In this essay we will discuss about the systems of irrigation practiced in India.
1. Essay on Surface Irrigation Systems:
In surface irrigation systems, water is either diverted from the rivers and streams when they are flowing (diversion works) for irrigating the standing crops or stored upstream side by constructing barrier across the How (reservoirs/tanks) for future use.
i. Diversion Systems:
The purpose of diversion work is, mainly, to raise the water level and divert the river or stream flow into canals to irrigate the standing crops of the ayacut. If necessary, small dams are used to raise the river water level to feed an off-taking canal and or some other conveyance system. A diversion dam is, generally, called a weir or barrage.
In the early stages of irrigation development, emphasis was on diverting river and stream flows during rainy season and to some extent in rabi period. Such schemes were largely developed in Indo-Gangetic plains due to snow melt from the Himalayas. Later, emphasis was on construction of storage tanks as in south India.
ii. Reservoir/Tank Systems:
Tank irrigation is storage irrigation scheme, which utilises the water stored on upstream side of earthen dam called a bund. These earthen bund reservoirs are known as tanks, especially in south India, where such works are very common. There is, thus, no technical difference between a reservoir and a tank, except that a large sized tank is termed as a reservoir.
Reservoirs are, generally, formed by dams of any material (masonry dam, concrete dam, earth dam etc.), where as tanks are formed by earthen dams only. These earthen bunds are called tank bunds. Most of the existing tanks have maximum depth of 4.5 m, while a few are as deep as 7 to 9 m and only a few exceptional ones exceed 11 m in depth. When the depth of tank exceeds 12 m, the tank is, generally, referred to as a reservoir.
Tank bunds are usually provided with sluices or outlets for discharging water from the tank for irrigation. As in the case of all dam reservoir projects, tanks are provided with surplus escape arrangements for spilling away the excess surplus water that may enter the tank to avoid over topping of the bund.
Most of the existing small sized tanks of south India form part of groups of tanks, which are connected together in series, such that any tank either receives the surplus water of the upper tank sends its own surplus into some lower tank or does both. If a tank neither receives water nor discharges into lower tank, it is called isolated tank.
Considerable economy of water can be obtained from the system of grouping, because surplus water of the upper tank drains into the next lower tank. Major disadvantage of grouping is that if a breach occurs in an upper tank, it exposes all the tanks below to the risk of similar failure.
iii. Storage Capacity of the Tank:
The gross capacity of a tank is the cubic content of water stored in the tank up to full tank level (FTL). Effective capacity of the tank is the cubic content of water stored between FTL and the bottom or sill level of the lowest supply sluice. The storage capacity can be computed using contour plan of the area of the water spread; total capacity being the sum of the capacities between successive contours.
When the contour plan is not available and only area of the tank at FTL is known, the effective cubic content of the tank can be roughly calculated: area of the tank at FTL is multiplied by one-third of depth from this level (FTL) to the deep bed of the tank, or the level of the sill of the lowest sluice, whichever is higher of the two. If the area of water spread at FTL contour is A and the difference in level between the lowest sluice sill and FTL is h, then the capacity is computed by the cone formula V = Ah/3.
iv. Decline on Tank Irrigation:
There are several reasons for decline in tank irrigation.
The following, among others, are the major causes for decline in tank irrigation:
1. Neglect of tank maintenance due to major thrust on major and medium irrigation projects after independence
2. Destruction of forests, excessive grazing of catchment area of tanks and cultivation of foreshore areas of tanks resulted in excessive soil erosion and silting of tanks
3. Farmers gave preference to more stable alternative well irrigation over relatively unstable tank irrigation because of frequent droughts leading to no water in the tanks.
Neglect of tank irrigation system may lead to unsustainable ecological balance. Tanks help in enhancing the total recharging of groundwater table. If tanks become inefficient, well irrigation also becomes unsustainable.
v. Canal Systems:
Surface water stored in major and medium irrigation projects or diverted directly is distributed for irrigation through the canal irrigation network to reach the fields to be irrigated.
Starting from storage to the point of use in the cropped fields, the movement of water in surface irrigation system can be viewed as four separate operations:
1. Diversion from storage system
2. Conveyance in main and branch canals
3. Distribution and conveyance by distributary canals
4. Field application by minor or farm canals.
The canal network consists of canals, distributaries, watercourses and field channels, which are termed according to their capacity and orientation with respect to the head works.
Main canal:
It takes its supply directly from the reservoir or the river and its capacity varies from 280 to 425 m3 s-1 in India.
