Unit - 3
Irrigation
1. Irrigation:
It is the supply of water to the land or crop, to help growth typically by the means of channels.
It is an artificial application of water to the crops for assured production.
It is a controlled supply of water, as per the requirement of the crop".
The irrigation water dissolves the nutrients in the soil, which are required for the plant growth through roots.
The irrigation water supplies moist to the soil. Which is required by the micro-life in the soil i.e., the de-composers in the eco-system
It helps to control the soil-temperature.
It helps to soften the tillage pav. i.e. the area to be irrigated
It provides a controlled and timely supply of water to be plant i.e., during sowing, growing and harvesting period.
On the basis of the availability of water and source of water, the methods of irrigation vary as given below:
A: Surface Irrigation Methods
B: Sprinkler Irrigation Methods
C: Sub-surface or sub-irrigation Methods
2. Necessity
3. Advantages of irrigation
Following are the advantages of irrigation:
(A) The water is made available, when even required for the crops.
(B) It has the controlled supply of water
(C) It can help to have more cropping season i.e.; the summer crops are possible only through irrigation.
(D) It helps to go for commercial crops.
Key takeaways:
1. Free flooding
Free flooding technique is likewise called irrigation with the aid of using plots.
The area is split into numerous plots of almost identical ranges and water is admitted to those plots on the better end.
The length of the plot relies upon the porosity of the soil.
Suitability:
Advantages:
Disadvantages:
2. Border flooding
In this method, the total land, which is to be irrigated, is divided into a number of long narrow strips.
These strips are separated by low levees or by flat dykes or borders.
Each of the strip is separately irrigated, by supplying water, at its upper end, by a supply channel or an underground pipe.
All the strips have a gentle slope in longitudinal direction but have no cross slopes.
So, when the irrigation water is applied to any strip, it gets spread, uniformly over its entire width and does not get accumulated on either side as it flows down the slopes.
The strips are laid down on a flat surface with a gentle slope and so, they are almost parallel to each other.
In case of the lands which have steep slopes, then the strips are laid across the general slope of the land and are aligned to follow the different contour lines, at a vertical interval of 0.30 to 0.60 metres.
These strips are called as contour strips while the parallel and strips are called as straight strips. The design criteria are same for contour strips and straight strips.
3. check flooding
This is one of the most common meth irrigation, in India and in most of the Agro based countries in the world. It is also called as 'Plot Irrigation'.
The land, which is to be irrigated, is divided into a number of plots (check basins). Each one of the plots is surrounded by checks or levees or bunds or low dykes and each plot or check has a leveled surface.
The irrigation water is applied by filling the plots with water, up to the desired depth (which is always less than the height of the levees). The water is retained and is allowed to infiltrate in the soil.
These levees (which separate the pots) are either temporary for one single irrigation as in the pre-solving irrigation of any seasonal crop, or it may be kept for the total cropping season to permit a number of irrigations to be applied (as required by the crop).
These levees can be semi permanent and can be used repeated e.g., Levees in the rice (paddy) fields.
The size of the levees, depends upon the depth of the water to be applied in each plot and also on the stability of soil, when becomes wet.
4. Basin flooding
It is a special form of check basin method of irrigation, which used for the tree crop. i.e., cinchers (Horticulture). In this for each tree, a separate basin of circulate shape is made and, so the method is called as Ring basin method.
Above mentioned for methods are the types of controlled flooding methods. Together with flooding there two other method of surface irrigation, one is Furrow method another is contour farming.
5. Furrow irrigation
In this method, the water is applied to the land, which is to be irrigated, by a series of long narrow field channels. They are called as Furrows as the method is called as Furrow Method.
These furrows are dug in the land with regular intervals. The water flows through the furrows and gets infiltrated into the soil and then it gets spread horizontally to irrigate the land.
In case of all the flooding methods, almost entire land is welted while in case of this furrow method, land which is wetted varies between 50% and 20%, so the evaporation losses are put under control.
This method is commonly used for the crops such as Maize, Cotton, Potatoes, Sugarcane, Beat roots, Tobacco, Groundnuts etc. They are known as raw crops.
Types of Furrows:
On the basis of the alignment the furrow is classified as under.
A: Straight furrows: They are aligned almost parallel to each other, in the form of straight lines and they are laid along the slope of the land. (The general Slope of the land should not be more than 0.75%.)
