Irrigation EfciencyTerry A. HowellUnited States Department of Agriculture (USDA), Bushland, Texas, U.S.A.INTRODUCTIONIrrigation efciency is a critical measure of irrigationperformanceintermsof thewater requiredtoirrigateaeld, farm, basin, irrigation district, or an entire watershed.The value of irrigationefciencyandits denitionareimportant to the societal views of irrigated agriculture andits benet insupplyingthehighquality, abundant foodsupplyrequiredtomeetourgrowingworldspopulation.Irrigationefciencyisa basic engineeringtermusedinirrigationsciencetocharacterizeirrigationperformance,evaluate irrigationwater use, andtopromote better orimproved use of water resources, particularly those used inagriculture and turf/landscape management.[14]Irrigationefciencyisdenedintermsof:1)theirrigationsystemperformance, 2)theuniformityof thewater application,and 3) the response of the crop to irrigation. Each of theseirrigation efciency measures is interrelated and will varywithscaleandtime.Fig.1illustrates severalofthewatertransport components involved in dening variousirrigation performance measures. The spatial scale canvaryfromasingleirrigationapplicationdevice(asiphontube, a gated pipe gate, a sprinkler, a microirrigationemitter) toanirrigationset (basinplot, afurrowset, asingle sprinkler lateral, or a microirrigation lateral) tobroader land scales (eld, farm, an irrigation canal lateral,awholeirrigationdistrict, abasinor watershed, ariversystem, or anaquifer). The timescale canvaryfromasingleapplication(or irrigationset), a part of thecropseason (preplanting, emergence to bloom or pollination, orreproductiontomaturity),theirrigationseason,toacropseason, or a year, partial year (premonsoon season,summer, etc.), or a water year (typically from thebeginning of spring snow melt through the end ofirrigationdiversion, orarainyormonsoonseason), oraperiodof years (adrought or awetcycle). Irrigationefciencyaffectstheeconomicsofirrigation,theamountof water needed to irrigate a specic land area, the spatialuniformityofthecropanditsyield,theamountofwaterthat might percolate beneath the crop root zone, theamount of water that canreturntosurface sources fordownstreamuses or togroundwater aquifers that mightsupplyotherwateruses, andtheamountofwaterlosttounrecoverablesources(saltsink,salineaquifer,ocean,orunsaturatedvadosezone).The volumes of the water for the various irrigationcomponentsaretypicallygiveninunitsofdepth(volumeper unit area) or simplythevolumefor theareabeingevaluated. Irrigation water application volume is difcult tomeasure, so it is usually computed as the product of waterowrateandtime. This places emphasis onaccuratelymeasuringtheowrate.Itremainsdifculttoaccuratelymeasure water percolation volumes groundwater owvolumes, and water uptake from shallow groundwater.IRRIGATION SYSTEM PERFORMANCEEFFICIENCYIrrigationwatercanbedivertedfromastoragereservoirandtransportedtotheeldorfarmthroughasystemofcanalsorpipelines;itcanbepumpedfromareservoironthefarmandtransportedthroughasystemoffarmcanalsor pipelines; or it might be pumped from a single well or aseries of wells through farm canals or pipelines. Irrigationdistricts often include small to moderate size reservoirs toregulate ow and to provide short-term storage to managethedivertedwater withtheon-farmdemand. Someon-farmsystemsincludereservoirsforstorageorregulationofowsfrommultiplewells.WaterConveyance EfciencyThe conveyance efciency is typically dened as the ratiobetweenthewater that reaches afarmor eldandthatdiverted from the irrigation water source.[1,3,4]It is denedasEc 100VfVt1The use of trade, rm, or corporation names in this publication is for theinformation and convenience of the reader.Such usedoes not constituteanofcialendorsementorapprovalbytheUnitedStatesDepartmentofAgriculture or the Agricultural Research Service of any product or serviceto the exclusion of others that may be suitable. This work was done by aU.S. Government employee and cannot be copyrighted. It may bereproducedfreelyfor anynoncommercialuse.This articlepreviouslypublishedintheEncyclopediaof Soil Science2002MarcelDekker, Inc.Encyclopedia ofWater ScienceDOI:10.1081/E-EWS120010252Published2003byMarcelDekker, Inc.Allrightsreserved.467This paperwas prepared by a USDA employee as part of the employee's official duties and cannot legally be copyrighted. The fact that the private publication in which the paper may appear is itself copyrighted does not affect the material of the U.S. Government, which can be reproduced by the public at will.where Ecis the conveyance efciency (%), Vfis thevolume of water that reaches the farm or eld (m3), and Vtisthevolumeofwaterdiverted(m3)fromthesource.Ecalsoappliestosegmentsofcanalsorpipelines,wherethewaterlossesincludecanal seepageorleaksinpipelines.The global Eccan be computed as the product of theindividualcomponentefciencies,Eci,whereirepresentsthe segment number. Conveyance losses include any canalspills (operational or accidental) and reservoir seepage andevaporation that might result from management as well aslosses resulting from the physical conguration orconditionoftheirrigationsystem.Typically,conveyancelossesaremuchlowerforclosedconduitsorpipelines[4]compared with unlined or lined canals. Even theconveyanceefciencyof linedcanals maydeclineovertimeduetomaterialdeteriorationorpoormaintenance.Application EfciencyApplication efciency relates to the actual