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 Irrigation water management
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May1988Bulletin249BasicIrrigation SchedulinginFloridaA.G.Smajstrla,B.J.Boman,G.A.Clark,D.Z.Haman, R T.Izuno,and R S. ZazuetaFlorida Cooperative Extension Service / Institute of Food and Agricultural Sciences University of Florida / JohnT.Woeste, Dean !mWRSHY OFFLORIDA LIBRARIES

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Contents Introduction ........ DeterminingWhento Irrigate CropWaterRequirements FieldWaterBalance...Water Budgeting for Irrigation Scheduling Understanding Evapotranspiration Estimating Evapotranspiration SoilWaterStorage Allowable SoilWaterDepletion.TheWaterBudget Soil Moisture Indicators for Irrigation Scheduling IrrigationWaterManagement.Summary. References .11346 678101114151617A.J.SmajstrlaisWaterMgt. Specialist, Agr. Eng. Dept., Gainesville; B.J. Boman is Citrus Irr. Specialist, Agr. Res.andEd. Center, Ft. Pierce; G.A. Clark isWaterMgt. Specialist, Gulf Coast Res.andEd.Center,Bradenton;D.Z.HamanisWater Mgt, Specialist, Agr. Eng. Dept., Gainesville; F.T. Izuno isWaterMgt. Specialist, Everglades Res.andEd. Center, Belle Glade;andF.S. Zazueta isWaterMgt. Specialist, Agr. Eng. Dept., Gainesville: respectively, Institute of FoodandAgricultural Sciences, University of Florida.

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IntroductionProper irrigation scheduling istheapplication ofwaterto crops onlywhenneeded and onlyintheamounts needed;thatis, deter miningwhento irrigateandhow muchwaterto apply. With proper irrigation scheduling, crop yields willnotbelimited bywaterstress from droughts, andthewaste ofwaterandtheenergy usedinpumping will be minimized.Otherbenefits include reduced leaching of nutrients from excesswaterapplications,andreduced pollution of groundwaterorsurface waters fromtheleaching of nutrients. Irrigation is practiced to providewaterwhen rainfall isnotsufficient or timely to meetwaterneeds of a crop.Formost agricultural crops, yield or quality reductions result fromwaterstress. Therefore, ifwateris available and ifitis relatively lowincost, as isthecaseinFlorida, irrigationsarenormally scheduled to avoid plantwaterstress. Despite Florida's relatively large average yearly rainfall of 52-60 inches, irrigation is practiced extensively. Irrigation is necessary because ofthenonuniform distribution of rainfall,thevery limited water-holding capacities of typical sandy soils,andtheextreme sensitivity of many specialty crops towaterstress. These factorsandtheeconomic implications of underor over-irrigation requirethatirrigations be scheduled as efficiently as possible. This publication discusses irrigation scheduling for Florida crops grown on soils wherethewatertable is substantially belowthecrop root zone sothatitdoes not contribute significantly to cropwateruse. Thus, irrigation eventsmustperiodically occur to replenishwaterinthecrop root zone. Water budgeting forwatertablemanagement(also called subirrigation or seepage irrigation) on poorly drained soilsinwhich irrigation occurs aswaterapplications to a highwatertable (immediately belowthecrop root zone), is discussedinIFAS Extension Circular769,"Water Budgeting for High Water Table Soils", available from IFAS County Extension Offices.DeterminingWhentoIrrigateBecausetheobjective of irrigation is to maintain a favorable environment for crops,theplants themselves arethebest indicators1

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oftheneed for irrigation. Instrumentation exists which could allowanirrigator to measure plantwaterstatus and to anticipatewaterstress. However, such instrumentation is expensive, requires specialtrainingfor use,andis generally applicable only for research purposes. Field scale use of such instruments is impractical. Another indicator of plantwaterstress isthevisual appearance oftheplant. Unfortunately, however, yield reduction has already occurred bythetime most agricultural crops show wilt symptoms. Growth processes cease in many crops before visual wilting occurs, and yield reduction may have occurred for some time before wilting is seen. Finally,therearetime lags associated with applying irrigation water. Because several zones might be irrigated from a single pump or other limiting distribution characteristics may exist, many irriga tion systems cannot quickly replenishwaterinthecrop root zone. Many hours or days may be required. Therefore,theneed to irrigatemustbe anticipated because of limitations oftheirrigation system. This problem is compounded in Florida bythelow water holding capacities of most agricultural soilsandbytheshallow root zones of many crops. When to irrigate can also be determined by calendar methods (for example every 5 days), by crop growth stage (for example, every 5 days during early vegetative growth stage,andevery 3 days during peak growth stage), or by similarmethods based on long-term average irrigation requirements. However, these methods fail to considerthetremendous effect of climatic variability on daily cropwateruse. Therefore,theuse of long-term average values may not be adetIuate during periods of hot, dry days, whileitmay result in overirrigation during periods ofcool,overcast days, especially if rainfall isnotconsidered. Day-to-day climatic conditionsarehighly variable during much oftheyearinFlorida because of cloud cover andtherandom nature of rainfall occurrences. Because ofthetheselimitations, irrigations are most often scheduled based onthesoilwaterstatus. Three procedures may be used:1)awaterbalance procedure based ontheestimated cropwateruse rate and soilwaterstorage,2)a direct measurement procedure based on instrumentation to measurethesoilwaterstatus, and3)a combination oftheabove two methods in which soilwaterstatus instrumentation is usedwitha water balance procedure. These procedures require a knowledge ofthecropwaterrequire-2

