Glossary of terms


Alkali Soil - A soil that contains sufficient exchangeable sodium to interfere with the growth of most of crop plants. The ESP is generally more than 15 and ECF less than 4 decisiemens per metre. The soil pH is higher than 8’2 except in systems with chloride and sulphate anions.

Alkaline Soil - Any soil having pH greater than 7.

Annual Crops - Crops which complete their life cycle from seed to seed within one year.

Available Soil Moisture (ASM) - It is the difference at any given time between the actual moisture content in the root zone soil and the wilting point.

Available Soil Moisture Capacity - Total amount of available soil moisture in the crop root zone that can be held by a soil for use by plants. Usually considered to be the moisture held between field capacity and wilting point.

Base Period - The number of days over which duty of water is reckoned, determined or measured. Base period equals the period between first and last irrigation to raise a crop.

Cash Crops - A high value marketable crop such as sugarcane, jute, spices, fruits, tobacco and plantation crops.

Coefficient of Uniformity (UC) - It is the ratio of average depth of irrigation water infiltrated into the soil minus the average deviation from this average depth divided by the average infiltrated water.

Crop Expenditure - Crop expenditure includes direct cost and common expenditure incurred in agricultural production. Direct cost is cost of those items which are used specifically for the production of a particular crop. Such items are : seeds, organic manure, fertilizers, insecticides and pesticides, irrigation charges, labour (family as well as hired), hired bullock labour, expenses on tractor (hired). The common item of expenditure include expenditure on owned bullock, repairs and maintenance, land revenue, interest on borrowed capital, payment to village functionaries, rent on leased-in land.

Crop Rotation - The sequence during a year or period of years in which different crops are grown (or planned) in the same land.

Cropping intensity - The percentage of the total crop area during a crop year or season to the culturable command area.

Cropping Pattern - Yearly sequence and spacious arrangement of the crops in a given area.

Daily Consumptive Use - It is the consumptive use of water by a crop in a day.

Deep Percolation Loss - It is the amount of water that percolates downward below the crop root zone.

Gross Cropped Area (GCA) (or Cropped Area) - Gross cropped area is the total cropped area under various crops during the whole agricultural year counting the area as many times as the number of crops grown on the same land. Mixed crops sown simultaneously on the same land are treated as one crop.

Gross Farm Income - Gross farm income implies the value of total output produced by a farmer. The value of total output is obtained by multiplying total physical production of agricultural product (including byproducts) by their corresponding market prices.

Kharif crops - Those crops which are cultivated in the monsoon season. The following are the principal kharif crops: Maize, rice, small millets, peas, groundnut, cotton, tobacco, and sesame.

Mixed Crop - Where more than one crop is raised on the same field in the same season simultaneously, without any definite row arrangement such as gram and wheat.

Monocropping - It is the practice of raising a single crop in an area in a crop year or period.

Mulching - Covering the soil with any material such as straw, plant residues or plastic film to reduce the evaporation from soil surface and/or to protect plant roots from extremely low or high temperatures.

Multiple cropping - It is the practice of growing more than one crop in an area in a crop year.

Net Cropped Area (Net Area Sown) - Net cropped area is the area sown (or cropped) during the agricultural season (July-June), counting the area only once even if two or more crops are grown in different seasons on the same land.

Net Income from Cropped Production - Net income from farm (crop) production is equal to gross farm (crop) income minus farm (crop) expenditure.

Operational Holding - 'Operational holding' implies the size of farm actually cultivated by a farmer, and it includes owned land, leased-in land and mortgaged-out land. The operational holding comprehends of irrigated area, dry area and current fallow.

Overlapping crop - An old crop which is harvested or removed after sowing of the new crop.

Perennial crops - Crops which last several crop years like plantation or orchard crops.

Rabi Crops - Those crops which are cultivated in the winter season. The following are the principal Rabi crops: Wheat, barley, gram, peas, potatoes, mustard, tobacco and linseed.

Relay Cropping - Seeding/planting of the succeeding crop after flowering And before the harvest of the standing crop.

Single Crop - Raising of only one crop in one season.

Soil Moisture - It is the water occuring in the voids of soil mantle.

Soil Moisture Content - It is usually defined as the quantity of water present in the soil, expressed either as the weightof water per unit weight of dry soil or the volume of water per unit volume of bulk soil.

Soil Moisture Deficit ( SMD ) - It is the difference between the field capacity and the actual soil moisture in the root zone soil at any given time. It is the amount of water required to bring the soil in the root zone to field capacity.

Summer Crop - Often represents an intermediate (third) crop between the Rabi and Kharif crops.

Wetland Crop - Crop which grows in standing water during most part of its life cycle.


Hydraulic Structures

Abutment - a) It is a wall constructed by the side of the waterway of diversion structures to: 1) retain the backfill, 2) protect the banks from erosion, 3) support the load from superstructure, and 4) confine the flow to the desired waterway at the structure.

b) The well defined bank on either side of the river flow particularly in a deep gorge is called the abutment of a river.

Afflux - The rise of water level above its orlginal level upstream of the diversion structure, in a channel or river.

Afflux Bund - An embankment or dyke designed to ensure that the structure is not outflanked during flood flows. In some cases, it also acts as an embankment to prevent flooding to the country side due to an afflux.

Aggradation of River Bed - A general increase in the bed level of a river over a sufficiently long length. This is caused either due to increase in sediment load, decrease in water discharge or reduction in the energy slope in a stream, which is otherwise in equilibrium.

Apron - A protective layer of stone of concrete block or other material, extending out from a structure on or extending beyond the toe on the bed of a channel, or situated at some other location in the bed of a channel, laid in order to prevent erosion.

Arch Dam - A curved solid masonry or concrete dam, convex upstream, that depends principally on arch action for its stability. The water load is mostly transferred by the arch to the canyon walls, or otherabutments. It is also called ‘Arched Dam’.

Arch Gravity Dam - A curved solid masonry dam which depends upon both arch action and gravity action for stability. Also called ‘Curved Gravity Dam’. Arch gravity dam has the base thickness between 0.5 to 0.6 times the height of dam.

Articulated Type Buttress Dam - Same as simply supported flat slab deck dam.

Backwater Curve - The upstream longitudinal profile of the surface of water in a stream or a conduit from a point where such water surface is raised above itsnormal level by a diversion structure.

Baffle Piers/Baffle Blocks or Friction Blocks - Obstruction set in path of high velocity water flow constructed for the purpose of energy dissipation or for controlling the position of hydraulic jump in a stilling basin.

Bag Dam - A low dam made of bags partly filled with soil or boulder concrete. Bags are partly filled to permit flexibility in placement.

Barrage - A structure built across a river, for diverting water into a canal or for providing a small storage pond. It comprises a series of gates for regulating the river flow and water level, while keeping the afflux during floods within, acceptable limits. The structure may or may not have a raised sill. It is constructed to regulate the water-surface level and to divert the water flow from upstream of the gates.

Basket Dam - A dam composed of boulders held in woven wire crates and piled up to form a barrier.

Bay - One of the main divisions of a diversion structure such as spillway, undersluice or regulator between two piers or a pier and abutment or a pier and a divide wall.

Beaver Type Dam - A fixed timber dam used for low heads, the bents being of round timbers or logs with butts pointing downstream, spacing logs laid transversely, crevices filled with gravel, covered with a plank deck, the whole structure being fastened together with drift bolts, and to the foundation with anchor bolts. It is seldom used as a permanent structure.