storage of waterin the root zone to meet the crop water needs in relation tothe water applied to the eld. It might be dened forindividual irrigation or parts of irrigations (irrigation sets).Fig. 1 Illustrationof thevarious water transport componentsneededtocharacterizeirrigationefciency.Table 1 ExampleoffarmandeldirrigationapplicationefciencyandattainableefcienciesFieldefciency(%) Farmefciency(%)Irrigationmethod Attainable Range Average Attainable Range AverageSurfaceGradedfurrow 75 5080 65 70 4070 65w/tailwaterreuse 85 6090 75 85 Levelfurrow 85 6595 80 85 Gradedborder 80 5080 65 75 Levelbasins 90 8095 85 80 SprinklerPeriodicmove 80 6085 75 80 6090 80Sideroll 80 6085 75 80 6085 80Movingbiggun 75 5575 65 80 6080 70CenterpivotImpactheadsw/endgun 85 7590 80 85 7590 80Sprayheadswo/endgun 95 7595 90 85 7595 90LEPAawo/endgun 98 8098 95 95 8098 92LateralmoveSprayheadsw/hosefeed 95 7595 90 85 8098 90Sprayheadsw/canalfeed 90 7095 85 90 7595 85MicroirrigationTrickle 95 7095 85 95 7595 85Subsurfacedrip 95 7595 90 95 7595 90Microspray 95 7095 85 95 7095 85WatertablecontrolSurfaceditch 80 5080 65 80 5080 60Subsurfacedrainlines 85 6080 75 85 6585 70aLEPAislowenergyprecision application.(From Refs.6,7,11.)Irrigation Efciency 468Applicationefciencyincludesanyapplicationlossestoevaporationor seepage fromsurface water channels orfurrows, any leaks from sprinkler or drip pipelines,percolationbeneaththeroot zone, drift fromsprinklers,evaporation of droplets in the air, or runoff from the eld.ApplicationefciencyisdenedasEa 100VsVf2where Eais the application efciency (%), Vsis theirrigationneededbythecrop(m3), andVfis thewaterdelivered to the eld or farm (m3). The root zone may notneedtobefullyrelled, particularlyif someroot zonewater-holding capacity is needed to store possible or likelyrainfall. Often, Vs is characterized as thevolume ofwaterstoredinthe root zone fromthe irrigationapplication.Someirrigations maybeappliedfor reasons other thanmeetingthe cropwater requirement (germination, frostcontrol, cropcooling, chemigation, fertigation, or weedgermination). The crop need is often based on thebenecial water needs.[5]In some surface irrigationsystems, the runoff water that is necessary to achieve gooduniformity across the eld can be recovered in a tailwaterpit and recirculated with the current irrigation or used forlaterirrigations,andVfshouldbeadjustedtoaccountforthe net recovered tailwater. Efciency values aretypicallysitespecic.Table1providesarangeoftypicalfarmandeldirrigationapplicationefciencies[68]andpotential orattainableefcienciesfordifferent irrigationmethodsthat assumesirrigationsareappliedtomeet thecropneed.Storage EfciencySincethecroprootzonemaynotneedtoberelledwitheachirrigation,thestorageefciencyhasbeendened.[4]ThestorageefciencyisgivenasEs 100VsVrz3whereEsisthestorageefciency(%)andVrzistherootzonestoragecapacity(m3). Theroot zonedepthandthewater-holding capacity of the root zone determine Vrz. Thestorage efciency has little utility for sprinkler ormicroirrigationbecausetheseirrigationmethodsseldomrell the root zone, while it is more often applied to surfaceirrigationmethods.[4]Seasonal Irrigation EfciencyTheseasonalirrigationefciencyisdenedasEi 100VbVf4where Ei is the seasonal irrigation efciency (%) and Vb isthe water volume benecially used by the crop (m3). Vb issomewhat subjective,[4,5]but it basically includes therequiredcropevapotranspiration(ETc)plusanyrequiredleachingwater (Vl) for salinitymanagement of thecroprootzone.Leaching requirement (or the leaching fraction)The leaching requirement,[9]also called the leachingfraction,isdenedasLr VdVfECiECd5whereLristheleachingrequirement,Vdisthevolumeofdrainagewater (m3), Vfisthevolumeof irrigation(m3)applied to the farm or eld, ECiis the electricalconductivityoftheirrigationwater(dS m21),andECdisthe electrical conductivity of the drainage water (dS m21).TheLrisrelatedtotheirrigationapplicationefciency,particularlywhendrainageistheprimaryirrigationlosscomponent. The Lrwould be required benecialirrigation use Vl; LrVi; soonly Vdgreater than theminimumrequiredleachingshouldreduceirrigationef-ciency. Then, theirrigationefciencycanbedeterminedbycombiningEqs.(4)and(5)Ei 100VbVf Lr_ _6Burtetal.[5]denedthebenecialwaterusetoincludepossibleoff-siteneedstobenet society(riparianneedsorwildlifeorsheryneeds). Theyalsoindicatedthat Vfshould not include the change in the eld or farmstorage of water, principally soil water but it couldinclude eld (tailwater pits) or farm water storage(areservoir)that wasnt usedwithinthetimeframethatwasusedtodeneEi.IRRIGATION UNIFORMITYThefractionofwaterusedefcientlyandbeneciallyisimportant for improved irrigation practice. The uniformityof the applied water signicantly affects irrigationefciency. Theuniformityisastatistical propertyoftheIrrigation Efciency 469appliedwatersdistribution.Thisdistributiondependsonmanyfactorsthatarerelatedtothemethodofirrigation,soil topography, soil hydraulic or inltrationcharacter-istics, andhydrauliccharacteristics (pressure, owrate,etc.) of the irrigation system. Irrigation applicationdistributions are usually based on depths of water (volumeperunitarea);however, formicroirrigationsystemstheyareusuallybasedonemitter owvolumes becausetheentirelandareaisnottypicallywetted.Christiansens Uniformity CoefcientChristiansen[10]proposed a coefcient intended mainly forsprinklersystembasedonthecatchvolumesgivenasCU 1001 2

