = Furrow slope (%).

Average depth of water applied during irrigation can be calculated with the relationship:

where, D = Average depth of water applied (cm)

Q = Stream size (lps)

t = Duration of irrigation (h)

w = Furrow spacing (m)

L = Furrow length (m).

The furrow stream (Q) calculated is allowed to flow until the water reaches furrow end is called initial stream. Its purpose is to wet the entire length of each furrow as quickly as possible. Reducing the difference in the time that water infiltrates at the upper and lower ends of the furrow gives more uniform distribution of water and improves the efficiency of irrigation.

After the initial stream has reached the lower end of the furrow, the stream is reduced or cut back to one that will just keep the furrow wet throughout its length with a minimum of waste at the end. This cut back stream flows until the required amount of water has been applied. To compute proper size of cut back stream, the average intake rate of the soil must be known.

Proper size of cut back stream is the intake per unit length of furrow times the length of run. For example, 800 ft long furrows on a silt loam with an intake rate of 2 gpm per 100 ft would take a cut back stream of 16 gpm (240 m long, 25 l min^-1 100 m^-1 60 l m^-1 or 0.06 m³ min^-1).

To determine how long to allow this stream to run, the amount to be applied in inches is divided by the intake rate in inches per hour. This intake rate is affected by spacing of furrows and the furrow intake rate converted to total intake for the irrigated area. In the example, if the planned application were 100 mm (0.01 m) and furrow spacing is 0.90 m, the cut back stream would need to run;

Sub-Irrigation:

Sub-irrigation (subsurface irrigation, seepage irrigation) has been used for many years in field crops in areas with high water tables. In the absence of high water table, water is applied beneath the soil surface by creating and maintaining an artificial water table at some depth, usually 30 to 75 cm below the ground surface.

It is a method of artificially raising the water table to allow the soil to be moistened from below the plants’ root zone. Often those systems are located on permanent grasslands in lowlands or river valleys and combined with drainage infrastructure. A system of pumping stations, canals, weirs and gates allows it to increase or decrease the water level in a network of ditches and thereby control the water table.

Sub-irrigation is also used in commercial greenhouse production, usually for potted plants. Water is delivered from below, absorbed upwards and the excess collected for recycling. Typically, a solution of water and nutrients floods a container or flows through a trough for a short period of time, 10-20 minutes and is then pumped back into a holding tank for reuse.

Sub-irrigation in greenhouses requires fairly sophisticated, expensive equipment and management. Advantages are water and nutrient conservation, and labor saving through lowered system maintenance and automation. It is similar in principle and action to subsurface drip irrigation.

Essay # 2. Micro-Irrigation:

The term micro-irrigation describes a family of irrigation systems that apply water through small devices. These devices deliver water onto the soil surface very near the plant or below the soil surface directly into the plant root zone. Micro-irrigation systems are immensely popular not only in arid regions and urban settings but also in sub-humid and humid zones where water supplies are limited or water is expensive.

In irrigated agriculture, micro-irrigation is used extensively for row crops, orchards, gardens, greenhouses and nurseries. In urban landscapes, micro-irrigation is widely used with ornamental plantings.

Emission Devices:

The actual application of water in a micro-irrigation system is through an emitter. Emitter is a metering device made from plastic that delivers a small but precise discharge (gph). Emission devices deliver water in three different modes – drip, bubbler and micro-sprinkler. In drip mode, water is applied as droplets or trickles. In bubbler mode, water bubbles out from the emitters (Fig 8.26). Water is sprinkled, sprayed, or misted in the micro- sprinkler mode.

Drip Irrigation:

Drip irrigation has been used since ancient times when buried clay pots were filled with water, which would gradually seep into the grass. Modern drip irrigation began its development in Afghanistan in 1866 when researchers began experimenting with irrigation using clay pipe to create combination of irrigation and drainage systems.

In 1913, EB House at Colorado State University succeeded in applying water to the root zone of plants without raising the water table. Perforated pipe was introduced in Germany in the 1920s and in 1934; OE Nobey experimented with irrigating through porous canvas hose at Michigan State University.

The modem technology of drip irrigation was invented in Israel by Simcha Blass and his son Yeshayahu. Instead of releasing water through tiny holes, blocked easily by tiny particles, water was released through larger and longer passageways by using velocity to slow water inside a plastic emitter.

The first experimental system of this type was established in 1959 when Blass partnered with Kibbutz Hatzerim to create an irrigation company called Netafim. Together they developed and patented the first practical surface drip irrigation emitter. This method was very successful and subsequently spread to Australia, North America, and South America by the late 1960s.

In the United States, in the early 1960s, the first drip tape, called Dew Hose, was developed by Richard Chapin of Chapin Watermatics. Beginning in 1989, Jain irrigation helped pioneer effective water management through drip irrigation in India.