Bed Sill - A sill in a river or canal the crest over which is at or very close to the normal bed of the river or the canal.

Blanket - Relatively thin layer of material covering material of a different properties.

Boulder Stage of River - The reach of a river characterised by steep bed slope and a bed comprising a mixture of boulder, shingle, gravel and sand.

Boule Dam - A movable dam similar to a Poiree Dam but with horizontal stop logs or panels, instead of needles which span from trestle to trestle.

Breaching Section - A low earth bund or dyke built acrossa saddle in the rim of the reservoir/different body of then dam, intended and designed to be washed out when the water reaches a predetermined elevation it is also called fuse plug spillway or emergency spillway.

Brush Dam - A check dam made of brush wood usually held in place with stakes and wire.

Bulk Head Buttress Dam - A type of buttress dam in which the face slab is replaced by flaring the upstream edge of the buttresses to span the distance between buttress walls. The flaring portion may be of various geometrical shapes, namely, (a) massive head; (b) round head or mushroom head; and © diamond head. The buttress dams comprising types of head classified above are named. Accordingly, It is also calIed ‘Round Head Buttress Dam’.

Buttress Dam - A dam consisting of a water supporting upstream face or deck, usually reinforced concrete slab, supported by buttresses generally in the form of equally spaced triangular reinforced concrete or masonry walls or counterforts, that transmit the water load and weight of the deck to the foundation.

Caisson - A chamber usually sunk by excavation within it for the purpose of gaining access under water to the desired foundation level.

Cantilever Buttress Dam - A flat deck dam where the slab is cantilevered from the buttresses.

Check Dam - A small low fixed dam, constructed of brush, logs, timber, loose rock, masonry, or concrete, in an eroded channel to reduce the slope of the water flowing therein during high stages, and also the resulting velocity, thereby preventing excessive scour and erosion and inducing deposition. A reduction in the size of flood peaks of erosion, and sometimes an increase in low-water flow due to increased ground storage often result from building a check dam. Such dams are also used to retain debris. They usually are built of inexpensive and temporary materials where dependence for ultimate protection is placed on vegetative cover.

Chute - Pipe, flume or open lined channel with a free surface flow.

Chute Blocks - Cement concrete blocks provided at suitable spacing at the toe of the downstream glacis of a structure for moderating the energy dissipation.

Closing Dyke - A structure built across the branch channel of a river in order to stop or reduce the flow entering that channel or separate dead branch from the main channel.

Coffer Dam - It is a temporary structure to exclude water or water and earth from a specific area to facilitate construction.

Columner Buttress Dam - A type of deck dam in which the massive buttresses are replaced by a series of inclined columns.

Composite Dam - A concrete/masonry wall with rockfill or earth-backing in downstream.

Composite Earth Dam - An earth dam consisting essentially of an inner or enclosed impervious section supported by two or more outer sections of relatively pervious material. It is also called ‘Multiple Zoned Earth Dam’ or Zoned Earth Dam’.

Concentration Factor - The factor by which the discharge per unit length of a barrage weir ( assuming uniform distribution ) is required to be increased for purposes of design, to allow for higher concentration of discharge in some parts of the structure.

Conoidal Dam - It is a modification of cupola arch buttress dam and has some of the characteristics of the hollow gravity dam and some of those of multiple arch dam. The upstream face is a plane slightly inclined to the vertical; the downstream face is a conoidal surface generated by straight lines, which where they touch the horizontal at the crest are orthogonal there-to and repose on a directrix drawn in a horizontal plane.

Constant Angle Arch Dam - It is a special type of variable centre dam in which the central angle of the arches are of approximately the same magnitude at all elevation

Constant Centre Arch Dam or Constant Radius Arch Dam - A type of arch dam whose arch centres for the upstream face and the downstream face are coincident with the axis centre at all elevation. A profile of these centres is a vertical straight line and the arches are of uniform thickness. All cantilevers are of identical shape, varying only in base elevation.

Contracted Weir - A weir with its length less than the width of the approach channel, and which is used to give a greater head for relatively low discharges in wide channels.

Core-Wall Type Rockfill Dam - A rockfill dam in which a core wall of steel, concrete ( plain or reinforced ) or compacted earth is placed in the centre, and the rockfill is dumped on both sides.

Cored Gravity Dam - It resembles hollow gravity dam in principle except that hollows are provided along contraction joints and not in the middle of the section. A timber dam consisting of a series of cribs or rectangular cells made of square or round timbers, drift-bolted together, filled with broken rock or boulders, with a upstream facing and deck covered with heavy planks to provide water tightness.

Crest - The line or area defining the top of a dyke embankment or weir.

Crib Dam - A type of construction consisting of pieces of timber or other suitable material fixed together to form bays or cells called crib which are filled with stone or other suitable materials.

Cupola Arch Buttress Dam - Here the upstream face of the buttress dam Consists of semi-cupolas of ovoidal form of reinforced concrete. Further modification of this dam is conoidal dam.

Cupola Arch Dam - Same as double curvature arch dam.

Curtain Wall, Cut-Off Wall - A wall-like structure, of masonry, plain or reinforced cement concrete or sheet pile, underthe floor of a hydraulic structure with the object of: a) dividing the work into suitable compartments, b) to reduce the percolation of water through permeable strata, c) to minimize the likelihood of undermining of the foundation by increasing the path of percolation and reducing the exit gradient, d) as a safeguard against erosion and undermining of the structures by scour, e) to intercept permeable strata in the foundation and/or, and f) to increase the resistance of the structure against sliding.

Cut Offs - A wall or diaphragm of concrete or steel, or a trench filled with impervious soil or a grout curtain extending into the foundation of a structure and providing a water tight anchor to the overlying structure. ‘Its purpose is the prevention or reduction of passage of water under the structure and foundation material or through the upper layers of the foundation material.

Differential Head - The difference in water level between the upstream and downstream of a structure or the difference in water level on either side of a pier or divide wall.

Diversion Ratio - The ratio of the diverted flow to the normal stream flow.

Diversion Work or Diversion Structure or Head Works - A collective term for all works required on the river or channel to divert, control or regulate the water level or water supplies in the river/channels or offtaking canals.

Divide Wall, Divide Groyne, Division Wall or Dividing Wall - A wall or groyne constructed normally at right angles to the axis of the weir or barrage extending well beyond the main structure to separate the under sluice bays/river sluice bays and barrage or weir bays for facilitating regulation.

Design Flood - Flood adopted for design purpose, which may be probable maximum flood or standard project flood or a flood corresponding to some adopted frequency of occurrence ( 50, 100, 200, 500 years, etc, ) depending on the standard of security to be provided.

Dewatering - Lowering the water table to facilitate construction of the substructure in fairly dry condition and to prevent free flow of particles below the foundation.

Dominant Discharge - A hypothetical constant discharge flowing through an alluvial river which will produce the same specific effect as is caused by a varying discharge flowing through the channel over a period of years. The specific effect can be the channel width, meander pattern, sediment load, etc.

End Sill - A vertical stepped, sloped or dentated wall, constructed at the downstream end of a stilling basin to help in dissipating residual energy and to reduce the length of the stilling basin.

Erosion - The lowering and wearing away of the land surface by weathering and removal of material by the action of wind, water or waves. The term also includes the erosion of the bed or banks of a river or channel or associated structures.

Embayment - A localized widening of a river or channel brought about artificially or naturally by recession or erosion of the bank.