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ments, effective root-zone, soil water-holding and irriga tion system capabilities in order to schedule irrigations effectively.CropWaterRequirementsWater is used in a cropped field in several ways:1)assimilation intotheplant and plant fruit,2)direct evaporation fromthesoil or other surfaces, 3) transpiration, which is loss ofwatervapor from plant leaves, and4)other uses such as leaching of salts, crop cooling, and freeze protection. Lessthan1%ofthewater used in crop production is assimilated intotheplants. Other uses (category4,above) maybesignificant,butthey depend on factors otherthanmaintaining adequate soilwatercontent, and they will not be considered in this publication. Most ofthewaterapplied to meetthewater requirements of a crop is usedinevaporation and transpiration. Evaporation and transpirationareimportant in cooling a crop, to maintain temperaturesintherangethatpermits photosynthetic activity and crop growthtooccur.Transpirationis also required to transport nutrients into and through plants. The combination of evaporationandtranspiration is called evapotranspiration (ET). Becausetheamount of water assimilated by a plant is very smallwithrespect to ET,ETis often considered to bethecropwaterrequirement--theamount of water required by a growing crop to avoidwaterstress. Deliveringwaterto a crop inthefield results in losses which increasetheamount ofwaterwhichmustbepumpedtosupplythecropwaterrequirement. Losses may occur because of inefficiencies intheconveyance system, evaporation and wind drift (if water is sprayed throughtheair), surface run-off, or percolation belowtheroot zone. These losses can be minimized through good management practices,butthey are impossible to completely eliminate, and theymustbe considered when determiningthetotal (or gross) irrigationwaterrequirement. The total irrigationwaterrequirement is the total amount of irrigationwaterwhich is requiredforcrop production, including ET, all losses incurred in delivering water tothecrop, and other needs such as leaching of salts, crop cooling, and freeze protection. In humid areas such as Florida, a largepartofthecropwaterrequire3

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ment can be provided by rainfall. Effective rainfall, rainfallthatis stored intheroot zone and available for crop use, proportionally reducestheamount of water whichmustbe pumped for irrigation.FieldWaterBalanceThe water balance of a field during and after irrigation is shown in Fig.1.In Florida, runoff losses are normally minimal because ofthehigh infiltration rates ofthesandy soils. Conveyance losses can be minimized by conveying water tothefieldinpipesratherthanopen channel. Application losses, including evaporation and wind drift, can occur during irrigation, especially from sprinkler irrigation systems. These losses are, however, relatively small during periods of low radiatipn, low wind velocities, and high humidities.Also,waterwhich evaporates during application, or. which is intercepted andlaterevaporates from soil, plant, or other surfaces is not entirely lost. Rather, some evaporation during application compensates for ET by reducing ETthatwouldhave occurred iftheintercepted water had not been evaporated. Evaporation and wind drift losses can be minimized by irrigationatnight, early mornings, and late afternoons when climatic condi tions are not severe. However, cultural aspects such as diseasemustbe considered for crops in whichwetfoliage may promote bacterial or other growths which could reduce yields. Deep percolation losses from well-designed irrigation systems can be minimized by good irrigation management.Ifwater is applied uniformly andthewater-holding capacity of a soil is not exceeded, water losses to deep percolation will be minimized.Ifsalinewateris used for irrigation,itmay be necessary to leach excess salts from the crop root zone by addingwaterin excess ofthesoil water holding capacity. However, excess irrigation for leaching should be required only during extended dry periods in Florida because rainfall normally leaches salts.Ifthelosses shown in Fig. 1 are kept to a minimum, most oftheirrigationwaterapplied will evaporate or transpire in proportion to the climatic demand. Unfortunately, rainfall is relatively unpre dictableanditsoccurrence immediately followinganirrigation reducesrainfall effectiveness. Irrigation canbeminimized by4

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THEWATERBALANCEOFA FIELDAPPLICATIONLOSSESCONVEYANCELOSSESEVAPOTRANSPIRATIONRAINFALL oRUNOF:> '.'\"1I. ,'r;:;. I 6 W=CHANGES IN SOIL:IWATERSTORAGEI -4 BOlTOM ..QF-.BOOL ZONE + nDEEP ""...7 PERCOLATION(DRAINAGE)WATERPUMPEDFORIRRIGATIONATTHEFARM1.Waterbalancecomponentsofanirrigatedfield.

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anticipating rainfall and providing soil storage capacity(thatis, irrigating to lessthanfield capacity to leave room for rainfall storage) to increase rainfall effectiveness.WaterBudgetingforIrrigationSchedulingTwo questionsmustbe answered in order to schedule irrigations:1)Whento irrigate,and2)How muchwaterto apply? A water budget procedure can be used to answer both questions.FromFig.1,thecrop root zone can be visualized as a reservoir wherewateris temporarily stored for use bythecrop. Inputs tothatreservoir occur frombothrainfall and irrigation. Ifthecapacity ofthesoil-water reservoir (the volume ofwaterstoredinthecrop root zone)andthedaily rates ofETextraction fromthatreservoirareknown,thedate ofthenextirrigation andtheamountofwaterto be applied can be determined. Thus,ETand soil-water storageintheplantroot zonearethebasic information needed to usethewater-budget method for irrigation scheduling.UnderstandingEvapotranspirationEvaporation involvesthechange of state ofwaterfrom a liquid to a vapor. Energy is required for evaporation to occur. If field surfaces, such astheleaves of well-watered plants orwetsoils,aremoist,theamountofwatervaporizing and moving intotheatmos phereina humid region such as Florida is mainly determined bytheenergy available from solar radiation. Thus,thesolar radiation level isthemain climatic factorthatdeterminestheET rate, althoughairtemperature, humidity, and wind also affect ET rates.Forthese reasons, ET ratesarehigherinsummerwhendaily sola.r radiation levelsandtemperaturesare high. Exceptionally low relative humidity and high winds will increase ET rates above normal.Hot dry winds may raisetheET rates of isolated irrigated fields by 25 percentormore abovethenormal, although such periodsareusually brief. The most significant crop factorsthataffect ET from a well watered croparethecrop species,thestage of growth, andtheplant sizeorleafareawithrespect totheground surface on which radiation is incident. Methods of expressingplantsizeandleafarea6