B: Contour furrows: These furrows are aligned along the contour lines i.e., each of the furrows will be at the same altitude, and so they are laid across the slope of the land. They are occurred in the plain. This method is adopted in a region where the slopes are steeper i.e., more than 0.75%
This method of irrigation has the following advantages:
In this method the water is supplied to land by spraying it (like an artificial rainfall) as it comes from above, it is also known as overhead Irrigation.
This method can be applied to any type of crops except Rice and Jute as both need standing water in the field. Rice is sown in the soil which is in a muddy form.
This method is applied to almost all types soils except, clayey soils which are heavy and the rate of infiltration of this soil is very low. so, it is best suitable for the very light soils in which deep percolation losses are very low.
As this method can be applied to a land which has rough topography, the leveling of land, before irrigation is not required.
Components System of Sprinkler Irrigation
Following are the important components of the sprinkler irrigation system:
i) Main pipe lines (also called as mains).
ii) Sub-main pipelines (also called as sub mains).
iii) Lateral pipe line (or laterals).
iv) Riser pipes (or Risers). v) Sprinklers
vi) Pumping unit
Working of sprinklers
The pump lifts the water from the source region and it is supplied to the sprinklers. These sprinklers spray the water.
Types of Sprinklers
In this system of irrigation three types of sprinklers are used they are as follows:
It consists of a lateral pipe which have a line of small holes. These holes are drilled at the top, with regular intervals, along their length. On each of these holes small nozzles are fixed.
A series of such pipes (have holes with the nozzle) are installed. They are kept parallel to each other, with an average distance of about 15 metres. These pipes are supported on raw posts.
The spraying of water is developed through these nozzles. By turning these pipes through 135", the entire 15 metres width between two pipelines is property irrigated. This is an old method of sprinkler irrigation. At present it is not commonly used.
b. Perforated Pipe
This type of sprinklers has the lateral pipes with small holes or perforators. They are drilled at the top and along the sides. They are designed in such manner to provide the water to all land uniformly,
A series of such pipes are laid on the ground keeping then parallel to each other’s Through the drilled holes, which are designed on both the sides of the pipe, can cover a strip of the land (which varies between 6 and 15 metres in width)
In this method rate of application of water is about 20 mm/ per hrs. As the application of water has high rate, these types of sprinklers are best suitable for the soils which have a moderately high rate of infiltration,
The operation pressure for such type of sprinklers is in the range between 0.49 and 0.245 N/mm² i.e., between 0.5 and 2.5 kg (0/cm³. This required pressure is low so no pumps are required, and the water can be supplied from an over head tank.
Such sprinklers are used to irrigate almost all crops. It has the advantage of low rate of application though large nozzle opening are in operation.
Another advantage of rotating sprinklers is that as it has large nozzle openings the clogging or caused by silt or debris is minimized.
The slow rate of water application is good for the soils having low infiltration rate.
In case of low water requirement, the single nozzle sprinklers are used. The people prefer to use two nozzles out of which one nozzle is used to apply water to a long (considerable) distance from the sprinkler, and the second nozzle is used to cover the area near the sprinkler.
For that the angles of these nozzles are adjusted, one in upper direction to provide water to long distance land and another in down ward direction for the closer areas. Both of these nozzles provide water very uniformly.
Depending upon the size, the presser for rotating sprinklers varies i.e., for a small size sprinkler it is 2.0 N/mm² i.e., 2kg(f) / cm² and for a large size sprinkler it is more than 0.7 N/mm² Le. 2 kg (1) /cm².
7. Drip irrigation
The regions having scarcity of water and problem of Stalinization of soil this method is best suitable.
This is one of the latest methods of irrigation and also a very popular method among the actual users of irrigation farming.
In this method small diameter plastic pipes are used together with drip nozzles which are known as Emitters or Drippers. They deliver water to the land surface, near the base of the plant.
In this method the application of water is low rate which varies between 2 and 10 lit/per hrs.
This keeps the soil moisture within the desired range for growth of the plant
Main Components of the Drip Irrigation System:
It has the following components
(a) A pumping unit.
(b) The main pipelines.
(c) The sub-main pipelines.
(d) The lateral pipelines.
(e) The drip nozzles or Emitters.
All the pipes used in this system are made up of Black PVC (Poly Vinyl Chloride) tubing's. The drip nozzles also are made up of PVC material.
The nozzles are designed in such manner no jet of water is given out but they allow water to seeps out or to trickles out.
The drip nozzles or the emitters are provided with regular intervals on the lateral pipelines.
