IRRIGATION ENGINEERING : Irrigation methods, design of irrigation canals

Types of Irrigation Systems

Major aim of irrigation systems is to help out in the growing of agricultural crops and vegetation by maintaining with the minimum amount of water required, maintenance of landscapes and re-vegetation of disturbed soils. Irrigation systems are also used for dust repression, removal of sewage, and in mining.

On the contrary, agriculture that relies only on direct rainfall is referred to as rain-fed or dry-land farming.

Techniques of Irrigation

In India, the irrigated area consists of about 36 percent of the net sown area. There are various techniques of irrigation practices in different parts of India. These methods of irrigation differ in how the water obtained from the source is distributed within the field. In general, the goal of irrigation is to supply the entire field homogeneously with water, so that each plant has the amount of water it needs, neither too much nor too little. Irrigation in India is done through wells, tanks, canals, perennial canal, and multi-purpose river valley projects.

A) Surface Irrigation

In this technique, water flows and spreads over the surface of the land. Varied quantities of water are allowed on the fields at different times. Therefore, it is very difficult to understand the hydraulics of surface irrigation. However, suitable and efficient surface irrigation system can be espoused after taking into consideration different factors which are involved in the hydraulics of surface irrigation. 

1. Surface slope of the field
2. Roughness of the field surface
3. Depth of water to be applied
4. Length of run and time required
5. Size and shape of water-course
6. Discharge of the water-course
7. Field resistance to erosion

If the surface irrigation method is perfectly selected, it fulfils following requirements:

1. It assists in storing required amount of water in the root-zone-depth.
2. It reduces the wastage of irrigation water from the field in the form of run-off water.
3. It reduces the soil erosion to minimum.
4. It helps to apply a uniform application of water to the fields.
5. Amount of manual labour required is less.
6. It is suitable to the size of the field and at the same time it uses the minimum land for making ditches, furrows, strips, etc.
7. It does not avert use of machinery for land preparation, cultivation, harvesting

Surface irrigation technique is broadly classified as

1. Basin irrigation - Basin irrigation is common practice of surface irrigation. If a field is level in all directions, is encompassed by a dyke to prevent runoff, and provides an undirected flow of water onto the field, it is herein called a basin. It may be furrowed or ridged, have raised beds for the benefit of certain crops, but as long as the inflow is undirected and uncontrolled into these field modifications, it remains a basin.

2. Furrow irrigation - In furrow irrigation technique, trenches or “furrows” are dug between crop rows in a field. Farmers flow water down the furrows (often using only gravity) and it seeps vertically and horizontally to refill the soil reservoir. Flow to each furrow is individually controlled. Furrow irrigation is suitable for row crops, tree crops and because water does not directly contact the plants, crops that would be damaged by direct inundation by water such as tomatoes, vegetables, potatoes and beans. It is one of the oldest system of irrigation. It is economical and low-tech making it particularly attractive in the developing world or places where mechanized spray irrigation is unavailable or impractical.

In different situations, different furrow methods are used (Surajbhan 1978). They are mainly of five types:

1. Sloppy Furrow
2. Levelled Furrow
3. Contour Furrow
4. Serial Furrow
5. Corrugated Furrow

There are numerous advantages of Furrow technique of irrigation:

1. Large areas can be irrigated at a time.
2. It saves labour since once the furrow is filled, it is not necessary to give water a second time.
3. It is a reasonably cheaper method.
4. Plants get proper quantity of water by this system.

Major drawback of furrow system of irrigation is ensuring uniform dispersal of water over a given field. To tackle this problem, some farmers engage in field levelling to remove any small hills that would have been bypassed by the gravity flow of the water. Other problem with furrow irrigation is the increased potential for water loss due to runoff. Building retention ponds along the edges of fields can help capture this runoff, allowing it to be pumped back to the upslope side of the field for use in further irrigation cycles.

• Uncontrolled flooding: There are many cases where croplands are irrigated without regard to efficiency or consistency. These are usually situations where the value of the crop is very small or the field is used for grazing or recreation purposes. Small land holdings are generally not subject to the range of surface irrigation practices of the large industrial farming systems. The assessment methods can be applied if desired, but the design techniques are not generally applicable nor need they be since the irrigation practices tend to be minimally managed.
• Free flooding - This flooding system of irrigation is used from ancient times. Flooding method consists in applying the water by flooding the land of rather smooth and flat topography. In current irrigation practice, several flooding methods have been developed. In free flooding method, water is applied to the land from field ditches without any check or guidance to the flow. The land is divided into plots or kiaries of suitable size depending on porosity of soil. Water is spread over the field from watercourse. The irrigation operation begins at the higher area and proceeds towards the lower levels. The flow is stopped when the lower end of the field has received the desired depth of water. The field watercourse is properly spaced, the spacing depends on the topography, oil texture, depth of soil and size of stream.

