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close this bookIrrigation Reference Manual (Peace Corps, 1994)
View the document(introduction...)
Open this folder and view contentsChapter 1 - Introduction
Open this folder and view contentsChapter 2 - Physical and biological resource base
Open this folder and view contentsChapter 3 - Developing water sources
Open this folder and view contentsChapter 4 - Estimating irrigation requirements
Open this folder and view contentsChapter 5 - Farm water delivery systems
Open this folder and view contentsChapter 6 - Farm water management
Open this folder and view contentsChapter 7 - Waterlogging and salinity
Open this folder and view contentsAppendix A - Math skills and tool use
Open this folder and view contentsAppendix B - Community organization and development
View the documentAppendix C - Summary of international irrigation center (IIC) training modules
View the documentAppendix D - Case studies
View the documentAppendix E - Annotated bibliography
View the documentAppendix F - Glossary of terms

Appendix F - Glossary of terms


ALKALINE: pH greater than 7.


AW: for given soil and climate, depth of soil water in the root zone readily available to the crop allowing unrestricted evapotranspiration; the fraction p of available soil water (see below); mm/m or inches/ft of soil depth.

AMENDMENT: chemicals added to soil or water in order to improve certain soil water properties such as infiltration rate or soil chemistry.

AMORTIZATION: gradual repayment of an amount or debt through regular payments over time. Depreciation is a form of amortization used to estimate the value of an asset over time at a given interest rate. Debt payments are often amortized over time. The capital recovery factor (see below) consists of an interest and an amortization component.

ANAEROBIC: the absence of oxygen.

ANNUAL EQUIVALENT: a series of equal annual amounts for a determined number of years that, when discounted at an appropriate interest rate, will sum to a specific present worth. The annual amount is calculated by multiplying the present value by the capital recovery factor (see below) for the appropriate interest rate and length of time.

AQUIFER: a water-bearing layer (stratum) of permeable rock, sand or gravel.

ARTESIAN WELL: a well that reaches water that, from internal pressure, flows up like a fountain.

ASSET: a business accounting term. Everything an individual or a company owns and that has a monetary value, such as cash, machinery, buildings, and land.

AVAILABLE HEAD: difference between the elevation of an upper water surface and a lower surface, such as a field or water surface.

AVAILABLE SOIL WATER, AW: depth of water stored in the root zone between field capacity and crop wilting point; mm/m or inches/ft of soil depth.

BASIC INTAKE RATE: rate at which water will enter soil when, after the initial wetting of the soil, the rate becomes essentially constant; mm/hr or in/hr.

BENEFIT/COST RATIO (B/C Ratio): selection criterion used in evaluating projects. The present worth of project's benefits is weighed against the present worth of its costs.

BENEFIT: in project analysis, any good or service produced by a project that benefits those for whom the project is being undertaken.

BHP: see Horsepower, Brake Horsepower.

BIT: a piece that operates at the bottom end of the tool string to loosen rock or soil in order to deepen a well being drilled.

BOTTOM SECTION: the part of a well that extends below the water table.

CAPACITY OR DISCHARGE: the rate of flow of liquid per unit time, as gpm or L/sec.

CAPITAL RECOVERY FACTOR: a factor used to calculate the annual value of an amount, machinery, or other asset over its expected life. It is used to calculate the equal installments necessary to repay (amortize) a loan over a period of time. The factor includes compound interest.

CASING: metal or plastic pipe used to keep open the drilled or excavated hole in a well.

CATION EXCHANGE: the interchange between one cation (positive charged ion) in solution and another on any negatively charged surface, such as clay or organic colloids.

CAVITATION: the vaporization of a pumped fluid as it goes through the pump impeller. The formation and collapse of vapor pockets as the liquid goes through the pump.

CEC (CATION EXCHANGE CAPACITY): the sum of all exchangeable cations that a soil can absorb; meq/100 gm.

CENTRIFUGAL PUMP: a pump in which water enters the center of the impeller and proceeds radially outward through the impeller.

CHECK DROP: check structure combined with a drop (see below) for dual-purpose function.

CHECK, CHECK STRUCTURE: structure built or placed across a channel at suitable points to control water levels and regulate water supply. Stop logs and check panels are the moveable sections placed in slots to control depths.

CHLOROSIS: general yellowing of plant tissue (to various degrees) caused by absence of chlorophyll. Can be due to absence of essential nutrient or other damage to plants.