It can be defined as the process of slow application of water in the form of discrete, continuous drops, tiny stream or miniature sprays through mechanical devices called emitters or applicators located at selected points along water delivery lines. Fertilisers and other chemical amendments can be effectively applied to individual or several plants using drip irrigation.

Drip system, also called as trickle system, has few characteristics in common with flood or sprinkler irrigation. Water advances in the soil around emitter only after the amount of water applied exceeds the infiltration rate at a point. Then it advances until the infiltration rate of ponded area equals the emitter flow rate. Typically, a wetted dia of less than 1.0 m, depending upon the soil properties and emitter application rate, will become saturated on the soil surface.

Depending on how the emitters are placed in the plastic polyethylene distribution line, the drip mode can be further delineated as a line source or a point source. The line source type emitters are placed internally in equally spaced holes or slits made along the line. Water applied from the close and equally spaced holes usually runs along the line and forms a continuous wetting pattern. This wetting pattern is suited for close row crops.

The point source type emitters are attached external to the lateral pipe. The installer can select the desired location to suit the planting configuration or place them at equally spaced intervals. Water applied from the point source emitter usually forms a round deep wetting spot. The point source wetting pattern is suited for widely spaced plants in orchards, vineyards and for landscape trees or shrubs.

Line source emitter:

Line source emitters are suitable for closely spaced row crops in fields and gardens.

Line source emitters are available in two variations:

1. Thin wall drip line, and

2. Thick wall drip hose.

A thin walled drip line has internal emitters molded or glued together at set distances within a thin plastic distribution line. The drip line is available in a wide range of diameters, wall thickness, emitter spacing and flow rates. The emitter spacing is selected to closely fit plant spacing for most row crops.

The flow rate is typically expressed in gallons per minute (gpm) along a 100-foot section. Drip lines are either buried below the ground or laid on the surface. Burial of the drip line is preferable to avoid degradation from heat and ultraviolet rays and displacement from strong winds. However, some specialised equipment to install and extract the thin drip distribution line is required.

The thick walled drip hose is a robust variation of the thin walled drip line. The internal emitters are molded or glued to the drip hose. It is more durable because of its considerable thickness. The diameter of the drip hose is similar to that of the thin walled drip line.

Unlike the thin wall drip line, the drip hose emitter spacing is wider and it operates at a higher pressure. The emitter discharges ranges from 0.2 to 2 gph. Thick walled drip hose is typically laid on the ground and retrieved at the end of the cropping season.

Point source emitters:

Point source emitters are typically installed on the outside of the distribution line. Point source emitters dissipate water pressure through a long narrow path and a vortex chamber or a small orifice before discharging into the air. The emitters can take a predetermined water pressure at its inlet and reduce it to almost zero as the water exits. Some can be taken apart and manually cleaned. The typical flow rates range from 0.5 to 2.0 gph.

Trickle irrigation, like other methods, will not fit every crop, specific site or objective. Presently, trickle system has greatest potential where water is expensive or scarce, soils are sandy, rocky or difficult to level and high value crops are produced. Principal crops under trickle irrigation are avocado, citrus, stone fruits, grapes, straw berry, sugarcane and tomato.

Suitable crops:

Drip irrigation is most suitable for row crops (vegetables), tree and vine crops where one or more emitters can be provided for each plant. Generally, only high value crops are considered because of the high capital costs of installing a drip system.

Suitable slopes:

Drip irrigation is adaptable to any farmable slope. Normally, the crop would be planted along contour lines and the water supply pipes (laterals) would be laid along the contour also. This is done to minimise changes in emitter discharge as a result of land elevation changes.

Suitable soils:

Drip irrigation is suitable for most soils. On clay soils, water must be applied slowly to avoid surface water ponding and runoff. On sandy soils, higher emitter discharge rates will be needed to ensure adequate lateral wetting of the soil.

Suitable irrigation water:

One of the main problems with drip irrigation is blockage of the emitters. All emitters have very small waterways ranging from 0.2-2.0 mm in dia and these can become blocked if the water is not clean. Thus, it is essential for irrigation water to be free of sediments. If this is not so then filtration of the irrigation water will be needed.

Blockage may also occur if the water contains algae, fertiliser deposits and dissolved chemicals which precipitate such as calcium and iron. Filtration may remove some of the materials but the problem may be complex to solve and requires an experienced engineer or consultation with the equipment dealer.

Drip irrigation is particularly suitable for water of poor quality (saline water). Dripping water to individual plants also means that the method can be very efficient in water use. For this reason, it is most suitable when water is scarce.

Each irrigation method has possible advantages and limitations with respect to technical, economical and crop production factors.