Branch canals:
These canals take off from main canal and convey water to different major parts of the irrigated areas. Branch canals, generally, carry a discharge from 4.0 to 8.5 m3 s-1. Direct discharge is, generally, not done from large branches.
Major Distributaries:
They take off from branch canals and sometimes from main canals and supply water to distributaries or outlets. They, generally, carry discharge between 0.75 and 5.5 m3 s-1.
Minor Distributaries:
These are smaller channels taking their supply from major distributaries and supplying water to outlets. The carrying capacity is less than 750 l s_1.
Canal Outlets:
They are provided in irrigation canal system at appropriate points. The size depends upon the irrigated area. Discharge of the outlet varies with the level of water in the channel.
Watercourse:
It is any channel which is supplied with water from a canal but which is not maintained at the cost of Government. They pass through common land and are maintained by farmers.
Field channels:
They carry water to individual fields from the watercourse. In irrigated rice areas of south, however, field channels are normally absent as field to field irrigation is practiced.
Canal water irrigation systems were scientifically planned. The water allowance factor, capacity factor and irrigation intensities were designed keeping in view the availability of canal irrigation water and irrigation demands of the cropping systems prevalent at that time.
Since then, a major shift has taken place in cropping pattern, groundwater development, cropping intensity, irrigation intensity etc. This has resulted in a mismatch between demand and supply during the crop season. This gap can be minimised by revising the water allowance and the capacity factors, keeping in view the irrigation requirements of existing crops, quality and availability of groundwater.
Tail-End Problems in Canal Systems:
Farmers who have land at the end of canal system are called tail-enders. They include farmers in the tail reach as well as those at the end of the upper and middle reaches of the system. It has been known that many get neither enough nor timely water.
Examples are:
1. The old Sardar canal project in the state of Gujarat, India, was designed with an irrigation intensity of 32 per cent, but at the upstream part the delivery was at an intensity of 42 per cent (131% of the design norm) and at the downstream end it was only 19 per cent (59% of the norm), although the project aimed at protective irrigation with equal rights for all.
2. Even in warabandi systems areas in Punjab and Haryana, 70 per cent of the tail-end farmers receive 54 to 70 per cent less water than they are entitled to.
3. The Sardar Sahayak Pariyojana irrigation project, an extension of the Sardar canal project with 1.7 M ha, the head farmers received 5 times more water than the tail- enders, although the project was designed for equal distribution of the scarce water.
4. The Ghatampur distributary canal in the Ramganga irrigation project in the state of Uttar Pradesh, India, delivered an amount of water equal to 155 per cent of the design discharge to the Kisarwal district canal near the head of the distributary and only 22 per cent to the Bairampur district canal at the downstream end.
5. In Tungabhadra system of Karnataka, farmers in the last reach get 91 per cent less water than they are entitled to even though the project performance was claimed as 90 per cent.
6. The uneven distribution of irrigation water over the Egyptian canal systems is exemplified in Aswan Dam irrigation for agriculture.
Major impact of such problems is low crop productivity of tail-end farmers, movement to low value crops or leaving the land fallow. The causes of tail-ender deprivation are excessive use by head-reach farmers, poor system maintenance, less fund allotment to tail- enders for maintenance, poor construction and design fault.
2. Essay on Groundwater Irrigation Systems:
Groundwater is the underground water that occurs in the saturated zone of variable thickness and depth, below the earth’s surface. About 46 per cent of total irrigated area in India gets its irrigation water from this source. Groundwater is utilised through wells using various lifting devices such as those using animal, manual, wind, diesel or electric power. Use of open wells is a traditional method of tapping groundwater. Use of tube wells, however, is a subsequent development.
The country had undertaken spectacular development of groundwater resources during sixth and seventh plans. During these plans, 5.82 and 7.8 M ha of irrigation potential, respectively, was created from groundwater sources alone. The country has made considerable progress in construction of dug wells and shallow tube wells that are mostly owned by individual farmers.
The causes for steep increase in well irrigation are:
1. Adequate availability of groundwater in the states of UP, Punjab, Bihar and Haryana
2. Availability of modern technology for drilling wells and lifting water
3. Increased crop productivity and higher monetary returns from well irrigation due to perfect control over water and absence of drainage problems
4. Financial support from private and Government agencies for development of well irrigation
5. Uncertainty of irrigation through tanks and other sources due to low and erratic rainfall both in time and space
6. Deterioration in tank irrigation due to silting and neglect of maintenance.
Water well is a hole, usually vertical, excavated in the earth for bringing groundwater to the surface.