Exit Gradient - The hydraulic gradient at the point where the seepage streamlines emerge at the end of an impervious apron.

Flank Walls - Retaining wall in continuation of abutments both upstream and downstream.

Flared Wall - Retaining wall with its profile gradually changing from one slope to another as required. Flared walls may be straight or curved.

Filter/Filter Bed - A layer or combination of layers of graded pervious materials designed and placed in such a manner as to provide drainage yet prevent the removal of soil particles by the seepage water, water currents or wave action.

Flexible Apron, Talus or Placed Riprap - A protection at the downstream or Upstream end of a weir, fall, etc, consisting of blocks of concrete or masonry or stones or stones in/wire crates ( gabions ). This is also referred to as a loose apron.

Free Board - The vertical distance between a specified water surface and the top of the non overflow section of a structure or embankment.

Fish Ladder/Lock - Device provided in the diversion structure to facilitate the migration of fish from upstream to downstream or vice versa.

Friction Block - Staggered R.C.C. blocks provided in the stilling basin for energy dissipation.

Glacis - The sloping portion of the floor upstream and downstream of the crest.

Gauge-Discharge Curve/Stage Discharge Curve - The curve indicating the various level/stages of the river for different values of discharge at a particular section.

Guide Bank - A protective and training embankment constructed at the side of weir/barrage, etc, to guide the flow through the waterway provided in the structure.

Intake - A structure to admit, control and regulate water supplies directly from source. Intakes may also be uncontrolled.

Hydraulic Gradient - The slope of the line of piezometric head ( that is sum of the pressure and elevation heads ) at any point in the flow path.

Lining - A protective covering ( over entire or portion of the perimeter ) of a water conductor system or reservoir to reduce seepage losses, to withstand pressure, to reduce and prevent erosion and improve conditions of flow.

Looseness Factor - The ratio of the overall waterway of a weir/barrage actually provided to the regime width of a river or channel at the design flood computed theoretically by using Lacey’s theory.

Nappe - A sheet of water overflowing a weir, fall, etc. The nappe has upper and lower surfaces.

Pitching - A protective covering of material on the earthen surface slope ( side pitching ) and beds ( bed pitching ) of rivers or canals.

Pier - Concrete or masonry structures constructed over the crest or floor of a structure supporting loads transmitted by the superstructure like bridge decking, gates and transmitted by the superstructure like bridge decking, gates and hoist operating mechanism. Sometimes loading piers are used on the downstream floor of a structure to counteract uplift pressure.

Pond Level - The level of water immediately upstream of a structure required to facilitate withdrawal into the canal or for any other purpose.

Riprap -Broken stone dumped or placed on the surface and the slopes of embankments for protection against the action of flowing water, wave wash and heavy rain.

Revetment - A protective surface of pitching, concrete blocks or matresses placed on the bottom or banks of a river to prevent or minimize erosion.

Retrogression/Degradation - A general decrease in the bed level of the river or a channel over a sufficiently long length downstream of a structure. It may be caused by a decrease in sediment load, increase in discharge or increase in the energy slope.

River Sluices - A set of sluices similar to the undersluices located in between the undersluices and spillway bays and separated from them by means of divide walls usually provided for still pond operation.

Seepage - Movement of water through pores and interstices of unsaturated packed soil material into or out of a surface or subsurface body of water, such as river, canal, reservoir, etc.

Sill - a) A structure built under water across deep pools of a river course for counteractingthe tendency to excessive scour.

b) A structure built at the outlet of a channel where certain minimum depth offlow is to be maintained in the channel, or a structure built at the head of a channel to prevent flow entering the channel until the main river stage reaches the crest of the structure.

C) The invert of a gate or sluice opening.

Slope Protection - Riprap, concrete blocks, brush or other material laid for protection of the sloping part of an embankment or levee to prevent erosion, slipping or caving.

Sludging - a) Flowing of mud, or b) The process of filling the crevices left in the dried clay of an embankment with sludge.

Stem bank or Shank - A transverse wall projecting from an abutment into the embankment acting as cut off and to intercept seepage.

Staunching Wall - Embankment connecting a groyne head to the river bank or marginal bund.

Stilling Basin - A short reach of paved channel to which a hydraulic jump, used for energy dissipation in hydraulic structures, is usually confined either partly or entirely.

Stone Mesh - A type of construction in which shingle, small boulders, or other form of stone, is held together by a wrapping of wire mesh, to give a heavy but more or less flexible structure used in various forms as groynes, aprons, low weirs, etc. Rounded stone is usually preferable to increase flexibility.

Stone Reserve - A quantity of stone kept as reserve on guidebanks, spurs or groynes for emergency use to prevent deep scour occurring and endangering the safety of the structure.

Silt Excluder - A device constructed for the purpose of preventing entry of excess sediment.

Stoplogs - Fabricated structural steel or wooden units or simple logs, planks, concrete slabs, etc, utilized for temporary closure of an opening for passage of water like anv bay of the barrage or regulator in order to faiilitate repairs of the gates and other components of the bay.

Toe Protection - Loose stones wire crates or concrete blocks laid or dumped at the toe of an embankment, groyne, etc, or masonry or concrete wall built at the junction of the slope of pitching and the bed in channels or at extremities of hydraulic structures to counteract erosion.

Toe-Wall - A shallow wall constructed below the bed or floor level to provide footing for the sloped pitching or the face of an embankment.

Training Wall - A structure built along or connected to the bank of a river substantially along the direction of flow, for example, an extension to a flank wall, intended to direct fast flow from a sluice or spillway away from erodible banks of a river or canal.

Trash Rack - A grid or screen made of steelbars, installed in front of the entrance to intakes, dam outlets, etc, to prevent entry of floating materials, debris, ice, etc. The size of the screen opening depends upon the maximum size or debris, etc. required to be excluded.

Under Sluices - The under sluices are bays in continuation of the weir with a crest at lower level on the same side as the canal to maintain a clear and well defined rives channel towards the canal head regulator, to scour the silt deposlted on the river bed in the pocket upstream of canal head regulator or to pass winter freshness and low floods without dropping the weir shutters.

Weep Holes - Opening provided in walls, return walls, aprons, linings, foundations, etc, to permit drainage and reduce pressure.

Weir or Anicut - An ungated barrier across a stream or a river for the purpose of:

a) measuring its discharge, or

b) raising, controlling and maintaining the water level, and/or,

c) diverting part or all the water from the stream/river into a canal or conduit.

Wing Walls - Walls joining the abutment of a structure to earth dyke or the banks to retain and protect the earth fill behind and provide a longer path of percolation around the end of a structure or for improving the flow conditions upstream and downstream of the controlling section.

Hydro Power

Access Tunnel - The underground approach for the power station.

Adit - It is an underground opening from hill face either for facilitating underground construction (construction adit) or for exploration instrumentation (exploratory adit).

Air-Vent or Air-Vent Pipe - Vent or pipe provided downstream of gate groove for entry of air into the conduit to prevent formation of negative pressures in the conduit and cavitation, and also to prevent possible collapse of the conduit when the conduit is drained with the gate closed.

Annual Load Factor – Annual load factor is defined as the ratio of the actual energy consumed during that year to the peak demand assumed to continue for one year.

Auxiliary Rooms or Auxiliary Bays - Portion of the power station annexe to the machine hall where the control and auxiliary station service equipment like cooling water supply, compressed air pumps, etc, are positioned.