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includethedegree of ground cover or percent canopy coverage. ET ratesaregreatestwhentheentire soil surface is covered bythecrop canopy. Many cropsdonottotally shadetheground, especially duringtheirearly stages of growth, and evaporation fromthedry soil surface between plants is low. This is especiallytrueof sandy soils which act as a mulch to greatly reduce evaporationwhenthesurface dries. Whenthecrop canopy is not complete,theET rate is strongly influenced bythearea of leaf surfacethatis intercepting sunlight,thatis,thepercent of soil surface shaded bythecrop.Forthis reason,ETfor row crops during early growth stages andthatof many orchardsandvineyards is considerably lessthantheETthatwould occur from a complete canopy. As growth increases, ET reaches its maximumatnearly complete ground cover. ET measure ments indicatethatwhenthepercent of ground covered bythecanopy is above 60-70 percent, full ground coverandfull ET rates can be assumed. Immediately afteranirrigation, evaporation fromthewetsoil occursatapproximatelythesame rate as full cover ET,butasthesoil dries, rates of evaporation are quickly reduced. Thus, frequency of irrigation playsanimportant role in determining evaporation losses fromthesoil, especiallywhentheentire soil surface is wetted. Thereareboth positive and negative aspects to evaporation from sandy soils--thesoilsareself-mulching and evaporation ratesarequickly reducedwhenthesoil surfaces dry, but, because oftheirlow water-holding capacities,thesurfacesmustbe wetted morefrequentlythanthoseof heavier-textured soils because more frequent irrigationsarerequired.EstimatingEvapotranspirationBecause climatic conditions largely determine ET, various methods based on meteorological factors have been developed to estimate ET rates. Asummaryanddiscussion of several ET equationsandtheirmodifications for Florida conditions were presented by a committee of IFAS researchers (Jonesetal., 1984). The ET estimation equations which canbeapplied on a daily basis for irrigation scheduling require inputs of measured or estimated solar radiation. ThePenmanequation, which is believed to bethemost accurate for Florida7

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conditions, is also mathematically complexanddifficult to use manually.Forthis reason, computer software which calculatesETfrom climaticandcrop factors was developed for IBM-compatible microcomputers (Zazuetaetal., 1987)andis available fromtheUniversity of Florida. One ofthesimpler methods of estimating dailyETinthefield is by measuring evaporation from a standardized free-water surface, since a correlation exists between crop ET and evaporation from free water. The standardwatersurface commonly used istheNational Weather Service Class A evaporation pan locatedinanirrigated grassed area. The ratio between potential ET (ET for a well-wateredshortgreen grass crop)andevaporation from a well-maintained evaporationpanis typically assumed to be about 0.7. Then cropETcanbeestimated by multiplying potentialETbywateruse coefficients(Kc)for specific crops, growth stages,andmanagementfactors.Kcvalues for many cropsthatare growninFlorida have been published by Doorenbos andPruitt(1977), Jonesetal. (1984), and SCS (1970). When a complete crop canopy exists,thedaily ET can be es timated by multiplyingthemeasured pan evaporation by 0.7. This procedure can be used as a "rule of thumb" if other specific crop coefficient dataarenot available.Soil-WaterStorageDuring irrigation,waterinfiltrates (penetrates)thesoil surface.Itisthendistributedinthesoil by gravity and soil capillary forces (attraction for water). Asthesoil becomes wetter, gravitational forces dominateandwaterdrains downward throughthesoil. Drainage is rapidatfirst,butafter one to several days (depending on soil type, layering, etc.)itdecreases to a very small rate, sothatfor practical purposesitmay be neglected. At this point, soil moisture intheroot zone may be considered to be in storage;itcan be depleted primarily byplanttranspiration or evaporation fromthesoil surface. This upper limit ofwaterstorage in soil is called "field capacity" (FC). Field capacityinsandy soilsinFlorida commonly occurs within one or two days afteranintense rainfallormaximum irrigation because oftherapid movement ofwaterinsandy soils.8

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A practical lower limit of soilwatermaybedefined asthesoil-water content below which severe cropwaterstressandpermanentwilting occurs. This lower limit has been defined asthepermanentwilting point (PWP). While plants may remove somewaterbelowthislevel, such extraction has littleorno significanceinirrigated agriculture, althoughitmay be crucial for plant sur vival. In fact, yield reduction typically occurs long before PWP is reached. The difference between FC andPWPis calledtheavailablewater(AW). Table 1 presents typical values of AW for various soil types. Most ofthemajor irrigated soilsinFloridaareinthetop category (Sandsandfine sands)inTable1.Local soil surveys and irrigation guides available fromtheSoil Conservation Service (SCS) provide information on specific Florida soil types. Availablewatermay also be estimatedinthefield by applying a known limitedamountofwatertothesoilwhentheprofilewatercontent isnearPWP, observingthevolume of soil wetted, and calculatingthevolume ofwaterstoredperunitvolume of soil. Table1.Available Water for Various Soil Types Available Water(AW)Type of Soil Sandsandfine sands Moderately coarse-textured sandy loamsandfine sandyloams Medium texturevery fine sandy loams to silty clay loam Fineandvery fine texture silty clay to clay Peatsandmucks9range (inches/ft)0.4to1.00 1.00to1.50 1.25to1.75 1.50to2.50 2.00to3.00average (inches/ft)0.75 1.25 1.50 2.00 2.50