Potential merits and drawbacks of trickle systems as stated by Bucks (1982) are:

Merits:

1. Increased beneficial use of available water

2. Enhanced plant growth and yield

3. Reduced salinity hazard to plant

4. Improved fertiliser and other chemical applications

5. Limited weed growth

6. Reduced operational labour

7. Decreased energy requirement

8. Improved cultural practices.

Drawbacks:

1. Persistent maintenance requirements

2. Salt accumulation near plants

3. Restricted soil water distribution and plant root development

4. Economic and technical limitations.

Components of Drip Irrigation System:

Irrigation pipeline systems are, generally, described as branching. Various branches are given names such as main, submain and lateral. Choosing the right size main, submain and lateral pipe to match the flow rates from the water source is important.

Basic components (Fig 8.25) listed in order from water source, can include:

1. Pump or pressurised water source

2. Water filter(s) – filtration systems: Sand separator like hydro-cyclone, screen filters, media filters

3. Fertigation systems (venturi injector) and chernigation equipment (optional)

4. Backwash controller (backflow preventer)

5. Pressure control valve (pressure regulator)

6. Main line (larger diameter pipe and pipe fittings)

7. Hand-operated, electronic or hydraulic control valves and safety valves

8. Smaller diameter polytube (often referred to as laterals)

9. Poly fittings and accessories (to make connections)

10. Emitting devices at plants (emitter or drippers, micro spray heads, inline drippers, trickle rings).

Note that in drip irrigation systems, pump and valves may be manually or automatically operated by a controller.

i. Head tank:

Water lifted for irrigation is stored in a head tank, usually 3 × 3 × 3 m, resting on a raised platform to maintain pressure head of 3 to 5 m. The head is connected to central water supply. Its functions are to regulate the pressure and the amount of water applied, filter the water and nutrient material. It is so located that the main lateral is conveniently taken in the middle of the field.

ii. Main lateral:

Plastic main lateral is usually of 20 to 40 mm dia, suitable for desired discharge over which water is to be carried. A water meter is installed in the main lateral close to the tank for measurement of discharge.

iii. Laterals:

In drip irrigation system, water is delivered to the plants via a set of plastic lateral tubes laid along the ground or buried at a depth of 15-30 cm and supplied from a field main. Small dia laterals, usually 10 to 20 mm in dia, fitted to the main lateral at a distance equal to the row spacing of the crop (usually 75 cm). The laterals are perforated at a distance equal to the spacing of the crop (usually 50 cm).

Nozzle:

Plastic nozzles having perforations are fastened to or constructed as a part of the drip line, to allow the water to leave the line at a very slow rate depending on crop needs. The trickling rate, generally, in the range of 2 to 10 l h^-1 per emitter, should not exceed soil intake rate to avoid runoff, if any.

The operating water pressure is usually 1-3 atm (15 to 45 psi). This pressure is dissipated by friction in flow through the narrow passage or orifices of the emitter such that the water emerges at atmospheric pressure in the form of drops.

Water tends to spread sideways and downwards in the soil from the point of introduction, under both suction and gravity forces. Generally, however, only part of the soil is wetted by drip irrigation, that part being affected by the configuration and density of the drip points as well as by the internal water spreading properties of the soil. In any case, the active rooting volume is usually confined to a fraction (often less than 50%) of what would be the normal root zone of a uniformly wetted soil.

Modifications of Drip Irrigation System:

In recent years, several modifications of drip system have been developed in the continuing effort to improve water use efficiency. Surface trickle irrigation, subsurface trickle irrigation, low-head bubbler irrigation, micro-spray irrigation, mechanical move irrigation and pulse irrigation assume significance. Some of these methods are adaptable to the needs of small farmers in developing countries.

i. Surface Drip Irrigation:

In this system, emitters and lateral lines are laid on soil surface. It is one of the most prevalent types of micro-irrigation systems, primarily, used on widely placed plants, but can also be used for row crops. Discharge rates are, generally, less than 12 l h^-1 for single outlet point source emitters. Its advantages are ease of installation, inspection, cleaning and changing emitters.

ii. Sub-Surface Drip Irrigation (SDI):

Sub-surface drip (SDI) is a low pressure, high efficiency irrigation system that uses burried drip tubes or drip tape to meet crop water needs. Sub-surface irrigation saves water and improves yields by eliminating surface water evaporation and reducing the incidence of disease and weeds. The SDI technologies have been a part of irrigated agriculture since the 1960s; with the technology advancing rapidly in the last two decades.

A typical system layout consists of a settling pond (where possible), pumping unit, pressure relief valve, check valve or back flow prevention valves, a hydrocyclone separator (when a pond is not feasible to take out the coarse materials), chemical injection unit, filtration unit equipped with back-flush control solenoid valves, pressure regulators, air vent valves, and PVC pipe lines delivery system to carry the water to the field.

The delivery system is composed of main, submain and manifold, to which the lateral drip tubes are attached. Items such as a flow meter and a pressure gauge are essential to monitor the performance of the system and provide early warning for leaks and blockages.