Balancing Reservoir - A reservoir created upstream of the forebay to cater for the diurnal or weekly fluctuations in water demand due to variations in power generation.

Base Load Power Station - A power station operating continuously at a constant or nearly constant power and which operates at relatively high load factors. It caters to power demand at base of the load curve.

Bell-Mouth Transition - Bell shaped transition provided at the entry of the penstock or a tunnel to ensure smooth inflow and minimize entry losses.

Bypass Channel ( or Bypass Tunnel ) - The channel ( or tunnel ) bypassing power station to permit direct flow of water from head race or surge tank to the tail race.

Cable Racks - Racks or trays supported by brackets or frames fixed in the walls, floors or ceiling for carrying the cables.

Capacity factor or Plant factor – It is defined as the ratio of average output of the plant for a given period of time to the plant capacity.

Chute - Pipe, flume or open channel on relatively steep slopes carrying a free surface flow.

Classification of Hydroplants on the basis of operating head on turbines

i) Low head scheme - A low head scheme is one which uses water head of less than 15 m.

ii) Medium head scheme – A medium head schem is one which uses water head varying between 15 to 60 meteres or so.

iii) High Head scheme – a high head scheme is one which uses water head of more than 60m or so.

Control Room - A room located near the units either just on the downstream or the upstream side of the unit blocks or at one end of the machine hall which houses the control panels.

Crane Beam - Beams over which the overhead crane traverses on the rails placed over the beam in the power station for carrying the loads.

Cut and Cover Conduit - A conduit usually of concrete/RC construction placed in a cut and covered with backfill to the required extent.

Dam power Station - A power station located at the toe of a dam thus using relatively small length of water conductor system.

Demand Factor – The ratio of the maximum demand at any particular time to the connected load is known as the demand factor.

Dependable Capacity – It is the load carrying capability of the power house with respect to the loade characteristics during a specified time interval, and is decided by the power factor, capability and the load on the power house.

Design Head – it is the net head under which the turbine reaches peak efficiency at synchronous speed. Generally, the design head = W.A.L.-M.W.L. The difference of N.W.L. and M.W.L. is 125% of design head.

Dewatering Sump - A pit provided in the power house for collecting the water to be pumped out from the turbine for evacuating it for inspection and maintenance.

Downsurge - Fall of water level in the surge tank/shaft below the static water level due to load acceptance.

Draft Tube – The draft tube is a conduit which connects the outlet of a reaction turbine to the tailrace.

Draft Tube Deck - A slab over the draft tube openings supported on draft tube pier above maximum tail water level for gantry cranes operating the draft tube gates.

Drainage and Inspection Galleries – Suitable galleries in the substructure of power station to facilitate drainage and inspection.

Drainage Sump - A pit provided in the machine hall for collecting and pumping out the water from inside the power station.

Expansion Gallery/Expansioa Chamber - It is a gallery ‘or chamber attached to the surge tank to provide additional storage capacity.

Fire Protection Wall - Protection walls provided in between equipments for protection against spread of fire.

Firm Power – the net amount of power which is continuously available from a plant without any break on firm or on guaranteed basis is known as firm power.

Flume - An artificial water channel of wood, metal, concrete or masonry usually supported above the surfaces of the ground.

Foreway – A foreway is a storage basin or any other large body of water situated just infront of intake. Its main function is to temporarily store the water which is rejected by the plant due to reduced load during off- peak hours, and also to meet the instantaneous increased demand when the load is suddenly increased.

Free-Flow Conduit - Channel, pipe or other enclosed structure carrying water partially full; the flow conditions are similar to those in open channel. Also called ‘Open Conduit’ or ‘Free-Flow Tunnel’.

Gantry Column - Columns ( RCC/steel ) which support the crane beams.

Generator Floor - The floor in the power house from where inspection, repairs and maintenance of the generator are carried out.

Gross head – gross head is the difference in the water level elevations at the point of diversion of water for the hydel scheme and the point of return of water back to the river.

Head Race Tunnel/Channel - A channel or a free-flow tunnel leading water to the forebay or a pressure tunnel leading the water to the surge tank.

Hydraulic Turbines – Turbines are machines which convert hydraulic energy into mechanical energy. The mechanical energy so developed by the turbine is then used to generate electric energy by directly coupling the shaft of the turbine with the generator.

Ice Removal System - An arrangement for removal of different types of ice formation in case of intakes located in very cold climates so as to prevent the blocking of the intake opening and also to prevent pieces of ice reaching the turbine runner.

Indoor power station - A power station where the machinery, namely, turbine, generator and control equipment, is housed in a permanent building with superstructure of conventional type.

Installed capacity of the Power House – The total capacity in kilowatts or million kilowatts of all the turbine-generator units installed in a power house, is called its installed capacity.

Intake Gates - The gates regulating the entry of flow into the power channel, tunnels or penstocks.

Intake Ports - The openings of the intake well ( tower ) which admit water into the tower.

Intake Structure – A structure to withdraw water from a surface water source to feed a power house.

Intake Tower - Intake structure constructed in the form of a tower in the reservoir with entry of flow at one level or at more than one levels, when there is wide variation of water level in the reservoir. This can be a submerged structure also.

Intermediate Structure - The portion of power station extending from the top of the draft tube top slab to the generator floor consisting of speed ring and its support, the generator supporting barrel, and the concrete around the scroll case and various floors.

Load factor (L.F.) – Load factor is defined as the ratio of the average load over a certain period of time to the peak load during the same period.

Mass Oscillation - Oscillation of relatively low frequency in the closed water conductor system caused by changes in the flow conditions.

Minimum water level (M.W.L) – The elevation of water level which produces minimum net head on the power units (i.e. 65% of design head H) is known as minimum water level.

Net Head or Effective Head – The effective head is the net head applied to the turbine, and is given by the difference of head at the point of entry and exit of turbine, and includes the respective velocity and pressure heads at both places.

Normal water level (N.W.L) – the highest elevation of water level that can be maintained in the reservoir without any spillway discharge, either with a gated gated or a non-gated spillway, is known as normal water level.

Operating Head – Operating head is the simultaneous difference between the elevation of water surface in the foreway and the tailrace, after making due allowance for the approach and exit velocity heads, operating head = T.E.L at foreway entrance - T.E.L. at tailrace exit.

Outdoor Power Station - A power station where the superstructure is eliminated and the generating equipment is protected against the weather by a suitable covering.

Outdoor Switch Yard - The area where outdoor switching and associated equipment are installed.

Peak Load Power Station - A power station primarily designed for the purpose of operating to supply the peak load of a power system. Also called ‘Peaking Station’.

Power Channel - A channel constructed to carry water for power generation.

Power Factor – it is defined as the ratio of actual power in kilowatts to the apparent power in kilo volt-amperes.

Power House – A power House is a building consisting of a substructure to support the hydraulic and electrical equipment and a superstructure to house and protect this equipment. It is the structure housing the generating and control equipments and service bay.

Power Station - Power station denotes the entire complex including power house, ancillary structure and switchyard.

Pressure Conduit - A closed conduit which entirely confines and guides the movement of water under pressure.

Pressure Drop - Decrease in the pressure head due to sudden increase of flow in the pipe.

Pressure Rise - Increase above normal condition in the pressure head due to sudden decrease of fl$ti in the pipe.