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Once AW is known,thetotal depth ofwateravailable, TAW (andthusthecapacity ofthesoil-water reservoir), can be obtained by multiplying AW bythecrop effective root zone depth.Forlayered soils, TAW is calculated by addingthemultiples of AWanddepths of all soil layers contained inthecrop root zone. The effective root depths of Florida agricultural crops can be estimated from8e8irrigation guides,butlocal conditions may affect root depths. Thebestway to determine effective root zone depths is by digging and observing where most oftherootsarelocated. The effective root zone isthatwhere most oftheroots actively involvedinwateruptake are located--this is normallytheupper 1 to 3 ft ofthesoil profile, depending onthecrop being grown. In a humid area such as Florida, irrigations should be concentratedinthis upper portion ofthecrop root zone wherethegreat majority ofthecrop roots are located.AllowableSoilWaterDepletionThe allowable soilwaterdepletion isthefraction oftheavailable soilwaterthatwill be' used to meet ET demands. As ET occurs,thesoilwaterreservoir begins to be depleted.Asthesoil dries,theremainingwateris bound more tightly tothesoil, makingitmore difficult fortheplant to extract it.Forthis reason ET willstartto decrease long beforethePWP is reached. This lower ET generally does not increase water-use efficiency becauseitalso reduces yield. For this reason, growers should irrigate beforetheroot zonewatercontent reaches a levelthatrestricts ET. The critical soilwaterdepletion level depends on several factors: crop factors (rooting density and developmental stage), soil factors (AWe and effective root depth),andatmospheric factors (currentETrate). Therefore, no single level can be recommended for all situations. Allowable depletions of1/3to2/3oftheavailable soilwaterarecommonly usedinscheduling irrigations. The smaller allowable depletionsarecommonly used for sensitive cropsatcritical stages of crop growth. The greater depletions are allowed for less sensitive cropsandatless-critical growth stages. As a "rule of thumb",anallowablewaterdepletion of1/2of AW should be used ifotherspecific dataarenot available.10

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TheWaterBudgetThe water-budget procedure is also called awaterbalance or bookkeeping procedure.Itis similar to keeping a bank account balance. Ifthebalance on a starting date andthedatesandamounts of depositsandwithdrawals are known,thebalance canbecalculatedatany time. Most importantly,thetime when all funds (or water) would be withdrawn can be determined sothatanover draft isavoided (oranirrigation can be scheduled). Thewaterbudget equation for irrigation scheduling on a daily basis canbewritten as follows: 68 =R+IET (D+RO) where 6 8= changeinavailable soilwater(inches), R= rainfall measuredatthefield site (inches), I= irrigation applied (inches), ET =evapotranspiration estimated frompanevaporation or other method (inches),(1)D + RO = drainageandrunoff, calculated as rainfall in excess ofthatwhich canbestoredinthesoil profile to field capacity (inches). The soilwatercontent on any day(i)can be calculatedintermsofthewaterstorage ontheprevious day (i-I), plustherainandirrigation, and minustheET, drainage,andrunoffthatoccurred sincetheprevious day as: 8(i) = 8Ci-l) + R +1-ET -(D+ RO)(2)Thestartingpoint for irrigation scheduling is often after a thorough wetting ofthesoil by irrigation or rainfall. This bringsthesoil reservoir to full capacity and 8(i) toTAW.If this doesnotoccur,theinitial available soilwatermustbe determined by direct observation (measurement or estimation). Daily measurements or estimates of ET are subtracted fromtheavailable soilwateruntilthesoilwaterhas been reducedtotheallowable depletion level. Atthatpointanirrigation should be11

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applied with anetamount equivalent totheaccumulated ET losses sincethelast irrigation. The soil reservoir isthusrecharged to full capacity, andthedepletion cycle begins again. Fig. 2 shows a sample of awaterbudget for a Florida sandy soil with a total availablewaterof 1.5 inches intheplant root zone.Itwas assumedthata management decision was made to irrigate when2/3oftheavailable soilwater(1.00 inch) was depleted. In this example,thatlevel of depletion occurred after 4 days. Atthattime,anirrigation should be scheduled to replenishtheI-inch of soilwaterdepleted. Thewaterbudget procedure also accounts for rainfall. Rainfall is entered into Fig. 2 inthesame waythatanirrigation application would be.Thatis,itrefillsthesoil profile and raisesthesoilwatercontent. If large rainfalls occur, onlythatportion required to restorethesoilwatercontent to field capacity will be effective. Greater amounts of rain will eitherrunoff ofthesoil surface or drain belowtheplant root zone. The management decision concerningthelevel of allowablewaterdepletion(AWD) is onethatwill need to be made by each irrigation manager.Itwill vary depending upon soil, crop, and climatic factors. Commonlyitwill vary duringthegrowing season.Forexample, AWDmay be setat2/3during non-critical crop growth stages,butitmaybedecreased to1/3during critical growth stages such as during fruit set. Decreasing AWDincreasesthefrequency of irrigation (but decreasestheamountperirrigation) to provide a more favorable crop root environment to reduce water stress during critical growth stages. Decreasing AWDwill generally result in greater irrigation requirements becausethesoil will be maintainedwetterandthusrainfall will be less effective. More frequent irrigations will also promote increased evaporation fromthesoil surface. The capacity oftheroot zone reservoir and allowable depletion levels can be estimated beforethestartof a growing season.Forannual cropsthecapacity will change astheseason progresses and astheirroots develop.Formature perennial crops such as citrus,theroot zone may be considered to be a constant for a givensetof soil conditions. The soil depthtobemanaged for irrigation must be refined by field experience.Forexample, experience in many parts oftheworld has shownthatthecitrus root zone to be irrigated should be much lessthanthe5 to 8ftdepths where some plant roots exist.12

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THEWATERBUDGETMETHODOFIRRIGATIONSCHEDULINGETLOSSTOTHEATMOSPHERESOILWATER RESERVOIR ALLOWABLE AVAILAOLE DEPLETION'" SOILWATER=1.0IN. =1.5IN.J1 _JL---_____ ETINCHES/DAYDAYS0.25I0.3020.1730.2841.004IRRIGATEI.WHEN ..... AFTER 4DAYS2.HOWMUCH'.APPLY1.0INCHESOFWATERPLUS LOSSES (EFFICIENCYCONSIDERATION)'"ALLOWABLE DEPLETION= 21:3 AVAILABLE SOIL WATER (MANAGEMENT DECISION)2.Illustrationofthewaterbudgetmethodofirrigationscheduling.