They are of two types:
1. Open Wells and
2. Tube wells.
i. Open Wells or Dug Wells:
Dugout wells up to water bearing strata are called open wells or dugout wells. They have comparatively bigger diameter with low discharges of the order of 1 to 5 l s-1. The diameter of open wells, generally, varies from 2 to 9 m and they are, generally, less than 20 m in depth.
ii. Tube or Bore Wells:
Tube wells are sunk by inserting pipes below ground surface through water bearing and non-water bearing strata. Deep bore wells are as deep as 70 to 300 m and tap more than one aquifer. Such wells may yield as high as 200 to 220 l s-1. Average yield, however, is of the order of 40 to 45 l s-1.
The diameter of the hole is 0.6 m up to 60 m depth and then 0.56 m below 60 m. Shallow tube wells having 20 to 70 m depth may yield as high as 15 to 20 l s-1, if located at proper place. Such wells can irrigate around 6 ha. Depending upon the entry of water through a cavity or a screen, tube wells are classified into two categories: cavity type and screen type.
A cavity type tube well draws water from the bottom of the well and not from the sides, as is done by a screen well. It essentially consists of a pipe bored through the soil and resting on the bottom of a strong clay layer. A cavity is formed at the bottom and water from the aquifer enters the well pipe through the cavity.
Screen type tube wells can easily tap a number of aquifers unlike a cavity well.
These are of two types:
1. Strainer tube wells and
2. Slotted pipe gravel-packed tube wells.
Strainer tube well uses strainer lengths lowered into the bore hole and locked opposite the water bearing formations, where as, plain pipe lengths are located opposite the non water bearing formations. A bail is provided at the bottom. Water enters the well through these strainers from the sides and the flow is radial.
A strainer essentially consists of a perforated or a slotted pipe with a wire mesh wrapped round the pipe with a small angular space between the two. The wire screen prevents sand particles from entering the well. Slotted pipe gravel- packed tube well, on the other hand, uses a slotted pipe without being covered by any wire mesh.
Such slotted pipe lengths are located opposite the water bearing formations, as is done with the strainers in a strainer bore well. These slotted pipes should strictly be referred to as screen pipes. After placing the assembly of plain and slotted pipes in the borehole, gravel pored into the borehole between the well pipe and casing pipe so as to surround the well pipes in the entire depth of the well below the top level of the shallowest screen.
A well in which surface of water in the water bearing formation surrounding the well is at atmospheric pressure is called gravity well. Water flows under gravity into the well and rises to the height of saturated material surrounding it. Pressure or artesian well is a well, in which water in the aquifer is at a pressure higher than atmospheric because the aquifer is confined or sandwiched between two impervious strata, one above and the other below, so that water flows under pressure into the well. In some cases, the pressure is so high that water from the well rises above the ground surface.
Advantages and Disadvantages of well Irrigation:
Advantages and disadvantages of well irrigation over canal irrigation are:
Advantages:
1. If a canal is not available for irrigation, such lands can be irrigated by well irrigation
2. Canal irrigation projects require huge funds as against lesser investment for well irrigation
3. Crops can be sown in optimum time leading to higher yields with well irrigation unlike sowing the crops late in the season due to late release of water in the canal
4. Cropping intensity can be greatly increased (2 or 3 crops annually) with well irrigation relative to canal irrigation
5. Unless drought continues for 2 to 3 years, well irrigation does not fail in drought years, while canal supply may fail even in a single drought year.
Disadvantages:
1. If power supply fails, well water cannot be made available to crops, unless diesel or solar power is available
2. Well water is more costlier than canal water as it has to be lifted by power
3. Frequent breakdown of power and motor parts cause large-scale interruptions in the working of wells.
Augmenting Groundwater Resources:
There has been a steady increase in irrigation potential from groundwater, which has gone up from 6.5 M ha in 1950s to 40 M ha in 2000s. Although, the present rate of annual gross withdrawal is less than 50 per cent of the available groundwater for irrigation, the signs of excess withdrawal are clearly visible in many districts in different states.
In these districts, the present rate of development exceeds the annual replenishable groundwater recharge. The number of districts where water table has declined more than 4.0 m constitutes 137 spread over various states.
The decline in groundwater levels has resulted in:
1. Decrease in well yield
2. Failure of wells/tube wells
3. Increased pumping costs and higher consumption of energy
4. Ingress of seawater in coastal areas.
Thus, depletion of groundwater needs to be arrested by maintaining hydrological equilibrium between annual replenishable recharge and groundwater recharge.
Groundwater resources can be augmented through artificial recharge. The success of groundwater recharge depends on the availability of good quality water, suitability of site and appropriate recharge technique.
Water resources, which can be used for artificial groundwater recharge include:
1. Surplus monsoon runoff
2. Canal water during rainy season
3. Treated sewage water.
Surplus Monsoon Runoff:
Out of 400 M ha-m average annual rainfall of the country, 115 M ha-m flows as surface runoff. Including stream regeneration, the annual surface runoff is 187.9 M ha-m. As per the current estimate, ultimate surface water resource likely to be developed has been computed as 69 M ha-m.
Therefore, available surplus water is 116 M ha-m. The monsoon surplus water for groundwater recharge is estimated as 87 M ha-m (assuming 75 per cent of surplus water is available for groundwater recharge). The basic requirement in planning and utilisation of surplus monsoon runoff for recharge is availability of surface storage space in different zones of the country.
The Central Groundwater Board has estimated the surface storage potential at 59 M ha-m for 20 major river basins. However, because of vide variation in the basin wise availability of monsoon runoff and surface storage potential, the feasible groundwater storage has been estimated as 23.39 M ha-m.
Surplus Canal Water:
During rainy days, water may not be required for irrigation but will have to be released for other requirements (hydropower, flood protection etc.). Canal water during this period, therefore, may be utilised for groundwater recharge. Surplus canal water available for groundwater recharge can be estimated, if the number of rainy days when irrigation is not required and daily canal water release during that period are known.
Assuming that 30 M ha-m water is released in a canal during the year and average number of rainy days as 40, the surplus canal water available for artificial groundwater recharge works out to be 30 x 40/365 = 3.5 M ha-m. However, the availability of surplus canal water in each zone can be estimated depending on actual canal water release and number of rainy days during which canal water is not required.
Sewage Water:
Treated sewage water is used for groundwater recharge in several countries. In India, the sewage is collected in temporary pumping stations and pumped either on to land for irrigation or into inland surface water or in some cases it is left to find its way into depressions where it stagnates.
For augmenting the water resources, the treated sewage water may be used for groundwater recharge wherever feasible. Assuming that 300 M people live in cities/towns having supply of water and water consumption of 200 I capita-1 day-1, the annual estimate of sewage comes to 2.19 M ha-m. If 50 per cent of the sewage water is retrievable, about 1.0 M ha-m will be available for possible recharge.
Artificial groundwater recharge can be accomplished by direct methods like flooding, ditch and furrows, recharge basins, percolation tanks, runoff conservation structures, injection wells, recharge wells, surface irrigation etc. and by indirect methods like pumping wells, collector wells, bore blasting and hydro-fracturing.
Conjunctive use of Surface and Groundwater:
Conjunctive use management of multisource waters can be defined as the management of multiple water resources in a coordinated operation such that the total water yield of the system over the period of time exceeds the sum of water yields of the individual components of the system resulting from uncoordinated operation.
As a result of conjunctive use of surface and groundwater resources, it is possible to have optimum utilisation of water resources as groundwater could act and function as a storage reservoir, regularisation agent and conveyance medium. Separate use of surface and groundwater in itself may not always constitute a conjunctive use.
Conjunctive use is planned and practiced with the following objectives:
1. Mitigating the effect of shortage in canal water supplies, often subject to steep variations in river flow, during different periods in the year
2. Increasing the dependability of existing water supplies
3. Alleviating the problem of high water table and salinity resulting from introduction of canal irrigation
4. Facilitating the use of high salinity groundwater, which cannot otherwise be used without appropriate dilution
5. Storing water in groundwater basins closer to the users to ensure water supply in case of interruption of surface water supply.
There are very few examples of conjunctive use in the irrigation commands. The only planned conjunctive use project is the augmentation of tube wells in the Western Yamuna Command in Haryana state. Construction of percolation tanks in Karnataka and Maharashtra can also be cited in this contest. However, major efforts have been voluntary and unplanned.
In Haryana and Punjab states, conjunctive use of saline and groundwater use is picking up to augment the irrigation water supply and to arrest the rate of rise in water table. Similar situation is developing in the Mahi Canal command in Gujarat, particularly in areas underlain with saline water.
Conjunctive use strategies could play an important role in solving the problem of waterlogging in the Indira Gandhi Nahar Pariyojana (IGNP). Problems of waterlogging and soil salinity could have been delayed by more than 20 years with conjunctive use than without it.