jX 2xj

X_ _7whereCUistheChristiansensuniformitycoefcient inpercent, X is the depth (or volume) of water in each of theequallyspaced catch containers in mm or ml, and xis themeandepth(volume) of thecatch(mmor ml). For CUvalues . 70%, Hart[11]and Keller and Bliesner[8]presentedCU 100 1 2sx_ _2p_ _0:5_ _8where s is the standard deviation of the catch depth (mm)or volume (ml). Eq. 8 approximates the normaldistributionforthecatchamounts.The CU should be weighted by the area represented bythe container[12]when the sprinkler catch containersintentionallyrepresent unequal landareas, asisthecasefor catch containers beneath a center pivot. Heermann andHein[12]revised the CUformula (Eq. 8) to reect theweighted area, particularly intended for a center pivotsprinkler,asfollows:CUH&H 100 1 2

SiVi2

ViSi

Si_ _ _ _

ViSi__________9where Siis the distance (m) fromthe pivot tothe ithequally spaced catch container and Vi is the volume of thecatchintheithcontainer(mmorml).Low-Quarter Distribution UniformityThe distribution uniformity represents the spatial evennessof theappliedwater acrossaeldor afarmaswell aswithin a eld or farm. The general form of the distributionuniformitycanbegivenasDUp 100