Advantages of Drip Irrigation:
A. The water application efficiently is very high, if properly managed.
B. The loss of fertilizers and nutrients is minimized.
C. The leveling is not required.
D. It can be applied for the fields having irregular shapes.
E. In this method, the non-potable water can be recycled.
F. The soil moisture in the root zone can be maintained.
G. The soil erosion is minimized.
H. It is an eco-friendly irrigation method.
I. As it reduces water as well as soil losses.
J. It has less evaporation losses.
Key takeaways:
1. The water application efficiently is very high, if properly managed.
2. The loss of fertilizers and nutrients is minimized.
3. The leveling is not required.
4. It can be applied for the fields having irregular shapes.
1. Crop water requirement:
The total amount of water needed by a crop from the time of its sowing up to the period of harvesting
If put together, the-sum total of water is known as 'The water requirement of crop'
This quantity of water required by the crop varies from crop to crop, also varies from sowing harvesting period i.e., summer crops, winter crops, rainy season crops and naturally varies from soil to soil, depending upon the capacity of soil to retain the water
For any crop, it is also necessary to maintain the quantity of readily available moisture in soil, by irrigation and so it is necessary to distribute the total amount of water required by any crop, in such a way that a part of it supplied at each irrigation, must be sufficient to meet the need of the crop, for a period in between two successive irrigation and is stored in the soil.
So, together with the quantity of water required by the crop, it is necessary to calculate, the required frequency of irrigation and also the total quantity of water to be irrigated during each application. All these points (qualitative
2. Duty
It is defined as, "it is the period to which the stated duty of water has reference." The total quantity of water is supplied to a crop, through a no. of devices at central interval.
The quantity of water to be given to a crop, during each watering varies and it is to be given a specific duration.
So, the term "Duty of water" is used to indicate the quantity of water required to be given during each watering and it is referred to a specific period when the water is applied, so it is called as the base of the duty of water. When the duty of water is stated for each watering, its base also needs to be stated.
3. Delta
It is total depth of water required by a crop to came to maturity is called as Delta. It is measured in meters or centimeters.
4. Base period
The long duration crops like sugarcane, as they stand in the field for a longer period of time (12 months or 18 months) they need more water and the duty of water will be less.
On the other hand, it is short duration crop like rice, wheat are four months crops, vegetable is the shortest duration crop (one and half month); need less water and so, the duty of water for the short duration crops, is always more.
5. Crop period
"It is the time interval between the sowing of the crop and its harvesting". (It is lim more period of time than Base Period d first watering to the last watering of crop).
Key takeaways:
Irrigation efficiency (n) is the optimum use water applied, for the growth of the plant. But it is fact that even with the best method of irrigation, there are always some losses through seepage, percolation and evaporation from the canals and also from the field.
The ratio of, water available for use to the water applied can be defined as the efficiency of irrigation (Theoretically the ratio should be 1:1, to indicate optimum efficiency, but it never observed in reality.)
Generally, the well irrigation has maximum efficiency as, the transit losses are almost nil as the well water is directly applied to the field (No chance of losses through seepage or percolation).
Various methods are used to calculate various types of irrigation efficiencies which expressed as below.
It is a ratio of the quantity of water delivered to the field (or to the irrigated land) to the quantity of water diverted into the canal system from the river or reservoir it is expressed in the following equation,
Where,
c = water conveyance efficiency.
W = Quantity of water delivered to the field.
2. Water application efficiency
It can be defined as, "The ratio of quantity of water stored in the root zone of the plant, to the quantity of water delivered into the field", This ratio is expressed in the following equation.
Where,
n₁ = Water application efficiency.
Wr = The quantity of water stored in the root zone.
Wf = The quantity of water delivered into the field.
3. Water Use Efficiency (nu)
It is the quantity of water used, beneficially, including the water required for leaching, to the quantity of water delivered. This ratio is expressed in the following equation.
Where,
nu = The water use efficiency,
Wu= The quantity of water used beneficially
Wf=The quantity of water delivered to the field.
4. Water Storage Efficiency (ns.)
It is the ratio of the quantity of water stored in the root zone (during the irrigation) to the total quantity of water needed to bring the moisture content of the soil to its field capacity. It is expressed in the following equation.
Where,
ns, = Water storage efficiency.
W, = Quantity of water stored in the root zone.
W, = Quantity of water needed to bring the moisture content of the soil to the field capacity (W = field capacity available moisture in the soil, before irrigation).