CHUTE: an inclined drop or fall in which the lowering of the water surface is achieved over a relatively short length of channel.

COLLOID: matter having very small particle size and large specific surface (surface area per unit mass of material). In soils, these can be clays or organic matter.

COMPOUNDING: calculating the future value of a present amount that is growing at a given interest rate.

CONSUMPTIVE USE: the total amount of water taken up by vegetation for transpiration or building of plant tissue, plus the unavoidable evaporation of soil moisture, snow and intercepted precipitation associated with vegetation (also see evapotranspiration).

CONVEYANCE EFFICIENCY, Ec: ratio between irrigation water received at the inlet to a block of fields and that released at the project's headworks; a fraction.

CRITICAL PERIOD: periods during crop growth when soil water stress will have a lasting effect on crop growth and yields.

CROP COEFFICIENT, kc: ratio between crop evapotranspiration (ETcrop) and the reference crop evapotranspiration (ETo) when crop is grown in large fields under optimum growing conditions; ETcrop = kc

ETo; fraction.

CROP EVAPOTRANSPIRATION, ETcrop: rate of evapotranspiration of a disease-free crop growing in a large field (one or more ha) under optimal soil conditions, including sufficient water and fertilizer, and achieving full production potential of that crop under the given growing environment; includes water loss through transpiration of the vegetation and evaporation from the soil surface and wet leaves; mm/day or in/day.

CROP WATER REQUIREMENTS: depth of water required by a crop or a group of crops for evapotranspiration (ETcrop) during a given period.

CROPPING INTENSITY: total cultivated area on a farm divided by total cropland. With multiple cropping, this value can be greater than 1.

DEEP PERCOLATION: the drainage of soil water by gravity below the maximum effective depth of the root zone.

DEFLOCCULATE: to disperse particles, such as clay particles in soil, by chemical or physical means.

DEPRECIATION: the reduction in value of an asset through wear and tear over time. Because actual depreciation cannot be measured until the end of the life of an asset, estimates are made using various accepted methods, including the "straight line method" and the "reducing-balance method."

DEPTH OF IRRIGATION, d: depth of irrigation, including application losses, applied to the soil in one irrigation application and that is needed to bring the soil water content of the root zone to field capacity; mm or inches.

DESILTING BOXES, SAND TRAPS: structures that reduce flow velocities so that sand and silt can settle and be removed.

DEVELOPMENT STAGE: for a given crop, the period between end of initial (emergence) stage and full ground cover, or when the ground cover is between 10% and 80%; days.

DISCOUNTING: the process of finding the present worth of a future amount.

DISTRIBUTION EFFICIENCY, Ed: ratio of water made directly available to the crop and that released at the inlet of a block of fields; Ed = Eb

Ea; fraction.

DRAINAGE STRUCTURES: structures used for removing excess water away from irrigated areas into a drainage system.

DRAWDOWN: the elevation difference between the static water level and the pumping level of a liquid.

DROP PIPE: that section of pipe in a deep well pump assembly that extends between the pump cylinder and the pump body.

DROP STRUCTURE: a structure designed to lower the water surface in a channel in a short distance with safe dissipation of energy.

DROP: a farm structure built to mitigate excess grade when the slope of a ditch is greater than the grade that should be used for the ditch. Erosive velocities are reduced upstream.

EFFECTIVE FULL GROUND COVER: percentage of ground cover (specific to crop) when ETcrop is approaching maximum generally 70 to 80% of surface area; percentage.

EFFECTIVE RAINFALL, or EFFECTIVE PRECIPITATION, Pe: rainfall useful for meeting crop water requirements; it excludes deep percolation, surface runoff, and interception; mm/period or in/period.

EFFECTIVE ROOTING DEPTH, D: soil depth from which the full grown crop extracts most of the water needed for evapotranspiration; m or ft.

EFFICIENCY, PUMP EFFICIENCY: the ratio in a pumping plant between power output (Water Horsepower -- WHP) and power input (Brake Horsepower -- BHP); percent.


EFFICIENCY, OVERALL PUMPING PLANT = output Water Horsepower (WHP)/Input Horsepower to motor.

EFFICIENCY, TRANSMISSION: efficiency of the gearhead, belt drivers, and other components of the pump.

ELECTRICAL CONDUCTIVITY, EC: the property of a substance to transfer an electric charge (reciprocal of resistance); used as a measure of the level of salinity.