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Rather,theirrigated zone should betheupper 2 to 3ftoftheroot zone wherethemajority oftherootsarelocated. This practice also hastheadvantage of allowing some soil capacity for rainfall whenitoccurs. Daily ET values for specificwateruse periods should be es timated frompanevaporation orETequations. If current dailyETestimatesarenot available,theuse of soil moisture measurement instrumentation ortheinstallation of evaporation pans should be considered. The use of long-term average ET data (Smajstrlaetal., 1984) will resultinscheduling errors because day-to-dayETratesarehighly variable. Long-term average ET data can be used as a guidefordailyETestimates,butthey will need to be modified for climatic variabilities.Thatis, they will need to be increased during long-term hot, dry periods,anddecreased during mildweatherperiods.Soil-MoistureIndicatorsforIrrigationSchedulingDevices for monitoring soil moisture have been available for morethan20 years. Among them, tensiometers aretheinstruments most commonly used for scheduling irrigations. Gypsum blocks are also being used on a limited basis. These devices registerthestatus ofwaterinthesoil,intermsof soil-water tension,atthedepthatwhichthedevice is placed. They havetheadvantage of providing a measurement ofthesoilwaterstatusratherthanrelying upon estimates of ET to calculatethesoilwatercontent. When placedintheplant root zonetheyindicatethesoilwaterstatusthattheplantsareexperiencing. Disadvantages of soil moisture sensors includetheircost, labor requirements for reading and servicing,andneed for periodic calibration. They also make pointratherthanfield scale measurements,thusmany instruments mayneed to be installedto accurately represent a given field. Details oftheuse, cost, advantages, and disadvantages of these andotherdevices which can be used for soil moisture measurement are giveninIFASExtension Circular 532, "Measurement of Soil Water for Irrigation Management". Details oftheuse of tensio meters are giveninIFAS Extension Circular 487, "Tensiometers for Soil Moisture Measurement and Irrigation Scheduling", available from IFASCounty Extension Offices.14

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No single soil-water tension level can be recommended as indica tingtheneed for irrigationwhenusing tensiometers.Forthesame reasonsthatallowable soilwaterdepletion is not constant for all cropsandconditions, critical soilwatertension also varieswithsoilandcrop conditionsandmanagement objectives. The level also varieswithdepth of placement ofthetensiometer. However, cropwaterstressisnormally avoided when irrigationsarescheduledintherange of 15-30 centibars (cb)intheupper portion ofthecrop root zone where most oftheroots actively involved in soilwaterextractionarelocated. Lower readings should be used for cropsthataremore sensitive towaterstress. Field experience is required to refinetheinterpretation ofinstrumentreadings for a given cropandmanagement system. Tensiometersorany other soil-moisture monitoring devicearemost effectively usedincombinationwithET data. The device is readtodeterminewhentoirrigate,andtheETdata are used to calculatethevolume ofwaterlost sincethelast irrigation.Fromthis,thevolume to be replaced can be determined.IrrigationWaterManagementGood on-farmwatermanagement practices include not only precise irrigation scheduling,butalso knowing (or being able to accurately measure)thevolume ofwaterapplied to each field. For example,ifthefield associatedwiththeirrigation scheduling example in Fig. 2 was 40 acres of citrus, irrigatedwithanoverhead sprinkler systemin4 sets of 10 acres each,andiftheapplication efficiency fortheoverhead system was75%(25%ofthewaterapplied isassumedtobelost to evaporation, wind drift, and nonuniform application during sprinkling),thedepth ofwaterto be pumpedateach irrigation would be 1.0"/0.75=1.33 inches. The volume ofwaterrequired for each 10 acre set would be (1.33 in.) (10 acres)=13.3 acre-inchesorapproximately 362,000 gal. Flow meters can accurately measure irrigationwatertoverifythatthecorrectamountis applied. They are availablewithregistersinunits of either gallons or acre-inches. Flow meters can easily pay for themselveswithsavings in fuel costs for irrigation pumping. More information on irrigation flow measurement is availableinIFAS Extension Bulletin 207, "Agricultural Water Measurement", availablethroughIF AS County Extension Offices. 15

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Good farm irrigation management requiresthatanirrigation system be capable of applyingwaterin sufficient quantities to meetthecrop'swaterrequirements and with high uniformity to minimize waste. Nonuniform irrigation will cause excesswaterto be applied in some areas while other areas will not get enough. Irrigation systems are more expensive if they are designed for a high degree of uniformity. Thus,thereis a temptation to sacrifice uniformitywhensystems are purchased onthebasis of competitive bids. The system manager should recognizethatoperating costs will be greateroryield losses will result when systems which apply waterand chemicals nonuniformly are operated. A lower initialsystemcost which sacrifices uniformity of water application maybefalse economy. A technique for field evaluation oftheuniformity of water application by trickle irrigation systems is available as IFASExtension Bulletin 197, "Field Evaluation of Trickle Irrigation Systems: Uniformity of Water Application", and as computer software fromtheUniversity of Florida.SummaryProper irrigation scheduling will help to assure efficient use ofwaterand energy in crop production. Irrigation scheduling methodsthatarecurrently applicable in Florida are1)awaterbudget method requiring estimation of daily ET and soil water content, and2)theuse of soil moisture measurement instrumentation. Techniques forestimatingET, determining soil water storage, determining allowablewaterdepletions, andwaterbudgeting were described. When properly used and combined with efficient methods ofwaterapplication, these techniques should also result in increased produc tion and profits.16