Pumped Storage Power Station - A power station which, during periods of high demand for energy, generates power from water stored in the upper reservoir; and which pumps the water from a lower reservoir back into the upper reservoir during periods of low demand utilizing low value energy from the system. Usually such stations follow the diurnal cycles but some may follow seasonal cycles.

Raking - Raking is a manual/electrical operation to clean the trash rack.

Rated Head – It is the head at which the turbine functioning at full gate opening will produce a power output, equal to that specified in the name plate of the turbine. This rated head should be equal to the design head of the turbine, so as to ensure maximum overall plant efficiency.

Reservoir - It is a water storage created by putting an obstruction across a stream or river.

i) Lower Reservoir ( Pumped Storage ) - A reservoir downstream of the draft tube usually created for pumped storage schemes.

ii) Upper Reservoir ( Pumped Storage ) - High elevation reservoir serving as head reservoir for pumped storage schemes for storage of water.

Run-of-the River Power Station - A power station utilizing the run-of-the river flows for generation of power with sufficient pondage for supplying water for meeting diurnal or weekly fluctuations of demand. In such stations, the normal course of the river is not materially altered.

Secondary Power – The excess power available over the firm power during the off peak hours or during monsoon, etc. is known as secondary power.

Semi-outdoor Power Station - A power station with a low superstructures over the machine hall with hatches in the roof for handling the generating equipment.

Semi-Underground Power Station - A power station located partly below the ground level and followed by a tail race tunnel.

Service Bay - Area of the power house in continuation of the machine hall where the assembly and maintenance of equipment may be carried out. This may also refer to the maintenance and repair area provided separately for transformers.

Stability of Surge Tank - Condition for damping the mass oscillations caused by the change in the steady flow condition of system due to the governor action.

Surface Power Station or Overground Power Station - A power station which is constructed over the ground with necessary open excavation for foundations.

Surge Tank or Surge Chamber – A surge chamber/tank is a device introduced in the system near the power plant and in a long pressure conduit to provide the required force of retardation in case of sudden load rejection and force of acceleration during the load acceptance.

i) Air Cushioned - It is a closed surge tank having a provision of air cushion to absorb change in pressure in the water conductor system

ii) Differential - A type of surge tank with a main chamber and central riser with port holes

iii) Multiple - The water conductor system having two or more shafts with free surface upstream, of power station

Tailrace – The channel into which the water is discharged after passing through the turbines is known as the Tailrace.

Trash Rack - A grill or screen cover at intake openings for preventing the entry of suspended or floating material into the water conductor system. Trash racks may have fine/coarse openings depending upon the nature of debris to be excluded.

Tunnel - An underground passage constructed for conveyance of water, equipment, materials and movement of traffic.

Underground Power Station - A power station located in a cavity in the ground with no part of the structure exposed to outside.

Up Surge - Rise of the water level in the surge shaft above static water level due to load rejection.

Utilization Factor or Plant use factor – it is defined as the ratio of water actually utilized for power production to the water available in the river.

Water Conductor System - It consists of one or any combination, of the means of conveyance of water from the reservoir or diversion structure to the turbine and thence to the exit of the tail race.

Water Hammer - The pressure wave set up due to change of kinetic energy to elastic strain energy caused by any change in the flow condition in a closed conduit.

Weighted Average Level (W.A.L) – The level above and below which equal amounts of power are developed during an average year (.e. 50% units between N.W.L and W.A.L., and 50% units between W.A.L and M.W.L.) is called weighted average level.


Hydrological Cycle

Hydrology cycle.png

1. Water Year - 1st June to 31st May

2. Monsoon Period - June to October

3. Non-Monsoon Period - November to May

Actual Evapotranspiration - Evapotranspiration from a particular crop under the given moisture and climate conditions.

Agricultural Drought - Continuous (period of) dry weather causing serious moisture deficits, for crop growth.

Anti-Cyclone - An area of relatively high pressure with closed isobars, the pressure gradient being directed from the centre so that the wind blows spirally outward in a clockwise direction in the northern hemisphere, counter-clockwise in the southern hemisphere.

Arid - Climate or regions having not enough rainfall (usually less than 250 mm) to support vegetation.

Channel Precipitation - Precipitation which falls directly on surface or lakes and streams.

Cloud Burst - Rain storm of high intensity and of a relatively short duration, usually over a relatively smaIl area.

Convective Precipitation - Precipitation resulting from the upward movement of air that is warmer than its surrounding. It is generally of a showery nature with rapid changes of intensities.

Critical Storm Period - The duration of that storm which causes the greatest peak at a station in a drainage basin.

Cyclone - A low atmospheric pressure area manifest on the synoptic chart having a very low central pressure and surrounded by a system of closed isobars, circular or oval in form. The wind circulation is anti-clockwise around the centre (in the northern hemisphere) and is associated with heavy rain and very high winds reaching above 6 Beafort scale (55 km/h), usually winds of 100 to 200 km/h is reached in severe cyclones.

Cyclonic Precipitation - The precipitation associated with the passage of depressions or cyclones.

Depression - The term is used for circulation on the synoptic weather chartdelineated by closed isobars with wind circulating around it anti-clockwise (northern hemisphere) with or without lateral motion. It is generally associated with cloudy to rainy weather and falling pressure in the direction of its motion.

Depth Area Curve (Rainfall Intensity Area Curve) - A curve which graphically expresses relation between progressively decreasing average depth of rainfall over a progressively increasing area from centre of maximum precipitation of a storm outward to its edges.

Depth Area Duration Curve - A curve which graphically indicates the precipitation amounts for various areas and durations for a particular rainstorm.

Depth Duration Curve (Rainfall intensity Duration Carve) - A curve which shows relationship between duration and depth of precipitation of storm for a specific area.

Depth of Run Off - The total run off from a drainage area or basin, divided by the area; expressed in either units of depth or units of volume per unit area of the basin.

Direct Run Off - The sum of surface run off, interflow and channel precipitation.

Effective Rainfall - It is the part of the rain that appears as run off in the stream or The portion of rainfall that replenishes the water availability in the crop root zone.

Estimated Evapotranspiration - An estimate of evapotranspiration by means of standard formulae using climatic factors.

Evaporation - Evaporation is the physical process by which a liquid is transformed into a gaseous state. In agriculture, it is the total water vapour loss from a given area over a given time period. It may be expressed as the total or the mean rate in units of depth or volume per unit area, for the period concerned.

Eye of the Storm - The small central region of a tropical cyclone usually extending to I5 km in diameter having the lowest pressure and associated with features like absence of rain, light winds and broken layers of clouds.

Groundwater runoff - The part of the run off which consists of water that has passed into the earth and entered the zone of saturation, and has later been discharged into a water body.

Hail - Small, roughly spherical lumps of approximately concentric shells of clear ice , and compact snow usually ranging from 5 to 10 mm or more in diameter which fall either separately or agglomerated into larger irregular Iumps precipitated during thunder storms.

Histograph - A map or chart of a river, drainage or sewer system, upon which a series of time lines are placed. These time lines give the time of transit of water, originating on a time line to flow down to the outlet of the system.

Hydraulic Conductivity - The rate of flow of water in litres/day through unit cross-section of soil under unit hydraulic gradient at a specified temperature.

Hydrograph - A graph showing the variation of gauge/river stage, discharge velocity, sediment concentration or sediment discharge or some other feature of flowing water with respect to time at a given place.

Infiltration - The downward entry of water from the surface into the soil.

Infiltration Rate - The rate at which a soil, in a given condition at a given time can take in water.