Advantages:

1. A SDI system is flexible and can provide frequent light irrigations. This is especially suitable for arid, semiarid, hot and windy areas with limited water supply. Farm operations also become free of impediments that normally exist above ground with any other pressurised irrigation system

2. Since water is applied below the soil surface, the effect of surface infiltration characteristics such as crusting, saturated condition of ponding water and potential surface runoff (including soil erosion) are eliminated during irrigation

3. With an appropriately sized and well-maintained SDI system, water application is highly uniform and efficient

4. Subsurface irrigation saves water and improves yields by eliminating surface water evaporation and reducing the incidence of disease and weeds

5. Some crops may benefit from the additional heat provided by dry surface conditions. When managed properly, water and fertiliser application efficiencies are enhanced and labor needs are reduced.

The drawback with subsurface drip system is that it requires higher initial investment and cost will vary due to water source, quality, filtration need, choice of material, soil characteristics and degree of automation desired.

iii. In-Ground Irrigation System:

Most commercial and residential irrigation systems in developed countries are in- ground systems, which mean that everything is buried in the ground. With all the components of an irrigation system (pipes, sprinklers, drippers, irrigation valves and other accessories) being hidden, it makes for a cleaner, more presentable landscape without garden hoses or other items having to be moved around manually. This does, however, create some drawbacks in the maintenance of a completely buried system.

iv. Bubbler Irrigation:

Bubblers, typically, apply water on a per plant basis. Bubblers are very similar to the point source external emitters in shape but differ in performance. Water from the bubbler head either runs down from the emission device or spreads a few inches in an umbrella pattern (Fig 8.26). The bubbler emitters dissipate water pressure through a variety of diaphragm materials and deflect water through small orifices.

Most bubbler emitters are marketed as pressure compensating. The bubbler emission devices are equipped with single or multiple port outlets. Most bubbler heads are used in planter boxes, tree wells or specialised landscape applications where deep localised watering is preferable. The typical flow rate from bubbler emitters is between 2 and 20 gph.

v. Micro/Mini Sprinkler Irrigation:

Micro-sprinklers (Fig 8.20) are emitters commonly known as sprinkler or spray heads. There are several types. The emitters operate by throwing water through the air, usually in predetermined patterns. Depending on the water throw patterns, the micro-sprinklers are referred to as mini-sprays, micro-sprays, jets, or spinners.

The sprinkler heads are external emitters individually connected to the lateral pipe typically using spaghetti tubing, which is very small (1/8 to 1/4 inch) dia tubing. The sprinkler heads can be mounted on a support stake or connected to the supply pipe. Micro-sprinklers are desirable because fewer sprinkler heads are necessary to cover larger areas. The flow rates of micro-sprinkler emitters vary from 3 to 30 gph depending on the orifice size.

vi. Mechanical-Move irrigation:

This system expands the bubbler concept to large scale row crops. It utilise liner-move sprinkler lines that have drop structures or tubes to deliver water as a continuously moving stream to each row. Operating pressures are lower than those of most conventional sprinkler systems and uniformity of water applications over the field are excellent. Potential advantages of this system include reduction in clogging problem and less expensive pipe network compared to solid set trickle systems.

vii. Pulse Irrigation:

It has a series of irrigation time cycles with an operating (water discharge) and rest phase (no discharge). Typical operating phases are 5, 10 or 15 min h¹. Primary advantage is reduction in clogging problems and disadvantage is the need to develop reliable inexpensive pulse solid set trickle system.

viii. Micro Jet:

In this system, water is applied in the form of discrete drops, fan type, full circle, part circle or quarter circle spray on the surface of soil from low height or low angle through air around the crop. Micro jet does not have moving parts but have greater discharge rate and greater coverage than drippers and bubblers.

ix. Misting or Fogging System:

It is the application of water in the form of very fine spray in order to create humidity in the air at low level or overhead. These systems are, generally, used in poly houses or green houses and in shade houses to control the humidity and temperature. Misting system has very low flow rates (10-30 l h^-1) and high pressure (3.5 to 10.0 kg cm^-2) is required to create the mist or fog.

x. Pop-Up Sprinkler:

This system is similar to sprinkler system but sprinklers are installed just below the ground level with cover and gets activated with pressure and comes up from ground level. When the system is shut off, sprinklers automatically go back to their original position. These systems are, generally, used for lawns and gardens such as golf courses, stud farms etc.

xi. Set Move Irrigation:

Set move sprinkler systems are moved from one set (irrigation) position to another by manually or mechanically. It remains stationary as water is being applied. After desired quantity of water is applied, the system is shut off and moved to the next set position.

Drip vs Sprinkler and Surface Irrigation:

Soil moisture regime:

Under drip, the wetted portion of the soil is maintained in a continuously moist (though unsaturated and hence well aerated) state and that soil volume is never allowed to be depleted or to approach the wilting point. This creates a uniquely favourable soil moisture regime and gives drip irrigation a distinct advantage over surface and sprinkle irrigation, most especially for sandy soils of low moisture storage capacity and in arid climates of high evaporativity.