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ReferencesDoorenbos,J.and W.O. Pruitt. 1977. Crop water requirements. FAOIrrigation and Drainage Paper No. 24. Food and Agric. Organiz. oftheU.N. Rome. Fereres,E.,D.W. Henderson, W.O. Pruitt, W.F. Richardson and R.S. Ayres. 1981. Basic irrigation scheduling. Leaflet 21199. Div. of Agric.ScL,Univ. Calif. Izuno, F.T. 1987. Water budgeting for highwatertable soils. Ext. Cir.769. IFAS, Univ. Fla. Izuno, F.T. and D.Z. Haman. 1987. Basic irrigation terminology. Agric. Engr. Dept. Fact Sheet AE-66. IFAS, Univ. Fla. Jones, J.W., L.H. Allen, S.F. Shih, J.S. Rogers, L.C. Hammond,AG.SmajstrlaandJ.D. Martsolf. 1984. Estimatedandmeasured evapotranspiration for Florida climate, crops, and soils. Bul. 840 (Tech.) IFAS, Univ. Fla. SCS Technical Staff. 1970. Irrigation water requirements. Tech. Release 21. U.S. Dept. of Agric.,Soil Conservation Service. Washington, D.C. Smajstrla, A.G., G.A. Clark, S.F. Shih, F.S. Zazueta and D.S. Harrison. 1984. Potential evapotranspiration probabilities and distributionsinFlorida. Ext. Bul. 205. IFAS, Univ. Fla. Smajstrla,AG.and D.S. Harrison. 1984. Measurement of soilwaterfor irrigation management. Ext. Cir. 532. IFAS, Univ. Fla. Smajstrla, AG., D.S. Harrison and F.X. Duran. 1985. Tensiometers for soil moisture measurement and irrigation scheduling. Ext. Cir. 437. IFAS, Univ. Fla. Smajstrla, AG., D.s. Harrison and F.S. Zazueta. 1985. Field evaluation of trickle irrigation systems: Uniformity ofwaterapplication. Ext. Bul. 195. IFAS, Univ. Fla. Smajstrla, AG., D.S. Harrison and F.S. Zazueta. 1985. Agriculturalwatermeasurement. Ext. Bul. 207. IFAS, Univ. Fla.17

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Zazueta, F.S., A.G. Smajstrla andD.Z.Haman. 1987. Evapotran spiration estimation utilities. Cir.744,Computer Series. IFAS, Univ. Fla. Zazueta, F.S., A.G. Smajstrla and D.S. Harrison. 1984. Glossary of trickle irrigation terms. Agric. Engr. Dept. Fact Sheet AE-45. IFAS, Univ. Fla.18

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MARSTONSCIENCE LIBRARY: COOPERAT'VE EXTENS'ON SERV'CE, UN'VERS'TY OF FLOR'DA, 'NST'TUTE OF FOOD AND AGRICULTURAL SCIENCES, JohnT.Woeste, Director, in cooperation with the United States Department of Agriculture, publishes this information to further the purpose of theMay8andJune30, 1914ActsofCongress;and is authorized to provide research, educational information and other services only to individuals and institutions that function without regard to race, color, sex, age, handicap or nationalI \origin. Single copies of extension publications (excluding 4-H and youth publications) "''''"''''''"",,, are available free to Florida residents from county extension offices. Information on bulk rates or copies for out-of-state purchasersisavailable from C.M. Hinton, Publications Distribution Center, IFAS Building 664, University of Florida, Gainesville, Florida 32611. Before publicizing this publication, editors should contact this address to determine availability. Printed 2/93.


Basic irrigation scheduling in Florida
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Title: Basic irrigation scheduling in Florida
Series Title: Bulletin
Physical Description: 18 p. : ill. ; 24 cm.
Language: English
Creator: Smajstrla, A. G ( Allen George )
Publisher: Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida
Place of Publication: Gainesville Fla
Publication Date: 1988
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Subjects / Keywords: Irrigation scheduling -- Florida   ( lcsh )
Genre: bibliography   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
non-fiction   ( marcgt )
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Bibliography: Includes bibliographical references (p. 17-18).
General Note: Title from cover.
General Note: "May 1988."
Statement of Responsibility: A.G. Smajstrla ... et al.
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Rights Management: All rights reserved by the source institution and holding location.
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ltuf - AJQ6978
oclc - 28923224
alephbibnum - 001832872
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Table of Contents
    Front Cover
        Front Cover 1
        Front Cover 2
    Table of Contents
        Table of Contents 1
        Table of Contents 2
    Introduction & Determining when to irrigate
        Page 1
        Page 2
        Page 3
        Page 4
        Page 5
    Water budgeting for irrigation scheduling
        Page 6
        Page 7
        Page 8
        Page 9
        Page 10
        Page 11
        Page 12
        Page 13
    Soil moisture indicators for irrigation scheduling
        Page 14
    Irrigation water management
        Page 15
    Summary
        Page 16
    Reference
        Page 17
        Page 18
    Back Cover
        Page 19
        Page 20
Full Text


Bulletin 249


Basic Irrigation

Scheduling in

Florida

A. G. Smajstrla, B. J. Boman, G. A. Clark,
D. Z. Haman, F. T. Izuno, and F S. Zazueta










Florida Cooperative Extension Service / Institute of Food and Agricultural Sciences
University of Florida / John T. Woeste, Dean
'V!ERSITY OF FLORIDA LIBRARIES