Interception - The process by which precipitation is caught and held by foliage. twig : and branches of the trees, shrubs and other vegetation, and lost by evaporation, never reaching the surface of the ground.

Isohyet - A Iine drawn on a map passing through places having equal amounts of rainfall recorded during the same period at these places (these lines are drawn after giving consideration to the topography of the region).

Isohyetal Map - A map showing isohyets.

Isopercental Map - A map showing lines connecting points of equal percentage of rainfall after showing the annual or monthly rainfall at each raingauge station as a percentage of the annual Iong-average figures for that station.

Leaching - The process of removal of soIuble salts in the soil by the passing water through it.

Leaching Requirements - A fraction of the water entering the soil that must pass through the root zone in order to prevent soil salinity from exceeding a specified value. Leaching requirement is used primarily under steady state or long time average conditions.

Low (Trough) - An area of comparatively low pressure with or without dosed isobars ( in case of troughs ), These low may be the result of movement of depression unequal healing or movement of fronts in the troposphere ( lower atmosphere).

Mass Rainfall Curve (Mass Precipitation curve) - A graph showing the accumulated precipitation against time.

Maximum Possible Precipitation - The maximum amount of precipitation that can theoretically occur for a certain duration in a drainage area or basin during the present climatic era.

Maximum Probable Precipitation - The amount of precipitation that is the physical upper limit for a given duration over a particular basin and in a designated length of time.

Mean Annual Precipitation - The mean of annual amount of precipitation observed over a period which is sufficiently long ( say 30 years or more) to produce a fairly constant mean value.

Mean Annual Run Off and Mean Monthly Runoff - The value of the annual volume of water discharged by the stream draining the area, the period of observation being sufficiently long to secure a fair mean, similarly mean monthly run off.

Meteorologically Homogeneous - If throughout any area the probability of occurrence of a storm of any given intensity over a very long period of time is the same at every point, the area is said to be meteorologically homogeneous.

Orographic Precipitation - Precipitation caused by dynamic cooling of air as an air current rises over a mountain barrier.

Overland Run Off - Water flowing over the land surface before it reaches a definite stream channel or body of water.

Percentage Run Off - The amount of runoff expressed as percentage of total rainfall on a given area.

Period of Concentration or Time of Concentration - The time taken by the runoff from the farthest point of the catchment to reach the point under consideration.

Point precipitation - Precipitation at a particular site in contrast to the mean precipitation over an area.

Potential Evaporation - The evaporation from a given surface when all surface atmospheric interfaces are wet (saturated), so that there is no restriction due to either biological control or soil water content on the water vapour loss from the surface area. Its magnitude will depend primarily on atmospheric conditions and surface albedo, but it will also vary with the geometric characteristics of the surface. These characteristic (aerodynamic roughness and vegetative structure and density) are governed by the type of vegetation present, its health and stage of growth.

Potential Evapotranspiration - Evapotranspiration from a particular crop with optimal plant density and soil fertility growing in a well-watered soil ( soil water not limiting plant growth at any time ) under large field conditions and typical weather situation.

Potential Transpiration - The amount of water transpired by a green crop of about the same colour as grass, which completely covers the ground and which has an adequate supply of water.

Precipitation - The total supply of water derived from the atmosphere in the form of rain, snow, dew, mist, frost, hail , sleet, etc. It is usually expressed as depth of liquid water on a horizontal surface in day, month, or year, and designated so daily, monthly or annual precipitation.

Rain Gauge - An instrument for measuring the quantity of rain that falls at a given place and time.

Rain Storm - Storm accompanied by rain.

RainfalI - The total liquid products of precipitation or condensation from the atmosphere as received and measured in a rain gauge.

Rainfall Area - Geographical extent of a given storm.

Rainfall Distribution Coefficient - The distribution coefficient for any storm is the ratio of the maximum rainfall at any point to the mean rainfall in the basin.

Rainfall Excess (Net Rainfall) - Part of “the rainfall that appears as run off in the stream ( same as effective rainfall ).

Rainfall intensity (Rainfall Rate) - The amount of rain occurring in a unit interval of time, generally expressed in mm per hour.

Rainfall Intensity Recurrence Interval - The most probable tIme interval between the occurrence of rainfall of a given intensity and that of an equal or greater intensity. Sometimes the term is referred as Rainfall Intensity Frequency which is actually the reciprocal of recurrence interval or return period.

Rainfall Penetration - The depth below the soil or rock surface reached by a given quantity of rainfall which has passed below such surface. The maximum extent of penetration would be up to groundwater table.

Rate of Run Off or Discharge - The volume of water flowing in the stream channel past any given section in a unit of time.

Recording Rain Gauge (Pluviograph) - A rain gauge which automatically records, usually in graphical form the cumulative amount of rainfall with reference to time.

Reference Crop Evapotranspiration - The evapotranspiration from a given well adopted crop selected for comparative purposes under given weather conditions and with adequate fetch ( sufficient to make boundary effects relatively unimportant ) and for a standardised watering regime appropriate for this crop and the region concerned.

Residual Mass Curve - A plotting of the year-to-year residual departure of rainfall or run off from the arithmetical average accumulated for the period under consideration.

Return Period or Recurrence Interval - Statistical parameter used in frequency analysis as measure of most probable time interval between occurrence of a given event and that of an equal or greater event.

Run Off - It is defined as that portion of the precipitation which is not absorbed by the deep strata but finds its way into the streams after meeting the persistent demands of evapo-transpiration including interception and other losses. It includes surface run off received into the channels after rainfall, delayed run off that enters the streams after passing through portion of the earth, and other delayed run off that has been temporarily detained as snowcover or stored in natural lakes or swamps.

Run Off Coefficient - The ratio of run off to precipitation.

Snow - Precipitation from the atmosphere in the form of branched hexagonal crystals or stars, often mixed with simple ice crystals, which fall more or less continuously from a solid cloud sheet. These crystals may fall either separately or in coherent clusters forming snowflakes.

Snow Pellets or Soft Hail - Precipitation of white opaque rain of ice, structure of which resembles to that of snow, the grains are spherical, or sometimes conical, about 2 to 5 mm in diameter.

Storm - Term commonly used for violent atmospheric motion, such as gale, thunderstorm, rain storm, snow storm or dust storm.

Storm Track - The path traversed by the centre of the storm.

Sub-Surface Run Off ( Interflow ) - Run off that moves through upper soil layers and returns to the surface or appears in streams without entering the water table in the zone of saturation.

Surface Run Off - The water which reaches the stream by travelling over the soil surface or falls directly into the stream channels.

Thiessen Polygon - The points of location of rain gauges on a map are joined by straight lines and their perpendicular bisectors are drawn. The polygon formed around each rain gauge station by these perpendiculars is called, after its originator, a Thiessen polygon.

Thunderstorm - A local and short-lived atmospheric disturbance accompanied by lightning and thunder and often by showery precipitation, gusts of wind, and sometimes by hail.

Tornado - A rotary storm, one of the most violent types of storms known, of small diameter, which travels across the country and leaves great devastation along a narrow path. Its chief characteristics are the following: a) Under a heavy cumulonimbus cloud there hangs a funnel shaped cloud which marks the vortex and, as the storm moves along, mayor may not touch the earth; b) Heavy precipitation and (usually) hail occur, with thunder. In addition to the thunder, there is the roar attending the tornado cloud when it touches the surface; c) The winds blow spirally upward around the axis of the tornado cloud; and d) The speed of the storm itself in travelling over the earth is comparatively slow 40 to 65 km an hour; its path is short, averaging about 480 km.