ELECTRICAL CONDUCTIVITY, IRRIGATION WATER, ECw: is used as a measure of the salt content of irrigation water; mmhos/cm or dS/m.

ELECTRICAL CONDUCTIVITY, SATURATION EXTRACT, ECe : is used as a measure of the salt content of an extract from soil that has been saturated with water; mmhos/cm or dS/m.

EVAPORATION, E: rate of water loss from liquid to vapor phase from open water or wet soil surface by physical processes; mm/day or in/day.

EVAPOTRANSPIRATION, ET: rate of water loss through transpiration from vegetation plus evaporation from the soil; mm/day or in/day.

EXCHANGEABLE SODIUM PERCENTAGE, ESP: the percent of the total Cation Exchange Capacity (CEC) of a soil occupied by sodium, i.e. the percent of the soil exchange sites occupied by exchangeable sodium.

EXTRA-TERRESTRIAL RADIATION, Ra: amount of solar radiation received on a horizontal at the top of the atmosphere; equivalent evaporation mm/day.

FARM GATE: the boundary of a farm; used in economics to delineate a boundary such as farm-gate price.

FIELD APPLICATION EFFICIENCY, Ea: ratio of water made directly available to the crop and that received at the field inlet.

FIELD CAPACITY, S: depth of water held in the soil after ample irrigation or heavy rain when the rate of downward movement has substantially decreased, usually 1 to 3 days after irrigation or rain.

FIELD SUPPLY SCHEDULE: stream size, duration, and interval of water supply to the individual field or farm.

FLOW RATE: the amount of water per unit time flowing past a point; L/sec or ft/sec.

FULL GROUND COVER: soil covered by crops approaching 100% when looking downwards from above.

GLEY: soil developed under conditions of poor drainage resulting in the reduction of metal elements and in a grey color with mottles at interfaces with better aerated soils.

GROUND COVER: percentage of soil surface shaded by the crop if the sun were directly overhead; percentage.

GROUND WATER TABLE: upper boundary of ground water where water pressure is equal to atmosphere, i.e. depth of water level in borehole when ground water can freely enter the borehole.

GROWING SEASON: for a given crop the time between planting or sowing and harvest; days.

HARDPAN: hardened soil layer caused by the cementing of soil particles due to physical processes such as compaction or chemical processes, for example, by sodium. The hardness does not change appreciably with changes in moisture content.

HEAD: the height of a liquid column above a specific point or the equivalent height for a given pressure.

HEAD, AVAILABLE HEAD: difference between the elevation of an upper water surface and a lower surface, such as a field or water surface.

HEAD, DISCHARGE HEAD: the head at the discharge of the pump. The pressure reading of a pressure gauge is converted to elevation of the liquid and velocity head (see below) at the point of gauge attachment.

HEAD, ELEVATION HEAD: the difference in elevation between two points in the system.

HEAD, FRICTION HEAD: the energy losses due to friction (resistance to water flow) between two points in the distribution system.

HEAD, HYDRAULIC HEAD: depth of water as referenced to a lower elevation. Height that water will stand in a tube. Energy available.

HEAD, LOSS: energy lost as a result of friction, impact or turbulence. Simply, the difference in head of two water surfaces connected by pipes or channels.

HEAD, NET POSITIVE SUCTION HEAD REQUIRED: the net positive suction head required to prevent cavitation.

HEAD, NET POSITIVE SUCTION HEAD: the total head at the suction flange of the pump less the vapor pressure of the liquid in the same units.

HEAD, PRESSURE: the pressure at a point expressed as an equivalent head of water, e.g. 10 psi = 23.1 ft of water or 1 kg/cm = 10 meters of water.

HEAD, STATIC: the elevation difference between a reference point on the system and the highest point on the system. The total static head is the difference between the pumping level (free water surface) and the highest point in the system.

HEAD, TOTAL DYNAMIC HEAD (TDH): the total head (energy) supplied by the pump to the liquid. It is the total discharge head at the discharge flange (including velocity head).

HEAD, VELOCITY: the kinetic energy of the flowing liquid in a pipeline.

HEADGATE: structure at the head of a watercourse, farm lateral, or field lateral that connects with the distributing channel. The turnout may be placed through the banks of the tertiary and quaternary canals for water delivery to fields.

HORSEPOWER, BRAKE HORSEPOWER: power required to drive a specific mechanical component.