May 1988








Contents


Introduction. ............... ...... 1

Determining When to Irrigate . . . . . . . . 1

Crop Water Requirements . . . . . . . . 3

Field Water Balance ................... 4

Water Budgeting for Irrigation Scheduling . . . . . 6

Understanding Evapotranspiration . . . . . . 6

Estimating Evapotranspiration . . . . . . . 7

Soil Water Storage... ................ .. 8

Allowable Soil Water Depletion . . . . . .... 10

The Water Budget .................... 11

Soil Moisture Indicators for Irrigation Scheduling . . ... 14

Irrigation Water Management . . . . . . .... 15

Summary .................. ....... 16

References . . . . . . . . . . . . . 17


A.J. Smajstrla is Water Mgt. Specialist, Agr. Eng. Dept.,
Gainesville; B.J. Boman is Citrus Irr. Specialist, Agr. Res. and Ed.
Center, Ft. Pierce; G.A. Clark is Water Mgt. Specialist, Gulf Coast
Res. and Ed. Center, Bradenton; D.Z. Haman is Water Mgt;
Specialist, Agr. Eng. Dept., Gainesville; F.T. Izuno is Water Mgt.
Specialist, Everglades Res. and Ed. Center, Belle Glade; and F.S.
Zazueta is Water Mgt. Specialist, Agr. Eng. Dept., Gainesville:
respectively, Institute of Food and Agricultural Sciences, University
of Florida.








Introduction


Proper irrigation scheduling is the application of water to crops
only when needed and only in the amounts needed; that is, deter-
mining when to irrigate and how much water to apply. With proper
irrigation scheduling, crop yields will not be limited by water stress
from droughts, and the waste of water and the energy used in
pumping will be minimized. Other benefits include reduced leaching
of nutrients from excess water applications, and reduced pollution of
groundwater or surface waters from the leaching of nutrients.

Irrigation is practiced to provide water when rainfall is not
sufficient or timely to meet water needs of a crop. For most
agricultural crops, yield or quality reductions result from water
stress. Therefore, if water is available and if it is relatively low in
cost, as is the case in Florida, irrigations are normally scheduled to
avoid plant water stress.

Despite Florida's relatively large average yearly rainfall of 52-60
inches, irrigation is practiced extensively. Irrigation is necessary
because of the nonuniform distribution of rainfall, the very limited
water-holding capacities of typical sandy soils, and the extreme
sensitivity of many specialty crops to water stress. These factors
and the economic implications of under- or over-irrigation require
that irrigations be scheduled as efficiently as possible.

This publication discusses irrigation scheduling for Florida crops
grown on soils where the water table is substantially below the crop
root zone so that it does not contribute significantly to crop water
use. Thus, irrigation events must periodically occur to replenish
water in the crop root zone. Water budgeting for water table
management (also called subirrigation or seepage irrigation) on
poorly drained soils in which irrigation occurs as water applications
to a high water table (immediately below the crop root zone), is
discussed in IFAS Extension Circular 769, "Water Budgeting for High
Water Table Soils", available from IFAS County Extension Offices.


Determining When to Irrigate

Because the objective of irrigation is to maintain a favorable
environment for crops, the plants themselves are the best indicators








of the need for irrigation. Instrumentation exists which could allow
an irrigator to measure plant water status and to anticipate water
stress. However, such instrumentation is expensive, requires special
training for use, and is generally applicable only for research
purposes. Field scale use of such instruments is impractical.

Another indicator of plant water stress is the visual appearance
of the plant. Unfortunately, however, yield reduction has already
occurred by the time most agricultural crops show wilt symptoms.
Growth processes cease in many crops before visual wilting occurs,
and yield reduction may have occurred for some time before wilting
is seen.

Finally, there are time lags associated with applying irrigation
water. Because several zones might be irrigated from a single pump
or other limiting distribution characteristics may exist, many irriga-
tion systems cannot quickly replenish water in the crop root zone.
Many hours or days may be required. Therefore, the need to
irrigate must be anticipated because of limitations of the irrigation
system. This problem is compounded in Florida by the low water-
holding capacities of most agricultural soils and by the shallow root
zones of many crops.

When to irrigate can also be determined by calendar methods (for
example every 5 days), by crop growth stage (for example, every 5
days during early vegetative growth stage, and every 3 days during
peak growth stage), or by similar methods based on long-term
average irrigation requirements. However, these methods fail to
consider the tremendous effect of climatic variability on daily crop
water use. Therefore, the use of long-term average values may not
be adequate during periods of hot, dry days, while it may result in
overirrigation during periods of cool, overcast days, especially if
rainfall is not considered. Day-to-day climatic conditions are highly
variable during much of the year in Florida because of cloud cover
and the random nature of rainfall occurrences.

Because of the these limitations, irrigations are most often
scheduled based on the soil water status. Three procedures may be
used: 1) a water balance procedure based on the estimated crop
water use rate and soil water storage, 2) a direct measurement
procedure based on instrumentation to measure the soil water status,
and 3) a combination of the above two methods in which soil water
status instrumentation is used with a water balance procedure.
These procedures require a knowledge of the crop water require-








ments, effective root-zone, soil water-holding capacity, and irriga-
tion system capabilities in order to schedule irrigations effectively.


Crop Water Requirements

Water is used in a cropped field in several ways: 1) assimilation
into the plant and plant fruit, 2) direct evaporation from the soil or
other surfaces, 3) transpiration, which is loss of water vapor from
plant leaves, and 4) other uses such as leaching of salts, crop
cooling, and freeze protection. Less than 1% of the water used in
crop production is assimilated into the plants. Other uses (category
4, above) may be significant, but they depend on factors other than
maintaining adequate soil water content, and they will not be
considered in this publication.