This technique is beneficial for newly established farms where making furrows is very expensive. This method is economical and can be effectively used where water supply is in plenty. This method is suitable for the fields with an irregular surface in which other techniques are difficult to apply.

The major drawback of this method is that there is no perfect control over the flow of water to attain high efficiency. Sometimes the flow of water over the soil is too rapid to fulfil soil moisture deficiency. On the other hand, sometimes water is retained on the field for a very long time and consequently, the water is lost in infiltration or deep percolation.

3. Border Strip Method - In this technique of irrigation, a field is divided into number of strips. The width of strip varies from 10 to 15 metres and length varies from 90 m to 400 m. Strips are separated by low embankments or levees. The water is diverted from the field channel into the strips. The water flows gradually towards lower end, wetting the soil as it advances. The surface between two embankments should essentially be level. It assists in covering the entire width of the strip. There is a general surface slope from opening to the lower end. The surface slope from 2 to 4 m/1000 m is best suited. When the slope is steeper, special arrangement is made to prevent erosion of soil.

Classification Based on Availability of Water

1. Gravity Irrigation:

Gravity or flow irrigation is the type of irrigation in which water is available at a higher level as to enable supply to the land by gravity flow. In flow irrigation water is supplied to the fields though the canals off taking from head works. Gravity flow irrigation is cheaper compared to lift irrigation. The gravity irrigation is further classified as under.

1.1 Perennial Irrigation

In this system assured the supply of water throughout the crop period to irrigation requirement of the crops is made available to the command area through storage of water done at the dam or diversion of supply made by means of head works at the off take point of the canal. Perennial irrigation may be either direct or indirect, as follows:

1.1.1 Direct irrigation:

In direct irrigation system, water is directly diverted from the river into the canal by the construction of diversion weir or barrage across the river without attempting to store water. This method is practiced where the river has the adequate perennial supply to feed the canal system at the times of crops periods.

1.1.2 Indirect irrigation:

It is also termed as storage irrigation. Here water is stored in reserved during monsoon period by the construction of a dam across the river for supply into the off taking canals. Evidently indirect irrigation is adopted where the river is non –perennial or flow in the river is inadequate during lean period. Storage irrigation has greater irrigation potential the direct irrigation but is costly due to the cost of construction of the dam.

1.2 Non –Perennial Irrigation:

 Also called restricted irrigation. Canal supply is generally made available in non-monsoon period from the storage in small dams as in Kandi areas which inadequate to feed all the year round, and/or canal water is not required during monsoon due adequate rainfall in the command area.

1.3 Inundation Irrigation:

Inundation irrigation is done by a canal taking off from a river in flood without any diversion work. It depends on the periodical rise in water level of the river and the supply is drawn through open cuts in the river bank or creeks which are called heads. Owing to changes in the river course the heads have often to be changed. A regulator is, however, provided at the canal about 5 km downstream from the off take, where the discharge passing below in the canal is controlled and the surplus supply is escaped back into the river. Inundation canals usually flow only during the summer months and bring in large quantity of silt beneficial to crops.

Design of Inundation Canal:

The bed level in the channel is kept low enough to draw about half the full supply discharge of the canal in low river, i.e., in the middle of April and again in October, and narrow enough to limit the excessive high floods to the minimum possible. Full supply level is fixed with due consideration to the steady water level in the river during about 1 ½ months, usually fair irrigation season. Beyond the head regulator, design considerations of unlined canals apply equally to the inundation canals. However, higher values of silt factor f and water surface slope are adopted than those allowed for canals off taking from permanent head works. Generally the slope ranges between 0.20 and 0.25 m/km. Manning’s, Chezy or Lac the design of inundation canal.

      Advantages:

(i)  Economical in cost being the simplest system of irrigation as permanent head works is not to be constructed,

(ii) Silty water carried by the canal has manorial value, and

      Disadvantages:

(i) Highs maintenance cost, and

(ii) Seasonal irrigation.

2. Tank Irrigation: Tanks on local streams form a significant source of irrigation especially in the peninsula area in the States of Karnataka, Maharashtra and Tamil Nadu. Tank irrigation belongs to category of storage irrigation. Tanks are small sized reservoirs formed by small earthen embankments to store runoff for irrigation. The site is selected within a watershed protected by vegetation and containing minimum of cultivated land so as to ensure minimum rate of sedimentation which lowers its storage capacity. Adequate soil conservation measures are essentially adopted to ensured quantity and quality of water inflow into the tank.

The essential components of irrigation that are tank embankment, surplus or escape weir, and outlet sluice. A suitable breaching section also sometimes provided to ensure that the tank embankment is not overtopped in the event excessive discharge from the catchment. The breaching section is a low level embankment of certain length designed to have a localized breach to escape excessive inflow.