HORSEPOWER, INPUT HORSEPOWER: the horsepower supplied to the prime mover (the power unit) of the pumping plant (may be electrical or other type of fuel).

HORSEPOWER, WATER HORSEPOWER: the horsepower that the pump imparts to the liquid.

HORSEPOWER: energy per unit time; 1 HP = 550 ft lb/see or 1 HP = 0.746 kw.

HYDRAULIC GRADE LINE: in an open channel, the water surface is the hydraulic grade line; in a closed pipe, the line joining the elevations to which water would stand in open gage tubes.

HYDRAULIC GRADIENT: slope of the hydraulic grade line.

HYDRAULIC HEAD: depth of water referenced to a lower elevation. Height that water will stand in a tube. Energy available.

HYDRAULIC RADIUS: area of the flowing water divided by the wetted perimeter. For pipes flowing full, this is equal to the diameter divided by four.

IMPELLER: the rotating components of the pump that impart energy to the liquid. Water enters the eye of the impeller and gains energy as it moves radially outward.

INITIAL DEVELOPMENT STAGE: for a given crop, the time between germination and early growth, when ground cover is less than 10%; days.

IRRIGATION EFFICIENCY: the ratio of the volume of water required for a specific beneficial use as compared to the volume of water delivered or actually used for this purpose. Commonly interpreted as the volume of water stored in the soil for evapotranspiration compared to the volume of water delivered for this purpose, but may be defined and used in different ways.

IRRIGATION INTERVAL, i: time between the start of successive field irrigation applications on the same field; days.

IRRIGATION REQUIREMENTS, LR: depth of water required for meeting evapotranspiration minus contribution by effective rainfall, ground water, and stored soil water; depth of water required for normal crop production plus leaching requirement, water losses, and operational wastes; sometimes called gross irrigation requirements. (See Net Irrigation Requirement.)

IRRIGATION SCHEDULING: the process of determining the amount and timing of water application or delivery to a farm or group of farms.

LATE SEASON STAGE: time between the end of the mid-season stage and harvest or maturity; days.

LEACHING REQUIREMENT: the fraction of water entering the soil that must pass through the root zone in order to prevent soil salinity from exceeding a specific value.

LEVEL: (adjective) perfectly horizontal; (noun) a device used to establish a perfectly horizontal line.

MARGINAL ANALYSIS: analysis of the effect of changing one variable upon another variable, other variables held constant. For example, varying the rate of fertilizer has an effect on yield; the additional cost of the fertilizer can be evaluated holding other costs constant. Marginal analysis is an important concept in economic analysis.

MEASURING STRUCTURES: weirs and other structures used to determine depth-discharge relationship.

NET BENEFIT: in project analysis, the amount remaining after all outputs are subtracted from all inputs, for example, the net cash flow.

NET IRRIGATION REQUIREMENT, In: depth of water required for meeting evapotranspiration minus contribution from precipitation, ground water, and stored soil water; does not include operational losses and leaching requirements.

NET PRESENT WORTH: in project analysis, a discounted measure of project worth, or the present worth of a stream of benefits minus the present worth of the stream of costs. Can be used as a selection criterion.

OPPORTUNITY COST: the benefit foregone by using a scarce resource for one purpose instead of for its next best alternative use.

OSMOTIC EFFECT: the force a plant must exert to extract water from the soil. The presence of salt in the soil water increases the force a plant must exert to withdraw water from soil.

PERCUSSION: a method of drilling a well by repeatedly dropping a bit.

PERMEABILITY: a measure of the speed at which water can move through a certain type of rock or soil. For example, sand is more permeable than clay because water moves faster through sand.

pH: a measure of acidity or alkalinity ranging from 1-14. It is the negative logarithm of the hydrogen ion activity.

PRECIPITATION: total amount of precipitation (rain, drizzle, snow, hail, fog, condensation, hoarfrost, frost, and rime), expressed in depth of water, that would cover a horizontal plane if there were no runoff, infiltration, or evapotranspiration.

PRESENT WORTH: the present value of an amount to be paid or received at some future date.

PRIMING: prefilling a structure, such as a suction tube or a centrifugal pump, with water before operation.

PROJECT CYCLE: the series of analytical phases through which a project passes, such as identification, planning, implementation, evaluation, and appraisal.

PROJECT: an investment activity upon which resources (costs) are expended in order to create assets that will produce benefits over an extended period of time.

PUMP: a device used to lift water or to provide pressure to water.

