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.
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.
There are numerous advantages of Furrow technique of irrigation:
- Large areas can be irrigated at a time.
- It saves labour since once the furrow is filled, it is not necessary to give water a second time.
- It is a reasonably cheaper method.
- 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. . Other problem with furrow irrigation is the increased potential for water loss due to runoff.
- 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 - 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.
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.
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.
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.
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.
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. 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:
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 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..
It is termed as subsurface irrigation because in this type of irrigation, water does not wet the soil surface. The underground water nourishes the plant roots by 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:
- 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.
Land levelling is not required, and thus avoiding removal top fertile soil, as happens in other surface irrigation methods.
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.
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.
Fertilizers can be uniformly applied because they are mixed with irrigation water itself.
This method leaches down salts and prevents waterlogging or salinity.
It is less labour oriented, and hence useful where labour is costly and scarce.
- 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:
- High winds may distort sprinkler pattern, causing non-uniform spreading of water on the crops.
- In areas of high temperature and high wind velocity, considerable evaporation losses of water may take place.
- They are not suited to crops requiring frequent and larger depths of irrigation, such as paddy.
- The initial cost of the system is very high, and the system requires a high technical skill.
- Only sand and silt free water can be used, as otherwise pump impellers lifting such waters will get damaged.
- It requires a larger electrical power.
- Heavy soil with poor intake cannot be irrigated efficiently.
- 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.
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).
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.
|Table: Average Approximate Values of Δ for Certain Important Crops in India|
|Crop||Delta on field(cm)|
- 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.
- Relation between Duty(D) and Delta(Δ)
Δ = 8.64B/D (metres)
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
- Climatic and season
- Useful rainfall
- Type of soil
- The efficiency of cultivation method
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 (ηc): 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 (ηa): 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 (ηs): 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 (ηu): 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
ηc = 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 (ηd ), which is defined below:
ηd = (1-d/D)x100
Where ηd = 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.
(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,
ηd = 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.
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.
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
It sates that the monthly consumptive use is given by
C u = K.(P/ 40 [1.8t + 32])
where, Cu = 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 =
= 137.41/177 = 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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