Effect of wind and topography of soil:

In contrast with sprinkle irrigation, drip irrigation is practically unaffected by wind conditions and unlike surface irrigation; it is little affected by slope, topography or surface roughness.

Brackish water use for irrigation:

With drip irrigation, it is possible to use somewhat brackish water (1,000 mg l^-1 of salts) for the irrigation of crops that are not too sensitive to salt. Unlike in sprinkle irrigation, the brackish irrigation water does not come in direct contract with the foliage, which is therefore not as prone to saline scorching.

As the soil in the wetted zone is kept constantly wet (rather than being allowed to dry periodically), the salts are prevented from concentrating and the osmotic pressure of the soil solution in the rooting zone is prevented from increasing to the point where it might affect the crop significantly.

With brackish water, there is an increased tendency for salts to accumulate at the peripheries of the wetted circles and these salts, if not leached by subsequent rainfall, should be leached periodically by means of a portable sprinkler system.

Water economy:

Drip irrigation can save water by reducing the portion of the soil surface that is wetted, thus decreasing the amounts of direct evaporation. The magnitude of this effect depends on the density of the grid of drippers, on the lateral spread of water beneath each dripper and on the degree of plant cover.

In addition to saving water, the reduction in wetted area also discourages weed growth in the inter-row strips of an orchard or row crop, thus reducing the need for frequent cultivation and the associated hazard of soil compaction by machinery.

Root growth:

The fact that the rooting volume is restricted under drip irrigation, so that the reservoir of soil moisture available to the crop is limited, makes the crop vulnerable to even short-term interruptions of the irrigation regime. Hence, the constant maintenance of the system in perfect working order is essential and frequent irrigation is not merely desirable but indeed mandatory. Since the supply of nutrients might also be limited in the restricted rooting zone, the injection of supplementary fertilisers into the water supply is also warranted.

Capital investment:

The capital investment costs of drip irrigation systems are relatively high because large quantities of pipes, tubes, emitters and ancillary devices are required to deliver water to specific sites in the field via closed conduits, rather than via the air or over the soil surface as in sprinkle and surface irrigation.

In addition, since the drip emitter orifices are narrow, expensive filtration equipment is necessary to prevent clogging. Labour saving automation adds to the capital costs, as does fertiliser injection into the water supply. Hence, drip systems tend to be more expensive initially than surface irrigation systems (except where costly land forming is needed for the latter) and often more expensive even than sprinkler system. The savings of water relative to surface irrigation and the savings of energy relative to sprinkle irrigation can reduce the long term comparative operating costs of drip systems.

Maintenance:

Drip irrigation can greatly reduce labour costs, but its successful operation demands continuous maintenance by skilled technicians with ready access to spare parts. It is certainly not the sort of system that can be installed once and for all, and that can continue to operate trouble free by itself.

Rather, it is a delicate system that needs constant attention and fine-tuning. The reliability of the system is critical, as the available soil moisture reservoir is very limited. If the system fails to deliver water even for a few days, the plants may suffer severe stress under the incessant transpiration demand. Drip emitters must be inspected regularly and cleaned or replaced whenever any fail by clogging or mechanical damage.

The most important aspect of maintenance is prevention of clogging by suspended particles (silt), by biological agents (algae) and by chemical precipitation of salts. Algae and other biological slimes can be controlled by chlorination. Special care is needed where the irrigation water is drawn from eutrophic open reservoirs filled with silt-laden runoff water.

Such salts as calcium carbonate can be prevented from precipitating by acidifying the water. Particles of various sorts can be removed from the irrigation water by means of screens, media filters (filled with gravel, sand or diatomaceous earth), and/or centrifugal separators. Filters, in fact, are integral components of drip irrigation systems.

Gravel and sand filters are effective in removing suspended solids from water, and are less expensive than screen filters. Their drawbacks are their large size and weight and appreciable pressure loss. As the pores of the gravel or sand medium become clogged with retained solids, pressure loss increases and flow rate diminishes.

Hence, these filters require frequent flushing by backflow. This action may not remove all of the fine material trapped in the filter, so the filtering medium itself must be replaced periodically. Screen filters are still more delicate and require even more rigorous inspection and servicing.

In principle, drip irrigation is most suited to orchard crops and to field and garden crops grown in rows and beds, as well as to ornamental plants, and least suited to close growing field crops requiring uniform wetting of the entire soil.

Apart from water saving, drip irrigation increases crop productivity by reducing moisture stress for crops (Fig 8.27). Productivity of crops under drip irrigation can be increased by 40 to 50 per cent over flood method of irrigation, especially in crops like bottle gourd, sweet potato, potato, tomato and chillies.