Key takeaways:
1. Soil moisture:
It is known fact that the growth of most of crops is determined by either excessive or deficient amount of soil moisture contents. e.g., If excessive moisture is available in the soil, it fills the soil pores and aeration capacity of soil is reduced from the root zone.
As the growth of the aeration, it affects the plant growth. On the other hand, if the soil has deficient amount of moisture, the soil in the root zone, holds it so tightly that the plants have to use an extra energy to get the moisture from the soil and it also affects the growth of the plant.
If the rate of intake of moisture by the plant is not sufficient to maintain the turgidity of the leaves, this condition brings the permanent wilting point.
So, between these two extreme moisture conditions, i.e., excessive moisture supply or deficient moisture supply there is a moisture content, which is designed as the optimum moisture content at which the growth of the plant is very rapid which finally results in the optimum growth of the crops.
2. Irrigation frequency relationship:
It is necessary that the amount of water applied to the soil, should be such that the moisture content of the soil is lifted up (raised up) to its field capacity. When the supply of water is stopped the moisture content of soil is reduced due to evaporation from soil and also due to transpiration.
The total water (moisture) lost by either evaporation or transpiration is known as Evapo transpiration or the consumptive use of water.
This reduction of soil moisture content should be allowed to go below the readily available moisture i.e., if and when the soil moisture content reaches the lower limit of the readily available moisture, it must be immediately, replenished by irrigation and the field capacity of soil should reached.
The minimum depth of water dw, is to be applied, during irrigation dw can be calculated by following formula.
Field capacity- weight of the moisture content held by the soil at the lower limit of the readily available moisture per unit weight of dry soil weight of the readily available]
Where in, w, = Specific weight of soil, w = Specific weight of water
The frequency of irrigation, [the time interval between any two successive irrigations to the same soil] depends upon the amount of readily available moisture in the root zone of the soil and rate of the consumptive use, through the evapo-transpiration.
Key takeaways:
1. Irrigation channels:
It is the name given to ditch or drain or trench which is constructed to carry water to the fields for irrigation either from river tank or diversion weir.
A canal can be defined on 'It is on artificial channel which has been constructed to carry water from a stream or bank or on artificial reservoir
There are different purposes to construct any canal like for irrigation for navigation or for hydel power generation. The purpose being the same the channel or canals are used as synonymous words. The canals have trapezoidal cross section. The canals are constructed on the basis of the purpose construction Le canals for irrigation.
2. Alignment
Canal alignment is the location of the centre line of canal in plan and fixing it on the ground.
Therefore, alignment should have given more concentration while fixing it. It should be done in such way that it should irrigate maximum area.
The length of canal should be minimum as shorter the length means head loss is less and smaller loss of discharge.
The alignment should be such that it has minimum cross drainage work and it should be such that cutting and filling of soil will balance.
3. Classification
Classification of canals based on alignment
1. Contour canals
2. Ridge canal/Ridge alignment
3. Side slope
4. Canal network
1. Contour canals:
When canals are to be aligned in hilly area's it is not possible to align along areas which is at top of the hill.
In such case canals are aligned generally along contours.
The canal taking off from reservoir follows contour alignment such contours which is aligned parallel to the contours of area is known a contour canal.
Canals for navigations, canals for perver generation etc. Among them the irrigation canal is constructed to carry the water up to the agricultural fields from their sources. The import canals constructed for irrigation purpose in India are, Ganga canal, western Yamuna canal, Bhakra canal, Rajasthan canal (known it is called as Indirs, canal).
Generally, the canal water is used to generate hydel power in the upstream and then the water is carried further through them canals to irrigate the fields under cultivation.
2 Ridge canal/Ridge alignment
It is also called as watershed canal. These types of canals are generally laid along ridge or natural watershed line as canal runs on watershed it can irrigate on both sides and thus irrigates area on both sides.
These canals are very economical. No cross-drainage work is required.
3. Side slope canals
In this type of alignment canal is not aligned along either ridge or contour but it is aligned across contour.
These canals run nearly parallel to natural grain similar to contour canal. It also irrigates areas on only one side.
4. Canal network
Key takeaways:
1. Distribution system
From the point of the head works where the water enters in the main channel up to the final point where the water is supplied to the agricultural fields, a considerable amount water is lost (not available for the field irrigation) is called as water losses or transit losses or transmission Losses.
In case of the un-lined channels these losses up to 25% to 30% of the total volume of water made available for irrigation from the head works. (In the lined channels their transit losses are less).