3. Lift Irrigation: 

In lift irrigation water is lifted from a river or a canal to the bank to irrigate the land which are not commanded by gravity flow. Lift irrigation is being increasingly practiced in India. Every State such areas exist where irrigation can be extended only by lift canals. Lift irrigation also includes tube well irrigation but the latter is not feasible in areas where scarcity of water exists, climate is dry and groundwater is low, i.e., groundwater is in insufficient quantity and unsuitable quality. Lift canal then constitutes the only means of extension of irrigation to sound perched lands. A lift canal can cater for much larger areas than a tube well and is suitable when supplies either from a river or a canal are available for lifting to higher elevation.

Lift area is defined as the area the level of which is too high to permit irrigation by gravity flow from the source, but which can be irrigated by lifting water to the necessary level by means of pump Gross lift area is the portion of gross irrigable area which can be irrigated only by pumping.

In lift Irrigation mechanical devices like pumps, or electric motors and pumps are required to be installed for lifting water. Electrical pumps are generally provided for lifting water. Diesel pumping sets are also installed as standby.

Lift Irrigation vs. Gravity Irrigation:

Lift irrigation

1.Costly means of irrigation

2.Less manorial silt in water

3.Working  dependent on the operation of machinery

4.Higher water rates.

5. Lift irrigation is a complex system and by and large costly.

Gravity flow irrigation

1.Cheapest means of irrigation

2. Silt in water has manorial value

3. Lifting equipment is not involved

4. Lowest water rates

5. Simple and economical system of irrigation.

4. Well Irrigation

Groundwater:

Groundwater is generally a more dependable source of irrigation than surface water and is free from seeds and plant organisms. The first cost of installation is, however, high. The best water bearing stratum or aquifer is coarse gravel free from sand but such formation are rare to find. An aquifer is a saturated formation which creates the ground water reservoir and yields sufficient quantity of water to wells or springs. These are made of unconsolidated formations like sand, gravel, fractured rocks.

Subsurface Irrigation:

It is termed as subsurface irrigation because in this type of irrigation, water does not we the soil surface. The underground water nourishes the plant roots by capillarity. It may be divided into the following two types:

Natural sub-irrigation; and Artificial sub-irrigation.

Natural sub-irrigation: leakage water from channels, etc., goes underground, and during passage through the subsoil, it may irrigate crops, sown on lower lands, by capillarity. Sometimes, leakage causes the water –table to rise up, which helps in irrigation of crops by capillarity. When underground irrigation is achieved, simply by natural processes, without any additional extra efforts, it is called natural sub-irrigation.

Artificial sub-irrigation: when a system of open jointed drains is artificially laid below the soil, so as to supply water to the crops by capillarity, then it is known as artificial sub-irrigation. It is a very costly process and hence, adopted in India on a very small scale. It may be recommended only in some special cases with favourable soil conditions and for cash crops of very high return. Sometimes, irrigation water may be intentionally collected in some ditches near the fields, the percolation water may then come up to the roots through capillarity.

5. Sprinkler Systems

In the sprinkler irrigation network, we have the mains and the subdomains, through which water under pressure is made to flow. Revolving sprinkler heads are then usually mounted on rising pipes attached to the laterals. The water jet comes out through the revolving sprinkler heads, with force. When sprinkler heads are not provided, perforations are made in the pipes, and they are provided with nozzles, through which water jets out and falls on the ground. Generally, such a perforated pipe system operates at low heads; whereas, the revolving heads sprinklers operate on high as well as low heads, depending upon the type of rotary head used.

The advantage of sprinkler irrigation are enumerated below:

1. Seepage losses, which occur in earthen channels of surface irrigation methods, are completely eliminated. Moreover, only the optimum quantity of water is used in this method.

2. Land levelling is not required, and thus avoiding removal top fertile soil, as happens in other surface irrigation methods.

3. No cultivation area is lost for making ditches, as happens in surface irrigation methods. It, thus, results in increasing about 16% of the cropped area.

4. In the sprinkler system, the water is to be applied at a rate lesser than the infiltration capacity of the soil, and thus avoiding surface run, and its bad effects, such as loss of water, washing of topsoil, etc.

5. Fertilizers can be uniformly applied because they are mixed with irrigation water itself. 

6. This method leaches down salts and prevents waterlogging or salinity. 

7. It is less labour oriented, and hence useful where labour is costly and scarce.

8. Upto 80% efficiency can be achieved, i.e. upto 80% of applied water can be stored in the root zone of plants/.

Limitations of sprinkler irrigation nare also enumerated below:

1. High winds may distort sprinkler pattern, causing non-uniform spreading of water on the crops.
2. In areas of high temperature and high wind velocity, considerable evaporation losses of water may take place.
3. They are not suited to crops requiring frequent and larger depths of irrigation, such as paddy.
4. The initial cost of the system is very high, and the system requires a high technical skill.
5. Only sand and silt free water can be used, as otherwise pump impellers lifting such waters will get damaged.
6. It requires a larger electrical power.
7. Heavy soil with poor intake cannot be irrigated efficiently.
8. Constant water supply is needed for commercial use of equipment.