Totalizer or Storage Rain Gauge - A type of rain gauge which totalizes the quantity of precipitation.

Transpiration - It is the process of release of water vapour to the atmosphere from aerial organs of the plant mainly through deep stomata.

Weighted Mean Monthly Precipitation - The weighted mean precipitation for each month for a large area.

Yield of Drainage Basin - Total volume or flow from a drainage basin for a long stipulated period of time, for example annual yield of drainage basin is the mean annual run off.

Zero Moisture Index - The index of moisture when the precipitation is just adequate to supply aII the water that would be needed for maximum evaporation and transpiration In the course of a year.


Actual Utilization - Actual utilization is the gross area actually irrigated each year.

Application Efficiency ( AE ) - It is the ratio of the average depth of the irrigation water stored in the root zone to the average depth of irrigation water applied.

Area Assessed - The area irrigated on which water rate have been levied.

Base Period - The number of days over which duty of water is reckoned, determined or measured. Base period equals the period between first and last irrigation to raise a crop.

Border Strip Irrigation Method - It is controlled surface flooding irrigation method where the field to be irrigated is divided into narrow strips by long parallel low bunds or levees along the general slope of land which shall normally be sufficiently gentle and each strip of land is irrigated by admitting a stream of water at the upper end.

Canal Irrigated Crop - A crop which is raised predominantly on canal irrigation.

Check Basin Method - The method consists of applying water to an area which is prepared level or nearly level and divided into basins or compartment, usually of rectangular shape with bunds. A supply channel is aligned on the upper edge of the area, and the laterals are dug in between two check basins.

Classification of Irrigation Projects - The irrigation projects can be classified as:

i. Major Irrigation Scheme - Culturable Command Area (CCA) more than 10,000 hectare (ha)

ii. Medium Irrigation Scheme- Culturable Command Area (CCA) more than 2000 hectare (ha) and upto 10,000 hectare (ha)

iii. Minor Irrigation Scheme - Culturable Command Area (CCA) upto 2000 hectare (ha)

Closure period - The period when the canal is closed for regular maintenance, repairs and other purposes.

Consumptive Use Efficiency - The ratio of consumptive water use by crop and the soil moisture stored in the root zone of the soil during the crop growth period.

Consumptive Water Use - The quantity of water cosumed in evaporation transpiration and metabolic processes during crops growth, including water consumed by accompanying weed growths. It is expressed in water-depth units per unit area, also called Consumptive Water Use or Evapotranspiration.

Conveyance - The movement of water from its source through the main or secondary canals or conduits to the tertiary or distributory offtakes.

Conveyance Losses or Transmission Losses - Losses of water in transit from the source of supply to the point of field turn out whether in natural channels or in artificial ones, such as canals, distributaries or watercourses. They comprise evaporation from the water surface, seepage and incidental transpiration by vegetation growing in or along the canals network. These also include the operation losses in the canal system.

Crop irrigation ratio – The crop irrigation ratio is the ratio of area irrigated under the crop to the total area (irrigated plus unirrigated) under the same crop expressed in percentage.

Crop Water Requirement - The total water needed for evapotranspiration from planting to harvest for a given crop in a specific climate regime, when adequate soil water is maintained, by rainfall and/or irrigation so that it does not limit plant growth or crop yield.

Culturable Command Area - 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 orTotal area in which cultivation is possible.

Culturable Irrigable area - The culturable command area less the area not available for irrigation due to high relief or other reasons.

Culturable Lift Irrigation - The culturable command area that can be irrigated only by lift irrigation.

Curve of Demand - A graph showing chronologically the amount of water needed for irrigation at various times during a crop season.

Curve of Supply - A graph showing chronologically the quantity of water available for irrigation during a time period from a given source.

Delta - It is the quantity of irrigation water expressed in depth units over the irrigated area. It is stated with reference to the place at which it is measured or reckoned, that is, delta at farm, delta at out-let, head of watercourses, or lateral head, delta at distributary head, delta at head of main canal.

Design Duty of Water - Duty of water assumed in a irrigation project for designing capacities of channels.

Distributary or Tertiary - Canal or conduit taking water from the con-veyance system and supply it to one tertiary unit.

Distribution Efficiency - It is the measure of uniformity of irrigation water distribution over a field.

Diversion Structure - The structure that diverts water from the water sources and supplies it to the irrigation system.

Division Structure - A structure in the conveyance system that divides the flow into two or more canals or conducts, or both.

Drip/Trickle Irrigation - It comprises the application of water in drops close to the plant. The entire space between the plants is not watered.

Dryland Crops - The crops which do not normally require irrigation because the prevailing quantity and incidence of the local rainfall is suitable and sufficient for cultivation.

Duty of a Well - The average annual area of land irrigated by well for maturing a crop.

Duty or Duty of Water - The relation between the area irrigated, or to be irrigated, and the quantity of water used, or required, to irrigate it for the purpose of maturing its crop. Duty is stated as area per unit area of flow.

Farm Field Inlet - A structure which supplies water to a farm field.

Farm Irrigation Efficiency - The percentage of the water consumed by crops in a farm to the water made available at the farm gate.

Farm Losses - Losses of water on the farm due to uneven distribution, evaporation and percolation into the subsoil due to over irrigation. These include surface run-off and deep percolation.

Field Application - The application of water from the field inlet to the field.

Field Application Efficiency - The field application efficiency is made up of two parts (1) the efficiency of water transport system in the field and (2) the efficiency with which the water is applied. It is, by definition, the relation between the quantity of water furnished at the field inlet and the quantity of water needed to maintain the soil moisture above some target level required for the crop.

Field Capacity ( FC ) - The moisture remaining in a soil following wetting and natural drainage until free drainage has practically ceased.

Field Channel - Channel usually taking water from the watercourse and supplying it to one or more forms or fields.

Field Irrigation Requirements - The requirements of irrigation water for crops at the diversion point of supply channel.

Flow Irrigated Area - Area which can be irrigated from the source of water, by flow under gravity alone.

Furrow Irrigation - It is a method of applying water to crops sown in rows through furrows.

Gross Command Area - The total geographical area which can normally be commanded or serviced from a irrigation project without consideration of water supplies available for irrigation. It is the total area covered by an irrigation project including unculturable area under habitation, road, tanks, waste land, forest land etc.

Gross Irrigated Area – The gross irrigated area is the total irrigated area under various crops during the whole agricultural year, counting the area irrigated under more than one crop during the same year as many times as the number of crops grown. Inter-cultured or mixed crops are treated as one crop.

Gross Irrigation Ratio – The gross irrigation ratio is the ratio of gross irrigated area to the gross cropped area in a year.

Intensity of Irrigation - The percentage of total area of normally irrigated crops in a year to total culturable command area.

Irrigation - The supply of water by artificial means for raising crops.

Irrigated Area - The area to which irrigation water has been applied.

Irrigation Method - The manner in which irrigation water is applied to the land for raising a crop.