Vp

Vf__10where DUpis the distribution uniformity (%) for the lowestpfractionoftheeldorfarm(lowest one-halfp 1=2;lowest one-quarter p 1=4), Vpisthemeanapplicationvolume (m3), and Vf is the mean application volume (m3)for the whole eld or farm. When p 1=2 and CU. 70%;thentheDUandCUareessentiallyequal.[13]TheUSDA-NRCS (formerly, the Soil Conservation Service) haswidely used DUlq p 1=4 for surface irrigation to accessthe uniformityappliedtoa eld, i.e., bythe irrigationvolume (amount) received by the lowest one-quarter of theeld fromapplicationsfor the whole eld. Typically, DUpis based on the postirrigation measurement[5]of watervolumethatinltratesthesoilbecauseitcanmoreeasilybemeasuredandbetterrepresentsthewateravailabletothe crop. However, the postirrigation inltrated waterignoresanywaterinterceptedbythecropandevaporatedand any soil water evaporation that occurs before themeasurement.Anywaterthatpercolatesbeneaththerootzoneorthesamplingdepthwillalsobeignored.The DUand CUcoefcients are mathematicallyinterrelated through the statistical variation (coefcientof variation, s/x, Cv) and the type of distribution.Warrick[13]presentedrelationships betweenDUandCUfor normal, log-normal, uniform, specialized power, beta-andgamma-distributionsofappliedirrigations.Emission UniformityFor microirrigation systems, both the CU and DU conceptsare impractical because the entire soil surface is notwetted.KellerandKarmeli[14]developedanequationformicroirrigationdesignasfollowsEU 1001 21:27Cvmn21=2