6. Drip irrigation Method

Drip irrigation, also called trickle irrigation, is the latest field irrigation technique and is meant for adoption at places where there exists acute scarcity of irrigation water and other salt problems. In this method, water is slowly and directly applied to the root zone of the plants, thereby minimizing the losses by evaporation and percolation.

This system involves laying a system of the head, mains, sub mains, laterals, and drop nozzles. Water oozes out of these small drip nozzles uniformly and at a very small rate, directly into the plant roots area.

The head consists of a pump to lift water, so as to produce the desired pressure of about 2.5 atmospheres, for ensuring proper flow of water through the system. The lifted irrigation water is passed through a fertilizer tank, so as to mix the fertilizer directly in the irrigation water, and then through a filter, so as to remove the suspended particles from the water, to avoid clogging of drip nozzles. 

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Water Requirements of Crops

Every crop requires a certain quantity of water after a certain fixed interval, throughout its period of growth. If natural rain is sufficient and timely so as to satisfy both these requirements, no irrigation water is required for raising that crop.

In a tropical country like India, the natural rainfall is either insufficient, or the water does not fall frequency of the rainfall varies throughout a tropical country, the certain crop may require irrigation in a certain part of the country. The area where irrigation is a must for agriculture is called the arid region, while the area in which inferior crops can be grown without irrigation is called a semiarid region.

• Crop Period or Base Period

The time period that elapses from the instant of its sowing to the instant of its harvesting is called the crop period. 

The time between the first watering of a crop at the time of its sowing to its last watering before harvesting is called the base period or the base of the crop.

Crop period is slightly more than the base period, but for all practical purposes, they are taken as one and the same thing, and generally expressed in days. 

• Delta of a Crop (Δ)

Each crop requires a certain amount of water after a certain fixed interval of time, throughout its period of growth.

The total quantity of water required by the crop for its full growth may be expressed in hectare metre (ha.m) or simply as depth to which water would stand on the irrigated area if the total quantity supplied were to stand above the surface without percolation or evaporation. This total depth of water (in cm) required by a crop to come to maturity is called its delta (Δ).

Explanation- The depth of water required every time, generally varies from 5 to 10 cm depending upon the type of the crop. If this depth of water is required five times during the base period, then the total water required by the crop for its full growth will be 5 multiplied by each time depth. The final figure will represent the total quantity of water required by the crop for its full-fledged nourishment.

Example 1: If rice requires about 10cm depth of water at an average interval of about 10 days, and the crop period for rice is 120 days, find out the delta for rice.

Solution: Water is required at an interval of 10 days for a period of 120 days. It evidently means that 12 no. of waterings are required, and each time, 10 cm depth of water is required. Therefore, the total depth of water required.

 = 12 x10 cm = 120 cm.

Hence Δ for rice = 120 cm. Ans.

Example 2: If wheat requires about 7.5 cm of water after every 28 days, and the base period for wheat is 140 days, find out the value of delta for wheat.

Solution: Assuming the base period to be representing the crop period, as per usual practise, we can easily infer that the water is required at an average interval of 28 days up to a total period of 140days.

This means that 5(140/28) no. of waterings are required 28days

The depth of water required each time = 7.5 cm.

Total depth of water required. In 140 days = 5 x7.5 = 37.5 cm

Hence, Δfor wheat = 37.5 cm. Ans.

• Delta for certain crops

The average values of deltas for certain crops are shown in the table. These values represent the total water requirement of the crops. The actual requirement of irrigation water may be less, depending upon the useful rainfall. Moreover, these values represent the values on the field, i.e. ‘delta on field’ which includes losses.

• Duty of Water (D)
  

The term duty means the "area of land" that can be irrigated with the unit volume of irrigation water. Quantitatively, duty is defined as the area of land expressed in hectares that can be irrigated with unit discharge, that is, 1 cumec flowing throughout the base period, expressed in days.

If water flowing at a rate of one cubic meter per second, runs continuously for B days and matures 200 hectares, then the duty of water for that particular crop will be defined as 200 hectares per cumec to the base of B days. Hence, duty is defined as the area irrigated per cumec of discharge running for base period B. The duty is generally represented by the letter D.

• Relation between Duty(D) and Delta(Δ)

Let there be a crop of base period B days. Let one cumec of water be applied to this crop on the field for B days. Now, the volume of water applied to this crop during B days.

Volume of water applied to crop = V = (1 x60 x60 x24 xB) m³. = 86400 B (cubic metre)

By definition of duty (D), one cubic metre supplied for B days matures D hectares of land.

This quantity of water (V) matures D hectares of land or 10⁴ D sq.m of area.

Total depth of water applied on this land = Volume/ Area = 86,400 B/ 10⁴ D . 8.64B/D metres

By definition, this total depth of water is called delta (Δ).

Δ = 8.64B/D (metres)

Where

Δ is in meter, B is in days; and

D is duty in hectares/cumec.

During the passage of water from these irrigation channels, water is lost due to evaporation and percolation. These losses are called Transit losses or Transmission or Conveyance losses in channels. 