2. Water losses in irrigation channels
(A) Losses due to Evaporation
(B) Losses due to Percolation
(C) Losses due to Transpiration
(A) Losses due to evaporation:
The loss of water through evaporation depends upon various factors like temperature. Surface are of the water bodies, relative humidity (if relative humidity is 100%. i.e., the air is totally saturated with moisture there will be almost no evaporation if the air is totally dry i.e., there is no moisture the rate of evaporation will be maximum), velocity of wind, cloudiness etc. In the summer season i.e., in the months of March, April and May in India as the duration of the day is higher the incoming solar radiation, through short waves, is more and, so during these months the rate of evaporation is more. In winter due to less temperature and during rainy season due to high relative humidity and over cast conditions (i.e., cloudiness) the rate of evaporation is less.
For these losses no special consideration is made while designing an irrigation channel.
(B) The losses due to percolation:
This type of loss of water is also known as seepage loss. The percolation losses depend upon the following conditions,
(a) Permeability of the soil in the bed and on the banks of the channel: If the soils are highly permeable the water losses will be very high.
(b) The depth of the water in the channel: If the depth is higher, higher will be the water losses, through percolation.
(c) Velocity of the flow: If the velocity of the water is high, the rate of percolation will be less.
(d) Amount of silt in the water: If the water has more silt in suspension, the rate of percolation will be less.
(e) Temperature of water: If the temperature of flowing water is high, the losses through percolations are high.
(f) Age of the channel: As the channel becomes old, the fine silt gets deposited on the bed of the channel and block the holes through which the water gets percolated and it reduces the speed of loss of water through percolation. In case of new channel, the holes in the bed are still open and it has less siltation, so the rate of percolation is always greater.
(g) The depth of the ground water: If the ground water level is just below the surface the rate of percolation is less.
(C) Losses through transpiration
There is a little loss of water through the plants, vegetation and weeds on the banks of the channel due to transportation. This can be controlled by keeping the banks clean from the growth of vegetation and weeds.
Key takeaways:
Comparison of Kennedy’s & Lacey’s Theories
Sr. no | Kennedy’s Theories | Lacey’s Theories |
1 | Kennedy has indicated that the vertical component of eddies generated from the bed of the channel keeps the silt carried by water, in suspension. So, he has given the relation between V and D while | Lacey believed that, the silt carried the water flowing in the channel, is kept is suspension by the vertical component of eddies, generated from the entire perimeter of the channel and so, he has given the relation between 'V' and 'R'. |
2 | For making his equation applicable to the channels flowing in different soils of different grades, Kennedy has introduced the concept of critical velocity ration 'm' but has not developed any method to calculate the value of 'm'. | Lacey has introduced the silt factor 'T, for making his equation applicable for the channels flowing in different soils having different grades and has provided a relation between 'T' and the mean particle size 'm', which is used to determine the value of 'T. |
3 | To determine the mean velocity of flow, Kennedy has used Katter’s equation (which has its drawbacks) own | Lacey has developed his own equation to determine the velocity of flow. |
4 | Kennedy has not developed any equation for the bed-slope of a channel and has used wood's table. | Lacey has developed his own equation for the bed slope of the channel |
5 | Kennedy has not mentioned anything about the shape of the section of a stable channel while | Lacey has made a mention about the shape of a channel (true regime channel has a
semi-elliptical section). |
6 | For the design procedure of a channel Kennedy has used, trial and error method | Lacey has not made the use of any trial-and-error method in his design procedure of a channel |
1. Irrigation canal lining:
It is the impervious layer which protects the bed and sides of canal.
Lining s generally a construction of a thin 2.5 to 15 cm thick layer of lining material, generally RCC or CC bricks, stones etc
Lining an irrigation canal or channel needs extra amount to be spent on the construction but in the long run, this lining which helps to control the seepage through the banks and bed and this extra water made available through the process of lining can justify the extra expenses made on the lining of the channel.
2. Types of Canal Lining
1. Cement/lime concrete lining
2. Cement mortar lining
3. Stone masonry lining
4. Brick lining
5. Concrete lining
6. Asphaltic lining
7. Precast concrete block lining
8. Sodium carbonate lining
1. Cement or lime concrete lining:
The lining which is called ca concrete lining done by connate in is very useful as it permits high redaction in seepage losses. It is suitable for both small and big canals. It is strong and durable and can be used to any thickness.