PUMPING LEVEL: the vertical distance from the centerline of the pump discharge to the free water surface from which the water is being drawn.

QUATERNARY CANALS (field laterals): canals branching from the minors and supplying water to outlets or turnouts. (head ditch, (USA); marwa (Egypt); watercourse (India and Pakistan)).

RATE OF RETURN: payment on an investment as a proportion or percentage of that investment.

REFERENCE CROP EVAPOTRANSPIRATION, ETo: rate of evapotranspiration from an extended surface of an 8 to 15 cm tall, green grass cover of uniform height, actively growing, completely shading the ground, and not short of water; mm/day or in/day.

RISK ANALYSIS: an analytical technique in which the probabilities of possible scenarios for all critical elements of a project are computed or evaluated.

ROTARY: a method of drilling a well by rotating a bit in a hole as the well is drilled.

SALINE SOIL: a non-alkali soil containing soluble salts in such quantities that they interfere with the growth of most plants.

SECONDARY CANALS, DISTRIBUTARY CANALS: canals that branch from main canals or branches and supply water to minors, outlets, and turnouts.

SENSITIVITY ANALYSIS: a technique by which a systematic analysis of the impact of different circumstances on the earning capacity of a project is undertaken. For example, uncertainty about future interest rates on loans would involve an analysis of a project using different interest rates.

SIPHON TUBES: pipes shaped in such a manner that they can be lain across a ditch bank and used to siphon water from that ditch into a field or furrow.

SODIC SOIL: an alkali soil containing exchangeable sodium in such quantities that it interferes appreciably with soil infiltration and structure and affects the growth of most plants.

SODIUM ABSORPTION RATIO, SAR: a ratio for soil extracts and irrigation water used to express the relative activity of sodium ions in exchange reaction with the soil; me/l.

SOIL AMENDMENTS: a substance or material that improves soil by modifying its physical properties rather than by adding appreciable quantities of plant nutrients.

SOIL INTAKE (INFILTRATION) RATE: instantaneous rate at which water will enter the soil.

SOIL STRUCTURE: arrangement of soil particles into aggregates that occur in a variety of recognized shapes, sizes, and strengths.

SOIL TEXTURE: characterization of soil in respect to its particle size and distribution.

SOIL WATER CONTENT: depth of water held in the soil; mm/m soil depth or in/ft soil depth.

STATIC LEVEL: the vertical distance from the centerline of the pump discharge flange to the free water surface while no water is being pumped.

STORED SOIL WATER, Wb: depth of water stored in the root zone from earlier rains, snow, or irrigation applications that partly or fully meets crop water requirements in the following periods; mm or in.

SUPPLY SCHEDULE: stream size, supply duration, and supply interval of irrigation water supply to field or irrigation block during the growing season.

TERTIARY CANALS OR FARM LATERALS: canals branching from secondary distributaries and supplying water to sub-minors, outlets or turnouts (laterals, (USA); meska (Egypt); minors (India and Pakistan)).

TRANSPIRATION: rate of water loss through the plant that is regulated by physical and physiological processes.

TURBINE PUMPS: a centrifugal pump designed for installation in a well. The bowls are usually set down in the water. Multistage assemblies may be set down at successive depths.

TURNOUT: structure that releases the water from a head ditch. Can be used to allow the water to pass through the banks of the head ditch onto a field, thus acting as a check gate and a headgate at the same time.

UNIFORMITY: the evenness with which a crop grows, water is applied, or water penetrates into the soil after an irrigation.

VOLUTE CASE: the case of a centrifugal pump in which the high velocity water coming through the impellers is converted to pressure head.

WATER CONTROL STRUCTURES: canals, flow measuring devices, check dams, diversion structures, and any structural methods employed in controlling the amount, direction, depth, or volume of water.

WATER SOURCE: any place where water can be obtained; for example a well, spring, river, lake, reservoir, tap, and faucet.

WATER TABLE: the upper limit of that portion of the ground that is wholly saturated with water.

WATER USER ORGANIZATION: an association of irrigators formed for the purpose of administering and operating an irrigation system.

WATERLOGGING: maintenance of saturated or near-saturated soil conditions in the root zone for an extended period of time.

WELL DEVELOPMENT: the process of re-arranging the soil particles around the intake section of a well to permit easier and better water flow into the well.

WELL: a hole in the ground that reaches below the water table and that is used as a source of water.

WHP: see Horsepower, Water Horsepower.