Layout of Canal System

Duty of water for a crop is the number of hectares of land which the water can irrigate. Therefore, if the water requirement of the crop is more, less number of hectares of land it will irrigate. Hence, if water consumed is more, duty will be less. It, therefore, becomes clear that the duty of water at the head of the watercourse will be less than the because when water flows from the head of the watercourse and reaches the field, some water is lost as transit losses.

Applying the same reasoning, it can be established that duty of water at the head of a minor will be less than that at the head of the watercourse; duty at the head of a distributary will be less than that at the head of a minor, duty at the head of a branch canal will be less than that at the head of a minor, duty at the head of the main canal will be less that the duty at the head of a branch canal.

Duty of water, therefore, varies from one place to another and increases as we move downstream from the head of the main canal towards the head of the branches or watercourses. The duty at the head of the watercourse (i.e. at the outlet point is generally the endpoint of Irrigation Department.

Factors Affecting Duty of Water

1. Climatic and season: As stated earlier, duty includes the water lost in evaporation and percolation. These losses will vary with the season. Hence, duty varies from season to season, and also from time to time in the same season. The figures for duties which we generally express are their average values considered over the entire crop period.
2. Useful rainfall: If some of the rain, falling directly over the irrigated land, is useful for the growth of the crop, then so much less irrigation water will be required to mature the crop. More the useful rainfall, less will be the requirement of irrigation water, and hence more will be the duty of irrigation water.
3. Type of soil: If the permeability of the soil under the irrigated crop is high, the water lost due to percolation will be more and hence, the duty will be less. Therefore, for sandy soils, where the permeability is more, the duty of water is less.
4. The efficiency of cultivation method: If the cultivation method (including tillage and irrigation) is faulty and less efficient, resulting in the wastage of water, the duty of water will naturally be less. If the irrigation water is used economically, then the duty of water will improve, as the same quantity of water would be able to irrigate more area. Cultivators should, therefore, be trained and educated properly to use irrigation water economically.

Importance of duty

It helps us in designing an efficient canal irrigation system. Knowing the total available water at the head of the main canal, and the overall duty for all the crops required to be irrigated in different seasons of the year, the area which can be irrigated can be worked out. Inversely, if we know the corps area required to be irrigated and their duties, we can work out the discharge required for designing the channel.

Irrigation Efficiencies

Efficiency is the ratio of the water output to the water input and is usually expressed as the percentage. Input minus output is nothing but losses, and hence, if losses are more, the output is es and, therefore, efficiency is less. Hence, efficiency is inversely proportional to the losses. Water is lost in irrigation during various processes and, therefore, there are different kinds of irrigation efficiencies, as given below.

(i) Efficiency of water-conveyance (η_c): It is a ratio of the water delivered into the fields from the outlet point of the channel to the water pumped into the channel at the starting point. It may be represented by ηc. It takes the conveyance or transit losses into account.

(ii) Efficiency of water application (η_a): It is the ratio of the quantity of water stored into the root zone of the crops to the quantity of water actually delivered into the field. it may be represented by ηa. It may also be termed as farm efficiency, as it takes into account the water is lost on the farm.

(iii) Efficiency of water storage (η_s): It is the ratio of the water stored in the root zone during irrigation to the water needed in the root zone prior to irrigation (i.e., field capacity –existing moisture content). It may be represented by ηs.

(iv) Efficiency of water use (η_u): It is the ratio of the water beneficially used, including leaching water, to the quantity of water delivered. It may be represented by ηu.

Example 3: Once cumec of water is pumped into a farm distribution system. 0.8 cumec is delivered to a turnout, 0.9 kilometres from the well. Compute the conveyance efficiency.

Solution: By definition

η_{c =} Output/ Input  x 100 = 0.8/1.0 . 100 = 80%

Example 4: 10 cumecs of water is delivered to a 32-hectare field, for 4 hours. Soil probing after the indicated that 0.3 metres of water has been stored in the root zone. Compute the water application efficiency.

Solution: Volume of water supplied by 10 cumecs of water applied for 4 hours =(10 x 4 x 60x 60)m³ = 1,44,000 m³

= 14.4  x10⁴ m³ = 14.4m x 10⁴m² = 14. 4ha.m.

Depth of water applied =

volume/area = 1,44, 000/32,0, 000 = 144/320  = .45

Input = 14.4 ha.m

Output = 32 hectares land is storing water upto 0.3 m depth,

Output = 32 x0.3 ha.m = 9.6 ha.m

Water application efficiency (ηa) = Output/ Input   x 100 =( 9.6/14.4) x 100= 67%

(v) Uniformity coefficient or Water distribution efficiency: 

The effectiveness of irrigation may also be measured by its water distribution efficiency (η_d ), which is defined below:

η_d = (1-d/D)x100

 Where η_d = Water distribution efficiency

D = Mean depth of water stored during irrigation.

d = Average of the absolute values of deviations from the mean.