High velocity is possible due to reduced cross section and strong pavement which prevents sitting tendency in canal. Its maintenance cost is less.
The disadvantage of such lining that is its initial cost is more and cracks may develop due to temperature and shrinkage.
2. Cement mortar lining:
In this type of lining seepage losses are reduced by 90 so 95%. It is commonly used due to its durability.
Impermeability and its hydraulic efficiency
Material used in this lining is Portland cement, sand gate, water. Sometimes for special purposes admixtures are added to it for required result.
3. Stone masonry lining
Such type of lining is suitable where stones are available in large quantities.
To make surface proper sometimes it is plastered.
4. Brick lining
When bricks are used for lining is called us brick lining.
It is economical and gives fair protection from seepage but it is not as impermeable as concrete lining and it has low resistance to erosion.
5. Concrete lining
In concrete lining survey of cement mortar is forced under pressure through nozzle on the surface of canal
It is a mixture of cement and sand. It is also used for resurfacing old lined surface. Thickness of this lining varies from 3.5 cm to 5 cm.
6. Asphaltic lining
In this type of lining asphalt cement is used with sand and gravel. It provides smooth flexible surface the only drawback of this lining is it absorb heat and encourages weeds growth a using rapid deterioration.
7. Precast concrete block lining
In such type of lining concrete blocks are casted in factories are used to prepare bed, it has high durability hydraulic efficiency the thickness of such lining may vary from 5116.5 cm.
3. Advantages
The following are the advantages of canal lining:
1. Reduction in losses due to seepage: Lining prevents seepage, so finally more area can be irrigated by same quantity of water and valuable water is saved.
2. Prevention of water logging: Seepage is main cause of water logging if canal is lined the seepage is reduced and thus helps in protection from water logging.
3. Low maintenance cost: The maintenance of lined canal is less as compared to unlined canal.
4. Prevention of weed growth: Lining prevents the growth of weeds.
Steeper bed slope can be provided Hard surface lining can sustain higher velocities, that means steeper bed slopes can be provided.
6. Less breaches: In lined canal possibility of breaching is less as section due to lining is more stable and strong.
Less silting because of higher velocity silting is less.
Low evaporation losses because of higher velocities water can reach quickly to area and thus evaporation losses are minimum.
Less salt problem as canal water does not comes in contact with harmful salt present in natural soil, salt problem reduced to some extent.
4. Economics & preliminary design
The extra cost paid for lining can be justified if extra cost is less or equal to the extra benefits that have been gained due to lining of the channel Le, saving the wastage of water, by preventing seepages is an additional benefit we have gains through proper lining of the channel.
By using the following equation, we can calculate the maximum rate of expenditure on lining. Let C Cost of lining in Rs; per sq.m; including the additional cost of dressing the banks
For lining and also accounting for saving resulting from small cross sections quantity of earth work and the structure required for lined sections.
This type of calculation can be made on new canal and not on the old unlined canal to be covered into a lines canal.
S and S Seepage losses in unlined and lined canals, in cubic m per sq.m of wetted surface, per day (i.e., 24 hrs.)
P and P = Wetted perimeter in m. of unlined and lined sections.
T = Total perimeter of lining (in m.)
d = Total no. of running days of a channel, in a year.
W= Value of water saved (in Rs.) per cu. m.
L = Length of the channel (in m.)
y = Life of the channel (in years)
M = Total annual saving (in Rs.) in operation and maintenance, due to lining.
B Annual estimated value (iu Rs.), of other benefits, for the length of the channel under consideration (like, prevention of water logging. reduction in the cost of drainage for the adjoining land, reduced risk of breaching etc.)
X = Present rate of interest.
The addition cost spent on the construction of a lined channel=TLC (In Rs).
This cost (TLC) must be recovered from the saving during the useful life of the lining (y) in years. If the prevalent rate of interest is X the Net Present Worth (NPW) of the total annual benefits 'a' over the life of the lining (y years) is determined by the following formula. NPW= (1+x)y -1/ X(1+x)y
To make the work of lining economical the additional initial cost of lined canal (TLC) must be equal or less than Net Present Worth (NPW) of the annual benefits i.e., TCL S NPW
In this method we must note that it is very difficult to have the actual benefits groups under item B and M. (mainly in case of new project work). So, these values are based on the experience of similar existing project.
It is assumed for the calculations, that the life expectancy, of concrete, brick tiles, boulder lining etc. is 60 years.
Key takeaways:
References:
