Chapter No 02 Water Resources Irrigation: Definition: The artificial application of water to land to assist in the production of crops. Classification of Irrigation projects:
Major project: This type of project consists of huge surface water, storage reservoirs and flow diversion structures. The area envisaged to be covered under irrigation is of the order over 10000 hectare. Medium project: These are also surface water projects but with medium size storage and diversion structures with the area under irrigation between 10000 hectare and 2000 hectare.
Minor project: The area proposed under irrigation for these schemes is below 2000Ha and the source of water is either ground water or from wells or tube wells or surface water lifted by pumps or by gravity flow from tanks. It could also be irrigated from through water from tanks. The major and medium irrigation projects are further classified as Direct irrigation method Storage irrigation method.
Few terms are necessary to define before going into the detail Commanded area (CA): is defined as the area that can be irrigated by a canal system, the CA may further be classified as under: Gross command area (GCA): Gross command area (or GCA) is the total area which can be economically irrigated from irrigation system without considering the limitation on the quantity of available water. Cultural Command Area(CCA):
It is the area which can be physically irrigated from the scheme and is fit for cultivation or The difference between the gross command area and the unculturable area falling under the command or Total area in which cultivation is possible. Direct and Indirect (Or Storage) Irrigation Methods Direct irrigation method:
In this project water is directly diverted from the river into the canal by constructing a diversion structure like weir or barrage across the river with some pondage to take care of diurnal variations. It also effects in raising the river water level which is then able to flow into the offtaking channel by gravity. The flow in the channel is usually controlled by a gated structure and this in combination with the diversion structure is also sometimes called the headworks.
If the water from such headworks is available throughout the period of growth of crops irrigated by it, it is called a perennial irrigation scheme. In this type of projects, the water in the offtaking channels from the river carries water through out the year. It may not be necessary, however, to provide irrigation water to the fields during monsoon. In some places local rainfall would be sufficient to meet the plant water needs. In case of a nonperennial river the offtaking channel would be carrying water only for certain period in a year depending upon
the availability of supply from the source. Another form of direct irrigation is the inundation irrigation which may be called river-canal irrigation. In this type of irrigation there is no irrigation work across the river to control the level of water in the river. Inundation canal off-taking from a river is a seasonal canal which conveys water as and when available in the river. This type of direct irrigation is usually practiced in deltaic tract that is, in areas having even and plane
topography. It is feasible when the normal flow of river or stream throughout the period of growth of crop irrigated, is never less than the requirements of the irrigated crops at any time of the base period. A direct irrigation scheme of irrigation using river water diversion head works typically be laid out as in Figure Storage irrigation method: For this type of irrigation schemes part of the excess water of a
river during monsoon which other wise would have passed down the river as a flood is stored in a reservoir or tank found at the upstream of a dam constructed across a river or stream. This stored water is then used for irrigation is adopted when the flow of river or stream is in excess of the requirements of irrigated crops during a certain part of the year but falls below requirements or is not available at all in the river during remaining part of the year. Since the construction site of a storage reservoir is possible in regions of undulating topography, it is usually practiced in non deltaic areas. A general layout of this
irrigation scheme may typically be laid out as shown in Figure 2. Another type of storage irrigation method envisages the storage of water at some place in the hilly terrain of the river where the construction of the dam is possible. A barrage is constructed at some downstream location, where the terrain is flatter and canals take off as in a usual direct irrigation method. A general layout of such scheme could be represented as in Figure
Methods of field water application Surface irrigation method 2. Subsurface irrigation method 3. Sprinkler irrigation system 4. Drip irrigation system 1. 1. Surface irrigation method
In this system of field water application the water is applied directly to the soil from a channel located at the upper reach of the field. It is essential in these methods to construct or designed water distribution systems to provide adequate control of water to the fields and proper land preparation to permit uniform distribution of water over the field. One of the surface irrigation method is flooding method where the water is allowed to cover the surface of land in a continuous sheet of water with the depth of applied water just sufficient to allow the field to absorb the right amount of water needed to raise the soil moisture up to field
capacity,. A properly designed size of irrigation stream aims at proper balance against the intake rate of soil, the total depth of water to be stored in the root zone and the area to be covered giving a reasonably uniform saturation of soil over the entire field. The flooding method applied in a controlled way is used in two types of irrigation methods as under: Border irrigation method
Basin irrigation method Border irrigation: Borders are usually long uniformly graded strips of land separated by earth bunds (low ridges) as shown in Figure. The essential feature of the border irrigation is to provide an even surface over which the water can flow down the
slope with a nearly uniform depth. Each strip is irrigated independently by turning in a stream of water at the upper end. The water spreads and flow down the strip in a sheet confined by border ridges. When the advancing water reaches the lower end of the border, the stream is turned off. For uniform advancement of water front the borders must be properly leveled. The border shown in the
figures above are called straight borders, in which the border strips are laid along the direction of general slope of the field. The borders are sometimes laid along the elevation contours of the topography when the land slope is excessive. Contour border method The straight border irrigation is generally suited to the larger mechanized farms as it is designed to produce long uninterrupted
field lengths for ease of machine operations. Borders can be 800m or more in length and 3 30 m wide depending on variety of factors. It is less suited to small scale farms involving hand labour or animal powered cultivation methods. Generally, border slopes should be uniform, with a minimum slope of 0.05% to provide adequate drainage and a maximum slope of 2% to limit problems of soil erosion. As for the type of soil suitable for border irrigation, deep homogeneous loam or clay soils with medium infiltration rates are preferred. Heavy, clay soils can be difficult to irrigate with border irrigation because of the time needed to infiltrate sufficient water into
the soil. Basin irrigation is preferable in such circumstances. Basin irrigation: Basins are flat areas of land surrounded by low bunds. The bunds prevent the water from flowing to the adjacent fields. The basins are filled to desired depth and the water is retained until it infiltrates into the soil. Water may be maintained for considerable periods of time. Basin method of irrigation can be formally divided into two,
viz; the check basin method and the ring basin method. The check basin method is the most common method of irrigation used. In this method, the land to be irrigated is divided into small plots or basins surrounded by checks, levees (low bunds); as shown in Figure. Check basin method Each plot or basin has a nearly level surface. The irrigation
water is applied by filling the plots with water up to the desired depth without overtopping the levees and the water retained there is allowed to infiltrate into the soil. The levees may be constructed for temporary use or may be semi permanent for repeated use. The size of the levees depends on the depths of water to be impounded as on the stability of the soil when wet. Water is conveyed to the cluster of check basins by a system of supply channels and lateral field channels or ditches. The supply channel is aligned on the upper side (at a higher elevation) of the field for every two rows of plot as shown in
the figure. Sometimes, basin sizes are made larger to include two more trees in one basin. Water to the basins is supplied from a supply channel through small field channels conveyed the basins with the supply channel. Trees which can be irrigated successfully using the ring basin method include citrus and banana. Ring basin irrigation method
Furrow Irrigation: Furrows are small channels, which carry water down the land slope between the crop rows. Water infiltrates into the soil as it moves along the slope. The crop is usually grown on ridges between the furrows, as shown in Figure . This method is suitable for all row crops and for crops that cannot stand water for long periods, like 12 to 24 hours, as is generally
encountered in the border or basin methods of irrigation. a. b. c. Using flexible pipes to siphon out water from the field channel Using the breach method to apply water to the furrows Pipe outlets to deliver water to the furrows
Water is applied to the furrows by letting in water from the supply channel, either by pipe siphons or by making temporary breaches in the supply channel embankment. The length of time the water is to flow in the furrows depends on the amount of water required to replenish the root zone and the infiltration rate of the soil and the rate of lateral spread of water in the soil. Furrow irrigation is suitable to most soils except sandy soils that have very high infiltration water and provide poor lateral distribution water between
furrows. 2. Sub-Surface irrigation method As suggested by the name, the application of water to fields in this type of irrigation system is below the ground surface so that it is supplied directly to the root zone of the plants. The main advantages of these types of irrigation is reduction of evaporation losses and less hindrance to cultivation works which takes place on the
surface. There may be two ways by which irrigation water may be applied below ground and these are termed as: Natural sub-surface irrigation method Artificial sub-surface irrigation method Natural Sub-surface irrigation method Under favorable conditions of topography and soil conditions, the water table may be close enough to the root
zone of the field of crops which gets its moisture due to the upward capillary movement of water from the water table. The natural presence of the water table may not be able to supply the requisite water throughout the crop growing season. However, it may be done artificially by constructing deep channels in the field which may be filled with water at all times to ensure the presence of water table at a desired elevation below the root zone depth. Though this method of irrigation is excellent from both
water distribution and labour saving points of view, it is favorable mostly for the following The soil in the root zone should be quite permeable There should be an impermeable substratum below the water table to prevent deep percolation of water. There must be abundant supply of quality water that is one which is salt free, otherwise there are chances of upward movement of these salts along with the moisture likely to lead the conditions of salt incrustation on the surface.
Artificial subsurface irrigation method ` The concept of maintaining a suitable water table just below the root zone is obtained by providing perforated pipes laid in a network pattern below the soil surface at a desired depth. This method of irrigation will function only if the soil in the root zone has high horizontal permeability to permit free lateral movement of water and low vertical permeability to prevent deep
percolation of water. For uniform distribution of water percolating into the soil, the pipes are required to be very closely spaced, say at about 0.5m. Further, in order to avoid interference with cultivation the pipes have to be buried not less than about 0.4m below the ground surface. This method of irrigation is not very popular because of the high expenses involved, unsuitable distribution of subsurface moisture in many cases, and possibility of clogging of the perforation of the pipes. 3. Sprinkler irrigation method
Sprinkler irrigation is a method of applying water which is similar to natural rainfall but spread uniformly over the land surface just when needed and at a rate less than the infiltration rate of the soil so as to avoid surface runoff from irrigation. This is achieved by distributing water through a system of pipes usually by pumping which is then sprayed into the air through sprinklers so that it breaks up into small water drops which fall to the ground. The system of irrigation is suitable for undulating lands,
with poor water availability, sandy or shallow soils, or where uniform application of water is desired. No land leveling is required as with the surface irrigation methods. Sprinklers are, however, not suitable for soils which easily form a crust. The water that is pumped through the pump pipe sprinkler system must be free of suspended sediments. As otherwise there would be chances of blockage of the sprinkler nozzles. A typical sprinkler irrigation system consists of the following
components: Pump unit Mainline and sometimes sub mainlines Laterals Sprinklers Fig. shows the typical layout of the sprinkler system The pump unit is usually a centrifugal pump which takes water
from a source and provides adequate pressure for delivery into the pipe system. The mainline and sub mainlines are pipes which deliver water from the pump to the laterals. In some cases, these pipelines are permanent and are laid on the soil surface or buried below ground. In other cases, they are temporary, and can be moved from field to field. The main pipe materials include asbestos cement, plastic or aluminum alloy. The laterals deliver water from the mainlines or sub mainlines to
the sprinklers. They can be permanent but more often they are portable and made of aluminium alloy or plastic so that they can be moved easily. 4. Drip irrigation method Drip Irrigation system is sometimes called trickle irrigation and involves dripping water onto the soil at very low rates (2-20 litres per hour) from a system of small diameter plastic pipes filled with outlets called
emitters or drippers. Water is applied close to the plants so that only part of the soil in which the roots grow is wetted, unlike surface and sprinkler irrigation, which involves wetting the whole soil profile. With drip irrigation water, applications are more frequent than with other methods and this provides a very favourable high moisture level in the soil in which plants can flourish. Figure shows a typical layout of the drip irrigation system.
A typical drip irrigation system consists of the following components: Pump unit Control Head Main and sub main lines Laterals Emitters and drippers The drip irrigation system is particularly suited to areas where water quality is marginal, land is steeply sloping or undulating
and of poor quality, where water or labour are expensive, or where high value crops require frequent water applications. Drip irrigation is most suitable for row crops (vegetables, soft fruit), tree and vine crops where one or more emitters can be provided for each plant. Generally only high value crops are considered because of the high capital costs of installing a drip system. Drip irrigation limits the water supplied for consumptive use of plants. By maintaining a minimum soil moisture in the root zone,
thereby maximizing the water saving. A unique feature of drip irrigation is its excellent adaptability to saline water. Since the frequency of irrigation is quite high, the plant base always remains wet which keeps the salt concentration in the plant zone below the critical. Irrigation efficiency of a drip irrigation system is more than 90 percent. The drip irrigated crops are mainly used to irrigate orchards of which the following crops are important ones (according to a 1991 survey):
Grapes (12000 hectare) Bananas (6500 hectare) Pomegranates (5440 hectare) Mangoes Drip irrigation was also used to irrigate sugarcane (3900 hectare) and coconut (2600 hectare).
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