The water distribution efficiency represents the extent to which the water has penetrated to a uniform depth, throughout the field. When the water has penetrated uniformly throughout the field, the deviation from the mean depth is zero and water distribution efficiency is 1.0.

Example 5: A stream of 130 litres per second was diverted from a canal and 100 litres per second were delivered to the field. An area of 1.6 hectares was irrigated in 8 hours. The effective depth of the root zone was 1.7 m. The runoff loss in the field was 420 cu. M. The depth of water penetration varied linearly from 1.7 m at the head end of the field to1.1 m at the tail end. Available moisture-holding capacity of the soil is 20 cm per metre depth of soil. It is required to determine the water conveyance efficiency, water application efficiency, water storage efficiency, and water distribution efficiency. Irrigation was started at a moisture extraction level of 50% of the available moisture.

Solution:

(i) Water conveyance efficiency (ηc)

=( Water delivered to the fields/ Water supplied into the canal at the head) x 100

          = 100/130 x 100 =77%

(ii) Water application efficiency (ηa)

 Water stored in the root zone during irrigation / Water delivered to the field  x 100

Water supplied to field during 8 hours @ 100 litres per second

= 100x8  x60 x 60 litres = 2880 cu. m.

Runoff loss in the field = 420 cu. M.

 the water stored in the root zone = 2880 –420 = 2460 cu. m.

(iii) Water application efficiency (ηa)

=   2460 /100 = 85.4% Ans. 2880

(iv) Water storage efficiency (ηs) = (Water stored in the root zone during irrigation /

Water needed in the root zone prior to irrigation)  x 100

Moisture holding capacity of soil

= 20 cm per m depth x1.7 m depth of root zone = 34 cm

Moisture already available in the root zone at the time of start of irrigation

 = 50/100  x  34 =17cm.

Additional water required in the root zone

= 34 –17 = 17 cm.

= 2720 cu. m.

But actual water stored in root zone = 2460 cu. m.

Water storage efficiency (ηs) =2460 /2720  x 100 90% (say)

(v) Water distribution efficiency

 Where D = mean depth of water stored in the root zone

D = ( 1.7+1.1 )/2  = 1.4m

d is computed as below:

Deviation from the mean at upper end (absolute value) =  |1.7 -1.4| = 0.3

Deviation from the mean at lower end = | 1.1 -1.4 | =0.3

d = Average of the absolute values of deviations from mean = 0.4 +0.3/2 = 0.35

Using equations, we have,

η_{d =} 75 or 75%    Ans.

vi) Consumptive Use or Evapotranspiration (Cu)

Consumptive use for a particular crop may be defined as the total amount of water used by the plant in transpiration (building of plant tissues, etc.) and evaporation from adjacent soils or from plant leaves, in any specified time. The values of consumptive use (Cu) may be different for different crops, and may be different for the same crop at different times and places.

In fact, the consumptive use for a given crop at a given place may vary throughout the day, throughout the month and throughout the crop period. Values of daily consumptive use or monthly consumptive use, are generally determined for a given crop and at a given place. Values of monthly consumptive use over the entire crop period are then used to determine the irrigation requirement of the crop.

Effective Rainfall (Re)

Precipitation falling during the growing period of a crop that is available to meet the evapotranspiration needs of the crop is called effective rainfall. It does not include precipitation lost through deep percolation below the root zone or the water lost as surface runoff.

Consumptive Irrigation Requirement (CIR)

It is the amount of Irrigation water required in order to meet the evapotranspiration needs of the crop during its full growth. It is, therefore, nothing but the consumptive use itself, but exclusive of effective precipitation, stored soil moisture, or ground water. When the last two are ignored, then we can write

CIR = Cu-Re

Net Irrigation Requirement (NIR)

It is the amount of irrigation water required in order to meet the evapotranspiration need of the crop as well as other needs such as leaching. Therefore, N.I.R. = Cu –Re + Water lost as percolation in satisfying other needs such as leaching.

Consumptive use or evapotranspiration depends upon all those factors on which evaporation and transpiration depend; such as temperature, sunlight, humidity, wind movement, etc.

Estimation of Consumptive Use: 

Although various methods have been developed in order to estimate evapotranspiration (consumptive use) value of a crop in an area, but the most simple and commonly used methods are:

(1)   Blaney –Criddle Equation, and

(2) Hargreaves class A pan evaporation method

Blaney-Criddle Formula: 

It sates that the monthly consumptive use is given by

C _u = K.(P/ 40  [1.8t + 32])

where, C_u  = Monthly consumptive use in cm.

k = Crop factor, determined by experiments for each crop, under the environmental conditions of the particular area. 

t = Mean monthly temperature in ^oC

p = Monthly pet cent of annual day light hours that occur during the period.

If   p/40  [1.8t +32]is represented by f, we get                                

          Cu   = k.f    

Example: The monthly consumptive use values for Paddy are tabulated in Table. Determine the total consumptive use. What is the average monthly consumptive use and peak monthly consumptive use?