Selection of an appropriate irrigation method for any combination of physical, agronomic and socioeconomic conditions involves complex and sometimes conflicting considerations. The decision of what irrigation system to adopt is still a matter of judgment, based on one’s evaluation of relative importance of the factors involved.

The advent of relatively inexpensive water application system has apparently removed some of the economic constraints to the widespread adoption of scientific innovations. Properly applied, the new irrigation methods can raise yields, while minimising waste, reducing drainage requirements and promoting the integration of irrigation with essential inputs.

Estimation of Water and Power Requirement for Installation of Drip Irrigation System:

Report of sub-committee on more crop and income per drop of water, Ministry of Water Resources, Government of India, New Delhi has given the following estimates:

A. Estimation of crop water requirement:

The daily water requirement for fully grown plants can be calculated as under:

V = Ep × Kc × Kp × Wp × Sp

Net depth of irrigation to be applied (Vn) = V × Re × Sp

The total water requirement of the farm plot would be Vn × No of plants

where, V = water requirement (1 per day per plant)

Ep = pan evaporation (mm per day)

Kc = crop coefficient

Kp = pan coefficient

Wp = wetted area (0.3 for widely spaced crops and 0.9 for closely spaced crops)

Sp = spacing (m²)

Re = effective rainfall (mm)

A = area of the plot (m²)

B. Estimation of horse power of pumping unit:

The ideal drip irrigation system is one in which all drippers (orifices) deliver the same volume of water in a given irrigation time. The dripper flow variation caused by water pressure can be controlled by hydraulic design.

Flow carried by each lateral line (de)

= Discharge of dripper × No of drippers per plant × No of plants along each lateral.

Flow carried by each submain line (ds) = de × No of lateral lines per sub main line

Flow carried by each main line (dm) = ds × No of submains.

The friction head loss in mains can be estimated by Hazen-Williams formula given below:

Hf = 10.68 × (Q/C) 1.852 × D 4.87 × (L + Le)

where, Hf = Friction head loss in pipe (m)

Q = Discharge (m³ sec^-1)

C = Hazen – William constant (140 for PVC pipe)

D = Inner dia of pipe (m)

L = Length of pipe (m)

Le = Equivalent length of pipe and accessories (Table C)

The design of lateral pipe involves selection of pipe for a given length which can deliver required quantity of water to the plant.

In designing the lateral, the discharge and operating pressure at drippers are required to be known and accordingly, the allowable head can be determined by the same formula as the main line.

Design Criteria:

1. It should be ensured that the head loss in the lateral length between the first and last emitter is within 10 per cent of the head available at the first emitter.

2. The friction head loss in the mainline should not exceed 1.0 m per 100 m length of the mainline. Friction head loss for various discharges is given in Table B and equivalent lengths of straight pipe in meters giving equivalent resistance to flow in pipe fittings in Table C.

After finalisation of dimensions of main, submains and laterals, the selection of pump consist of the following steps:

Total pressure head drop in meters due to friction (Hf) = Friction head loss of main + Friction head loss of submains + Friction head loss of laterals.

Operating pressure head required at the dripper = He (m).

Total static head = Hs (m).

Total pumping head (H) = Hf + He + Hs

Discharge of main = dm (1 s^-1)

Efficiency (overall) = (60% in the case of electric pump, 40% in the case of diesel engine).

Example (1.0 ha citrus garden):

Basic Data Analysis:

1. No of plants:

Area = 1 ha = 100 × 100 m = 10,000 m²

Spacing (m) = 6 × 6

2. Estimation of water requirement:

Normal monthly pan evaporation data (mm):

From the above data, seasonal and daily pan evaporation (mm) is given below:

Therefore, drip irrigation system has to be designed for maximum requirement of 51.6 lpd plant^-1 during summer. Water requirement works out to 14.3 m³ day^-1 ha^-1 of plantation.

If the pump set works 4 h per day, the discharge required would be as below:

Pumping rate = 13 lph plant^-1

Pumping rate per ha

277 plants × 13 lph × 4 h = 14.404 1 day^-1 = 14.4 m³ day^-1 ha^-1

= 14.4/4h = 3.6 m³h^-1ha^-1 = 0.97 or 1 lps

Alternatively, a tank of 14.4 m³ capacity can be provided so that uninterrupted irrigation may continue for 4 h even in areas where power shut offs are frequent.

3. Selection of drippers:

Number of drippers:

For a pressure head of 10 m and discharge at 4.0 1 h^-1, number of drippers required is:

No of drippers per plant = Rate of pumping h^-1 plant^-1

Average discharge of one dripper = 13/4 or 3.22 say 3.0

As the plot is 1.0 ha square, the mainline would be 100 m long and laterals would also be 100 m in length. For a plant spacing of 6 × 6 m, a total of 17 laterals would be required. Each lateral would serve approximately 16 plants and there would be 3 drippers per plant. Thus, the total number of drippers per lateral would be 16 × 3 = 48.