qm q_ _11where EU is the design emission uniformity (%), Cvm is themanufacturers coefcient of variability in emissiondeviceowrate(1/h), nis thenumber of emitters perplant, qm is the minimum emission device ow rate (1/h) atthe minimum system pressure, and qis the mean emissiondeviceowrate(1/h).ThisequationisbasedontheDUlqconcept,[4]and includes the inuence of multiple emittersper plant that each may have a ow rate from a populationof randomow rates based on the emission devicemanufacturing variation. Nakayama, Bucks, and Clem-mens[15]developed a design coefcient based morecloselyontheCUconceptforemissiondeviceowratesIrrigation Efciency 470fromanormaldistributiongivenasCUd 1001 20:798Cvmn21=2 12where CUdis the coefcient of design uniformity inpercentandthenumericalvalue,0.798,is2p_ _0:5fromEq.8.Many additional factors affect microirrigation uni-formityincludinghydraulicfactors, topographicfactors,andemitterpluggingorclogging.WATER USE EFFICIENCYThe previous sections discussed the engineering aspects ofirrigation efciency. Irrigation efciency is clearlyinuencedbytheamountofwaterusedinrelationtotheirrigationwaterappliedtothecropandtheuniformityofthe applied water. These efciency factors impactirrigationcosts, irrigationdesign, andmoreimportant,insomecases, thecropproductivity. Water useefciency(WUE) has been the most widely used parameter todescribeirrigationeffectiveness interms of cropyield.Viets[16]denedWUEasWUE YgET13whereWUEiswateruseefciency(kg m23), Ygistheeconomicyield(g m22), andETisthecropwater use(mm). Water useefciencyis usuallyexpressedbytheeconomicyield, but it hasbeenhistoricallyexpressedaswell interms of thecropdrymatter yield(either totalbiomass or aboveground drymatter). These twoWUEbases (economicyieldor drymatter yield) haveledtosome inconsistencies in the use of the WUE concept. Thetranspirationratio(transpirationperunit drymatter)isamore consistent value that depends primarily on cropspecies and the environmental evaporative demand,[17]andit issimplytheinverseofWUEexpressedonadrymatterbasis.Irrigation Water Use EfciencyThe previous discussion of WUE does not explicitlyexplainthecropyieldresponsetoirrigation. Water useefciency is inuenced by the crop water use (ET). Bos[3]denedatermforWUEtocharacterizetheinuenceofirrigationonWUEasWUE Ygi2YgdETi2ETd14whereWUEisirrigationwateruseefciency(kg m23),Ygi is the economic yield (g m22) for irrigation level i, Ygdis the dryland yield (g m22; actually, the crop yieldwithout irrigation), ETi is the evapotranspiration (mm) forirrigationleveli, andETdistheevapotranspirationofthedrylandcrops(oroftheETwithoutirrigation).AlthoughEq. 14 seems easy to use, both Ygd and ETd are difcult toevaluate. If the purpose is to compare irrigation and drylandproductionsystems,thendrylandratherthannonirrigatedconditionsshouldbeused. If thepurposeistocompareirrigated regimes with an unirrigated regime, thenappropriate values for Ygd and ETd should be used. Often,in most semiarid to arid locations, Ygd may be zero. Bos[3]dened irrigation WUE asIWUE Ygi2YgdIRRi15where IWUEis the irrigation efciency (kg m23) and IRRiis the irrigation water applied (mm) for irrigation level i. InEq. 15, Ygd may be often zero in many arid situations.CONCLUSIONIrrigation efciency is an important engineering term thatinvolves understanding soil and agronomic sciences toachieve the greatest benet from irrigation. The enhancedunderstanding of irrigation efciency can improve thebenecial use of limitedanddecliningwater resourcesneeded to enhance crop and food production from irrigatedlands.REFERENCES1. Israelsen, O.R.; Hansen, V.E. Irrigation Principles andPractices,3rdEd.;Wiley:NewYork,1962;447.2. ASCE; DescribingIrrigationEfciency andUniformity.J.Irrig.Drain.Div.,ASCE1978,104(IRI),3541.3. Bos, M.G. Standards for IrrigationEfciencies of ICID.J.Irrig.Drain.Div.,ASCE1979,105(IRI),3743.4. Heermann, D.F.; Wallender, W.W.; Bos, M.G. IrrigationEfciency and Uniformity. In Management of FarmIrrigation Systems; Hoffman, G.J., Howell, T.A., Solomon,K.H.,Eds.;Am.Soc.Agric.Engrs.:St.Joseph,MI,1990;125149.5. Burt, C.M.; Clemmens, A.J.; Strelkoff, T.S.; Solomon,K.H.; Bliesner, R.D.; Hardy, L.A.; Howell, T.A.;Eisenhauer, D.E. IrrigationPerformance Measures: Ef-Irrigation Efciency 471ciencyandUniformity.J.Irrig.Drain.Eng. 1997,123(3),423442.6. Howell, T.A. Irrigation Efciencies. In Handbook ofEngineering in Agriculture; Brown, R.H., Ed.; CRC Press:BocaRaton,FL,1988;Vol.I,173184.7. Merriam, J.L.; Keller, J. FarmIrrigationSystemEvalu-ation: A Guide for Management; Utah StateUniv.: Logan,UT,1978;271.8. Keller, J.; Bliesner, R.D. SprinkleandTrickleIrrigation;TheBlackburnPress:Caldwell,NJ,2000;652.9. U.S. Salinity Laboratory Staff. Diagnosis and Improvementof Saline and Alkali Soils; Handbook 60; U.S. Govt.PrintingOfce:Washington,DC,1954;160.10. Christiansen, J.E. Irrigation by Sprinkling; CaliforniaAgric. Exp.Bull. No. 570;Univ.ofCalif.:Berkeley, CA,1942;94.11. Hart,W.E.OverheadIrrigationbySprinkling.Agric.Eng.1961,42(7),354355.12. Heermann, D.F.; Hein, P.R. Performance Characteristics ofSelf-Propelled Center-Pivot Sprinkler Machines. Trans.ASAE1968,11(1),1115.13. Warrick, A.W. InterrelationshipsofIrrigationUniformityTerms. J. Irrig. Drain. Eng., ASCE 1983, 109 (3), 317332.14. Keller,J.;Karmeli,D.TrickleIrrigationDesign;RainbirdSprinklerManufacturing:Glendora,CA,1975;133.15. Nakayama, F.S.; Bucks, D.A.; Clemmens, A.J. AssessingTrickle Emitter Application Uniformity. Trans. ASAE1979,22(4),816821.16. Viets, F.G. Fertilizers and the Efcient Use of Water. Adv.Agron.1962,14,223264.17. Tanner, C.B.; Sinclair, T.R. Efcient Use of Water in CropProduction: Research or Re-Search? In Limitations toEfcient Water Use in Crop Production; Taylor, H.M.,Jordan,W.R.,Sinclair,T.R.,Eds.;Am.Soc.Agron.,CropSci. Soc. Am., Soil Sci. Soc. Am.: Madison, WI, 1983;127.Irrigation Efciency 472