Solution: The summation of consumptive uses

= 29.69+8.76+14.38+22.73+21.29+25.50+15.06 = 137.41 cm

Hence, total consumptive use for paddy = 137.41 cm.

Average daily consumptive use =

      137.4/Period of growth in days =

     = 137.41/31+31+31+30+31+24

= 137.41/177 = 0.77 cm. = 0.77x30=23.1  mm.           

Average monthly consumptive use = 0.77 × 30 = 23.1 mm.

Peak monthly consumptive use = 26.69 cm.            (Highest value is given)

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Lacey's and Kennedy's Theory for Canal Design

Canal Design in General

Design Parameters

• The design considerations naturally change according to the type of soil.
• The velocity of the flow in the canal should be critical.
• Design methodologies of canals which are known as ‘Kennedy’s theory and ‘Lacey’s theory’ are based on the characteristics of sediment load (i.e. silt) in canal water.

Important Terms Related to Canal Design

• Alluvial soil: The soil which is formed by the continuous deposition of silt is known as the alluvial soil. The river has to carry a heavy charge of silt in the rainy season. When the river overflows its banks during flooding, the silt particles get deposited on the adjoining areas. This deposition of silt continues year after year. This type of soil is found in the deltaic region of a river. This soil is permeable, soft, and very fertile. The river passing through this type of soil has a tendency to frequently change its course.

• Non-alluvial soil: The soil which is formed by the disintegration of rock formations is termed non-alluvial soil. It is found in the initial mountainous region of a river. The soil is hard and impermeable in nature. This is not fertile. The river passing through this type of soil does not have a tendency to change its course.

• Silt factor: During his investigative work in various canals in alluvial soil, Gerald Lacey established the importance of the effect of silt on the determination of discharge and the canal section. He introduced a factor to account for this which is known as the ‘silt factor’. It depends on the mean size of the particle of silt. It is denoted by ‘f’. The silt factor is determined by the expression,

where d_mm id the mean particle size of silt in mm

• Co-efficient of rugosity: The roughness of the canal bed has an effect on the velocity of flow. This roughness is caused due to the ripples formed on the bed of the canal. So, a coefficient was introduced by R.G Kennedy to calculate the mean velocity of flow. This coefficient is known as the coefficient of rugosity and it is denoted by ‘n’. The value of ‘n’ depends on the type of material constituting the bed of the canal.
• Mean velocity: It is found by observations that the velocity at a depth of 0.6D from the surface represents the mean velocity (V), where ‘D’ is the depth of water in the canal or river.
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• Critical velocity: When the velocity of flow is such that there is no scouring or silting action in the canal bed, then that velocity is known as critical velocity. It is denoted by ‘Vo ’. Its value was given by Kennedy according to the following expression,

V_o = C.Dⁿ

Where V_o = Critical velocity

D = Depth of channel

C & n = Constants

He found the values of C & n are 0.55 and 0.64

Therefore V_o = 0.55 × D^0.64

 Later he found that the critical velocity ratio has a huge impact on Critical velocity and he incorporated some changes in the above equation.

• Critical velocity ratio (c.v.r), m: The ratio of mean velocity ‘V’ to the critical velocity ‘Vo ’ is known as the critical velocity ratio (CVR). It is denoted by m i.e.

CVR (m) = V/Vo  

When m = 1, there will be no scouring or silting.

When m > 1, scouring will occur

When m < 1, silting will occur

So, by calculating the value of m, the condition of the canal can be predicted whether it will have scouring or silting.

• Regime channel: When the character of the bed and bank materials of the channel is the same as that of the transported materials and when the silt grade and silt charge are constant, then the channel is said to be in its regime and the channel is called regime channel. This ideal condition is not practically possible.

• Hydraulic mean depth: It is the ratio of the cross-sectional area of flow to the wetted perimeter of the channel. It is generally denoted by R.

R = A/P

Where,

A = Cross-sectional area

P = Wetted perimeter

• Full supply discharge:  Full supply discharge is the maximum capacity of the canal for which it is designed. The water level of the canal corresponding to the full supply of discharge is known as the full supply level (F.S.L).

• Economical section: For the canal section such that the earth obtained from cutting (i.e. excavation) can be completely utilized in forming the banks, then that section is known as an economical section. Again, the discharge thus obtained will be maximum with minimum cross-section area. Here, no extra earth is required to be used from the borrow pit and no earth is in excess to form the spoil bank. This condition only arises in the case of partial cutting and partial banking. Sometimes, this condition is designated as the balancing of cutting and banking. Here, the cutting depth is called balancing depth.

Unlined Canal Design on Non-alluvial soil

The non-alluvial soils are stable and nearly impervious. For the design of a canal in this type of soil, the coefficient of rugosity has a very important role, but the other factors like the silt factor have no role. Here, the velocity of flow is considered to be very close to critical velocity. So, the mean velocity given by Chezy’s expression or Manning’s expression is considered for the design of the canal. The following formulae are adopted for the design.