4. Main line and laterals:

Main Line:

The main line is designed to carry the maximum discharge required for total number of plants in the farm plot.

Maximum discharge required = No of plants × peak discharge per plant

= 277 × 13 = 360 lph or 1.0 lps

Friction head loss in pipes (m)

Total length = 100.0 (m)

Equivalent length of 17 straight connection = 8.5

Equivalent length of Tee bends, etc. = 6.0

Total = 114.5 or say 115 m

From Table B, it would be seen that for a discharge of 1 lps through a pipe of say 40 mm dia, the friction loss would be 2 m per 100 m length of 2.3 m for 115 m equivalent length.

Friction head loss = 2.3 × 0.88 = 2.02

Conversion factor = (0.88)

As the proposed system uses multiple openings, the friction loss is taken as 1/3 of the total friction loss (2.03/3 = 0.67 m). Thus, the loss in mains is within 1.0 m per 100 m and a pipe of 40 mm dia will be ideal in the layout.

Laterals:

A lateral is so selected that the pressure difference from the proximate end to the last dripper does not exceed 10 per cent of the normal operating head which in the present case is 10 × 10/100 = 1.0 for lateral of 100 m length. The land slope is 0.5 m per 100 m. Thus, the total friction loss allowable is 1.0 + 0.5 = 1.5 m.

In addition to 100 m length of laterals, there is additional loss due to connectors. This is generally taken as 0.1 to 1.0 m (on an average 0.5) of the equivalent length of a dripper. The equivalent length of 48 drippers would be 48 × 0.5 = 4 m. Thus, total equivalent length for calculation of friction loss in laterals would be 24 m. The total flow in laterals is 192 lph (4 × 3 × 16).

A perusal of Table A shows that for 200 lph, flow the friction loss in 13.9 mm inner dia pipe would be 1.7 m per 100 m length. Therefore, in 124 m length it would be 2.20 m. It is a general practice that friction losses are taken at 1/3 of the total equivalent length of pipes with multiple dripper/connections. Thus, the friction loss works out to 1/3 × 2.2 = 0.74 m which is within the maximum permissible limit of 0.9 m. Therefore, 14 mm (OD) lateral pipe of 100 m length is suggested in this scheme.

5. Horse power of pump set:

The HP of pumpset required is based upon design discharge and total operating head. The total head is the sum of total static head and friction losses in the system.

Static head:

(i) The total static head is the sum total of the following (m):

(a) Depth to water (bgi)

(b) Drawdown

(c) Outlet level above ground level

(d) Friction loss in pipes, bends, and foot valves etc.

Total =15+ 3+1+2 = 21

(ii) The friction loss in the drip unit is as under:

(a) Friction loss in main pipe

(b) Friction loss in laterals

(c) Minimum head required over drippers

Total = 0.67 + 0.75 + 10 = 11.42 m

Total Head = Static Head + Friction head loss

= 21.00 + 11.42 m

= 32.42 or say 33 m

HP of pump set = Q × H/75 × e

where, Q = discharge (lps)

H = head (m)

e = pumping efficiency (0.6)

HP = 1 × 33 / 75 × 0.60

= 0.73 or say 1.0 HP.

Maintenance of Micro-Irrigation Systems:

Drip System Maintenance:

Most important aspect of maintenance in drip is prevention of clogging by physical and chemical methods.

Physical Methods:

Lateral flushing:

During irrigation season, laterals should be flushed every 2-3 weeks. Flushing is done by opening the lateral end for 30-60 seconds until the water coming out of the lateral is clear. Lateral flushing flushes out debris that accumulates in drip line and can eventually clog the dripper’s water inlet or labyrinth. Flushing sub-main or with a lateral flush valve will reduce costs of manual labour and guarantee frequent flushing.

Chemical Methods:

Acid treatment:

Application of acid is recommended as part of a routine maintenance procedure. Acid injection reduces clogging caused by low solubility salts, such as calcium carbonate.

Following chemicals are usually recommended for conducting acid treatment:

1. Hydrochloric acid (HCl 35%)

2. Nitric acid (HNO₃ 33%)

3. Sulfuric acid (H₂SO₄ 65 %)

4. Ortho phosphoric acid (H₃PO₄ 85%)

Determination of acid quantity to be injected:

Take a 10 l bucket and gradually start adding acid in small portions and measuring the accepted pH. Once you reach the required pH of 2.0, calculate the amount of acid required for receiving this value in your system by multiplying the acid quantity by 100 and injecting this amount per 1 m³ of the system discharge.