Irrigation Potential Created - (As per Planning Commission)

a. The irrigation potential created by a project at a given time during or after its construction is the aggregate gross area that can be irrigated annually by the quantity of water that could be made available by all connected and completed works upto the end of the water courses or the last point in the water delivery system upto which the Government is responsible for construction.

b. Before an area is included and reported under 'Potential Created', it may be ensured that the storage, head-works as well as the distribution system including irrigation outlets to serve the area are completed together with necessary water courses covering chaks or blocks upto 40 hectares in area and that works completed will make available the requisite water for the purpose in a design year for the assumed cropping pattern. The irrigation outlets should be of a capacity of about 0.03 cumec. The capacity may, however, vary depending on local conditions relating to topography, crop pattern, etc. but it should not normally exceed 0.06 cumec. The figures of the potential which relate to the gross irrigated 'new area' and 'old area stabilized' should be reported separately. The potential which refers to the 'old areas stabilized' should, however, not be considered as adding to the total irrigation potential created since this area would have already counted earlier once.

Irrigation Potential Utilized - (As per Planning Commission)

a. The irrigation potential utilized is the total gross area actually irrigated by a project during the year under consideration. The figures relating to the stabilization of 'old area' should be furnished separately in this case also since these will not be additive to the gross area irrigated.

b. As, generally, the utilzation of irrigation potential created can take place only in the year following the creation of such potential, it will be appropriate if the irrigation potential utilized in a particular year is considered with the potential created upto the end of the preceding year for the purpose of comparison.

Irrigation Return Flow - It is the leakage or seepage or both of water from irrigation works, namely, canals and dams or regenerated flow from fields which could be used for irrigation areas downstream.

Irrigation System - It includes storage and diversion structure, main canal, distributory, minors, water courses, field channels, and allied structures including head regulator, cross drainage works and control structures.

Irrigation Water Requirement - The amount of crop water requirement that is not provided by effective rainfall, utilization of stored soil moisture or upward flow of water to the root zone from a saturated zone.

Irrigation Works - The works related to storage, diversion, conveyance and delivery of irrigation supplies to the project command.

Lift Irrigated Area - That area where the level is too high to allow irrigation by gravity flow, but which can be irrigated by lifting water to the necessary level at some point in the supply system.

Lift Irrigation - It is the method of irrigation in which the water is lifted with mechanical or manual means.

Main Canal - Principal canal for the conveyance of water supplied to the branch canal/distributary.

Management Allowed Deficit ( MAD ) - It is the desired soil moisture deficit at the time of irrigation.

Net Irrigated Area – The net irrigated area is the area irrigated during the year counting the area only once, even if two or more crops are irrigated in different seasons on the same piece of land.

Net Irrigation Ratio – The net irrigation ratio is the ratio of net area irrigated to the net area sown in a year expressed in percentage.

Net Water Requirement - The consumptive use requirements of crops minus the effective rainfall.

Nominal Duty or Normal Duty - The duty sanctioned as per the schedule of an irrigation department.

Non-beneficial Consumptive Use - The water consumed by native non-crop vegetation, evaporated from bare and ideal land surfaces and from water surfaces.

Non-perennial Area - The area which does not receive perennial irrigation.

Outlet Command Area - The area, in irrigation practice, for distribution of water from an outlet. It is the area that can be served by an individual outlet.

Outlet or Turnout - A structure that supplies water to a block in which different farmers use the flow in rotation.

Overall Irrigation Efficiency of the System - It is the ratio of the average depth of irrigation water which is beneficially used to the average depth of irrigation water supplied from the headwork.

Panchnama - A written statement executed by canal authority in presence of and witnessed by the irrigators or members of water committee of the concerned canal system against an offender found misusing, wasting or taking water unauthorizedly.

Peak Period Consumptive Use - It is the average daily water used during the period of highest consumptive use.

Perennial Irrigated Area - The area served by a perennial canal.

Reservoir Storage Efficiency ( Es ) - It is the ratio of the volume of water released from the reservoir for irrigation, to the volume of water received in the storage reservoir (surface or underground) for irrigation.

Ring/Basin Method of Irrigation - It consists of applying water in level basins either of rectangular or circular shape, generally made around each tree or group of trees.

Rostering of Channels - It is the sequencing of water delivery in different channels as a part of regulation.

Rotational Distribution Water Supply - It is a time table of water supply to individual fields from a particular outlet during one rotation.

Pre-Sowing Irrigation - Water application to a field before sowing of a crop to provide the required moisture in the soil for germination of the seed.

Sprinkler Irrigation - The method of applying water over the land by spraying it under pressure. This is often done by rotating sprinkler heads with one or more nozzles or by using perforated pipes.

Sub-Surface Irrigation - This is the method of applying water to crops below the ground surface through porous tiles or similar other material. This can also be done through low level open ditches. It is generally applicable to layered soil.

Surface Irrigation - Method of irrigation where the water flows on to the field surface by gravity from the head to the tail end.

Surface Irrigation Method - It is the application of water by surface method such as wild flooding, border strip, check basis, and furrows for raising crops.

Ultimate Irrigation Potential -(As per Planning Commission)

i. It is the gross area that can be irrigated from a project in a design year for the projected cropping pattern and assumed water allowance on its full development. The gross irrigated area will be aggregate of the areas irrigated in different crop seasons, the areas under two-seasonal and perennial crops being counted only once in the year.

ii. The following considerations have to be taken into account in estimating the ultimate irrigation potential expected from a project in terms of area:

a. It will not be correct to assume the culturable command area as an arbitrary percentage of the gross command area. The CCA should be assessed from actual and by consulting land records.

b. A part of the area being proposed to be brought under irrigation from a project may be already receiving irrigation from other sources, whether major, medium or minor irrigation works, which might have been commissioned earlier. The benefits from the new project may be by way of an additional water allowance to irrigation more secure or to stabilize irrigation the area. Such area should not be counted in new irrigation potential but considered only as stabilize of irrigation in an old area. The Ultimate irrigation potential should indicate only figures of gross irrigation of new area whether in the new command area or in the existing command (by increasing the intensity of cropping). The old area stabilized may be reported separately.

Unit Irrigation Efficiency (Eu) - It is the ratio of the volume of irrigation water used in evapotranspiration in the specified irrigated area, plus that necessary to maintain a favourable sal concentration in the soil solution, to the volume of water delivered to the area.

Utilization Ratio - The utilization ratio is the ratio of the actual irrigated area to the irrigation potential.

Water Conveyance Efficiency (EC) - It is the ratio of the volume of water delivered by an open or closed conveyance system to the volume of water delivered to the conveyance system at the supply source.

Well Irrigated Crop - A crop which is raised on by well irrigation.

Wild Flooding - It is a method of irrigation by uncontrolled flooding of the area.

Water Use Efficiency ( WUE ) - It is defined as the marketable crop produced per unit of water consumed in evapotranspiration.

Wilting Point (WP) - It is the moisture content of the soil below which plants can no longer extract moisture at a rate sufficient for its growth.


1. IS 4410-1 (1991): Glossary of terms relating to river valley projects, Part 1: Irrigation practice [WRD 6: Water Resources Planning, Management and Evaluation]

2. IS 4410-11-2 (1972): Glossary of Terms Relating to River Valley Projects, Part XI: Hydrology, Section 2: Precipitation and Run off [WRD 3: Ground Water and Related Investigations]

3. IS 4410-12 (1993): Glossary of terms relating to river valley projects, Part 12: Diversion works [WRD 22: River Training and Diversion Works]

4. IS 4410-8 (1992): Glossary of terms relating to river valley projects, Part 8: Dams and dam section [WRD 9: Dams and Spillways]

Water Resources

  • Note: This page content is in under progress.
QR Code
QR Code glossary_of_terms (generated for current page)