Note: 

• If the value of K is not provided, then it may be assumed as follows,

For unlined channel, K = 1.30 to 1.75.

For line channel, K = 0.45 to 0.85

• If the value of N is not provided, then it may be assumed as follows,

For unlined channel, N = 0.0225

For lined channel, N = 0.333

Example 1: 

Design of Unlined Canal on Alluvial soil by Kennedy’s Theory

R.G Kennedy established a theory which states that the silt carried by flowing water in a channel is kept in suspension by the vertical component of eddy currents which are formed over the entire bed width of the channel and the suspended silt rises up gently towards the surface.

His theory makes the following assumptions:

1. The eddy currents are developed due to the roughness of the bed.
2. The quantity of the suspended silt in the stream is proportional to bed width.
3. This theory is applicable to those channels which are flowing through the bed consisting of sandy silt or same grade of silt.

He established the idea of a critical velocity ‘Vo ’ that will make the channel free from silting or scouring. From long observations, he established a relationship between the critical velocity and full supply depth as follows,

V_o = C Dⁿ

The values of C and n were found to be 0.546 and 0.64 respectively, thus

V_o = 0.546 D^0.64

He also observed that the critical velocity was affected by the grade of silt and introduced another factor (m) which is known as the critical velocity ratio (C.V.R).

V_o = 0.546 m D^0.64

Drawbacks of Kennedy’s Theory

1. The theory is limited to the average regime channel only.
2. The design of the channel is based on the trial and error method.
3. The value of m was fixed arbitrarily.
4. Silt charge and silt grade are not considered.
5. There is no equation to determine the bed slope and it depends on Kutter’s equation only.
6. The ratio of ‘B’ to ‘D’ has no significance in his theory.

Design Procedure

1. Critical velocity, V_o =0.546 m D^0.64 .
2. Mean velocity, V = C (RS)^1/2

Where,

m = critical velocity ratio,

D = full supply depth in m,

R = hydraulic mean depth of radius in m,

S = bed slope as 1 in ‘n’.

The value of ‘C’ is calculated as per Kutter’s formula

Where,

 n = rugosity coefficient which is taken as an unlined earthen channel.

3. The B/D ratio is assumed between 3.5 to 12.

4. Discharge, Q = A*V

Where,

A = Cross-section area in m² ,

V = mean velocity in m/sec

5. The full supply depth is determined by trial to satisfy the value of ‘m’. Generally, a trial depth between 1 m to 2 m is assumed. If the conditions are not satisfied within these limits, then a different value may be assumed accordingly.

Example 2:

.

Try Yourself

Design of Unlined Canals in Alluvial soil by Lacey’s Theory

Lacey’s theory is based on the concept of the regime condition of a channel which will be satisfied if,

• The channel is flowing uniformly in unlimited incoherent alluvium of the same character which is being transported by the channel.
• The silt grade and silt charge remain constant.
• The discharge remains constant.

Lacey observed that the silt carried by the flowing water is kept in suspension by the vertical component of eddies. These eddies are generated at all the points on the wetted perimeter of the channel section. Again, he assumed the hydraulic mean radius (R) as the variable factor and he recognized the importance of silt grade and he introduced a factor to account for it which is known as silt factor ‘f’.

Thus, he deduced the velocity as;

V = (2/5f R)^0.5

Where,

V = mean velocity in m/sec,

f = silt factor,

R = hydraulic mean radius in meter

Then he derived the relationship between A, V, Q, P, S and f are as follows:

Example 3:

Drawbacks of Lacey’s Theory

1. The concept of the true regime is a theoretical one and can not be achieved practically.
2. The various equations are derived by considering the silt factor f to be a constant which is not at all constant.
3. The concentration of silt is not considered.
4. Silt grade and silt charge is not taken into account.
5. The equations are empirical and based on the available data from a particular type of channel and may not hold true for a different type of channel.
6. The characteristics of the regime channel may not be the same for different cases.

Comparison between Kennedy’s and Lacey’s theory

Design of Lined Canal

As the section of the canal is rigid, the lined canals are not designed by the use of Lacey’s and Kennedy’s theory and Manning’s equation is used for designing. The design considerations are,

• The section should be economical (i.e. cross-sectional area should be maximum with a minimum wetted perimeter).
• The velocity should be maximum so that the cross-sectional area can be minimized.
• Silting does not reduce the capacity of the lined section.

Section of Lined Canal:

The following two-lined sections are generally adopted:

• Circular section: The bed is circular with a radius equal to full supply depth ‘D’ and its centre at the full supply level. The sides are tangential to the curve. However, the side slopes are generally taken to be 1:1.

• Trapezoidal section: The horizontal bed is connected to the side slope by a curve of radius equal to full supply depth D. The side slope is generally kept as 1:1.

Note: For the discharge of up to 50 cumecs, a circular section is suitable and for the higher discharges trapezoidal section is suitable.

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