Precautions to be followed in acid treatment:

1. While preparing the acid solution, always add the acid to the water and not vice versa

2. Treatment should be carried out 1-2 times during the irrigation season or when system discharge drops by 5 per cent

3. Flush all sub-mains and laterals before starting the treatment

4. Check the discharge of the system before the treatment as well as after so as to compare with the discharge of the treated system

5. Solution preparation: The solution volume (water + acid) should be equal to one quarter of the hourly discharge of the injector. This way, the injection will last for 15 minutes. Recommend working with the maximum injector discharge in order to avoid working with a highly concentrated solution

6. Start the injection only after the system is full of water and the drippers are emitting

7. Control: Using a litmus indicator strip, check the pH at the furthest lateral for residual acid (pH 2.0). A second application is recommended if no residual acid is detected

8. Inject during 15 minutes

9. Continue irrigation for 30-60 minutes to ensure the complete flushing of the system

10. Check the discharge of the system.

Acid treatment example:

1. Acid needed for receiving pH (2.0) in the 10 l bucket = 12 cc

2. 12 cc × 100 = 1200 cc = 1.2 l

3. Inject 1.2 l of acid per 1 m³ of the system discharge

4. System discharge (of the treated sector) = 30 m³ h^-1

5. System discharge during the 15 minute treatment= 7.5 m³

6. Acid required = 1.2 l × 7.5 = 9 l

7. Max. injector discharge = 200 l h^-1

8. Total solution volume required (4 of 200 l) = 50 l (200/4)

9. 50 l of solution = 9 l of acid + 4 l of water

10. Injection time = 15 minutes (50 l injected with a 200 l h^-1 injector).

Iron and Manganese Treatments:

If water shows higher amount of iron and/or manganese, following measures can be taken, oxidation by aerations allows iron to precipitate faster. Store the water in settlement tank after stepped aeration to allow iron to precipitate down and then pump the water for your system. Chlorination along with aeration can enhance rate of oxidation. Please note that manganese impurities react slowly with chlorine hence they coagulate after the main filters.

In such cases, either allows some additional reaction and precipitation time or use plot filters as secondary fine filter to avoid dripper clogging which cannot be cleaned by any chemical means.

Chlorination Treatment:

Chlorine is a biocide that kills micro-organism viz. bacteria, algae etc. Chlorine injection will reduce clogging and help keeping the irrigation lines clean. It is recommended as an intermittent treatment or as an ongoing preventive treatment in systems that use water that contains a high concentration of organic materials.

Most commonly used materials available in three forms:

1. Sodium hypochlorite NaOCl, (10% chlorine)

2. Cl₂ gas (100 % chlorine),

3. Calcium hypoclorite Ca(OCl)₂ (50 to 65% chlorine).

Precautions for chlorination:

Do not have direct contact with skin, eyes, nose, mouth with any chlorine substance or chlorine (Cl₂) gas as it is poisonous for human and animals. Wear goggles, hand gloves, safety shoes etc. during chlorination treatment. Vessels for the solution should be thoroughly washed to avoid accident by reaction.

Never use fertigation of nitrogenous fertiliser during chlorination to avoid formation of sublime compound like ammonium chloride etc. Never mix acid in chlorine solution; use another device of injection for acid prior to chlorine. For making/diluting solution of chlorine, add chlorine product into water but do not pour water in chlorine substance/solution.

Treatment instructions:

Find out the required dose, treatment frequency and duration.

Refer to the chart below:

Water at pH above 7.5 reduces the chlorination effectiveness. Acidified to a pH of 6.5 will maximise the effectiveness of the chlorine treatment.

Contact Time:

A minimum contact time of 30 minutes is required for the effective chlorine treatment in order to kill the micro-organism. This time is measured from the moment you detect free chlorine in the emitters.

Concentration of free chlorine:

1. Measure active free chlorine concentration (residual chlorine), using a color comparison set. This is the same set that is used to monitor the chlorine level in swimming pools. The residual chlorine concentration depends on the water chlorine demand

2. Flush all sub-mains and laterals before starting the treatment

3. Dosing and injecting:

Use the following formula to determine injection rate and stock solution concentration:

If the injector can be manipulated to inject at different discharge levels, you may do so, according to your requirements. If not, you can adapt the stock solution concentration.

Adapting the Stock Solution Concentration to a Fixed Injection Rate:

Example:

1. System discharge (of the treated sector) = 30 m³ h^-1

2. Chlorine concentration required at injection point = 10 ppm

3. Chlorine quantity required: 10 ppm × 30 m³ h¹ 10%^-1 10^-1 = 3.0 1

4. Injector discharge = 200 l h^-1

5. Solution preparation:

Mix the 3.0 l with 197 l of water. This volume will be injected now in 1.0 h at 10 ppm of chlorine.

Warning:

Active chlorine is dangerous.

Follow the manufacture instruction:

1. Storage:

Sodium hypochlorite should be stored under a shaded area in a clean dark tank without any fertiliser residues

2. Concentration will degrade over time.