Most of the water applied to meet the water requirements of a
crop is used in evaporation and transpiration. Evaporation and
transpiration are important in cooling a crop, to maintain tempera-
tures in the range that permits photosynthetic activity and crop
growth to occur. Transpiration is also required to transport
nutrients into and through plants.

The combination of evaporation and transpiration is called
evapotranspiration (ET). Because the amount of water assimilated by
a plant is very small with respect to ET, ET is often considered to
be the crop water requirement -- the amount of water required by a
growing crop to avoid water stress.

Delivering water to a crop in the field results in losses which
increase the amount of water which must be pumped to supply the
crop water requirement. Losses may occur because of inefficiencies
in the conveyance system, evaporation and wind drift (if water is
sprayed through the air), surface run-off, or percolation below the
root zone. These losses can be minimized through good management
practices, but they are impossible to completely eliminate, and they
must be considered when determining the total (or gross) irrigation
water requirement.

The total irrigation water requirement is the total amount of
irrigation water which is required for crop production, including ET,
all losses incurred in delivering water to the crop, and other needs
such as leaching of salts, crop cooling, and freeze protection. In
humid areas such as Florida, a large part of the crop water require-








ment can be provided by rainfall. Effective rainfall, rainfall that is
stored in the root zone and available for crop use, proportionally
reduces the amount of water which must be pumped for irrigation.


Field Water Balance

The water balance of a field during and after irrigation is shown
in Fig. 1. In Florida, runoff losses are normally minimal because of
the high infiltration rates of the sandy soils. Conveyance losses can
be minimized by conveying water to the field in pipes rather than
open channel.

Application losses, including evaporation and wind drift, can
occur during irrigation, especially from sprinkler irrigation systems.
These losses are, however, relatively small during periods of low
radiatipn, low wind velocities, and high humidities. Also, water which
evaporates during application, or which is intercepted and later
evaporates from soil, plant, or other surfaces is not entirely lost.
Rather, some evaporation during application compensates for ET by
reducing ET that would have occurred if the intercepted water had
not been evaporated.

Evaporation and wind drift losses can be minimized by irrigation
at night, early mornings, and late afternoons when climatic condi-
tions are not severe. However, cultural aspects such as disease must
be considered for crops in which wet foliage may promote bacterial
or other growths which could reduce yields.

Deep percolation losses from well-designed irrigation systems can
be minimized by good irrigation management. If water is applied
uniformly and the water-holding capacity of a soil is not exceeded,
water losses to deep percolation will be minimized. If saline water
is used for irrigation, it may be necessary to leach excess salts from
the crop root zone by adding water in excess of the soil water-
holding capacity. However, excess irrigation for leaching should be
required only during extended dry periods in Florida because rainfall
normally leaches salts.

If the losses shown in Fig. 1 are kept to a minimum, most of the
irrigation water applied will evaporate or transpire in proportion to
the climatic demand. Unfortunately, rainfall is relatively unpre-
dictable and its occurrence immediately following an irrigation
reduces rainfall effectiveness. Irrigation can be minimized by







THE WATER BALANCE OF A FIELD


EVAPOTRANSPIRATION


APPLICATION
LOSSES
A


WATER PUMPED
FOR IRRIGATION
AT THE FARM



CONVEYANCE
LOSSES


1. Water balance components of an irrigated field.


RUNOFF

I 'I
W = CHANGES IN SOIL
WATER STORAGE
BOTTOM OF ROOT ZONE
DEEP
PERCOLATION
( DRAINAGE)








anticipating rainfall and providing soil storage capacity (that is,
irrigating to less than field capacity to leave room for rainfall
storage) to increase rainfall effectiveness.


Water Budgeting for Irrigation Scheduling

Two questions must be answered in order to schedule irrigations:
1) When to irrigate, and 2) How much water to apply? A water-
budget procedure can be used to answer both questions.

From Fig. 1, the crop root zone can be visualized as a reservoir
where water is temporarily stored for use by the crop. Inputs to
that reservoir occur from both rainfall and irrigation. If the capacity
of the soil-water reservoir (the volume of water stored in the crop
root zone) and the daily rates of ET extraction from that reservoir
are known, the date of the next irrigation and the amount of water
to be applied can be determined. Thus, ET and soil-water storage in
the plant root zone are the basic information needed to use the
water-budget method for irrigation scheduling.


Understanding Evapotranspiration

Evaporation involves the change of state of water from a liquid
to a vapor. Energy is required for evaporation to occur. If field
surfaces, such as the leaves of well-watered plants or wet soils, are
moist, the amount of water vaporizing and moving into the atmos-
phere in a humid region such as Florida is mainly determined by the
energy available from solar radiation. Thus, the solar radiation level
is the main climatic factor that determines the ET rate, although air
temperature, humidity, and wind also affect ET rates. For these
reasons, ET rates are higher in summer when daily solar radiation
levels and temperatures are high.

Exceptionally low relative humidity and high winds will increase
ET rates above normal. Hot dry winds may raise the ET rates of
isolated irrigated fields by 25 percent or more above the normal,
although such periods are usually brief.

The most significant crop factors that affect ET from a well-
watered crop are the crop species, the stage of growth, and the
plant size or leaf area with respect to the ground surface on which
radiation is incident. Methods of expressing plant size and leaf area








include the degree of ground cover or percent canopy coverage. ET
rates are greatest when the entire soil surface is covered by the
crop canopy.

Many crops do not totally shade the ground, especially during
their early stages of growth, and evaporation from the dry soil
surface between plants is low. This is especially true of sandy soils
which act as a mulch to greatly reduce evaporation when the
surface dries.

When the crop canopy is not complete, the ET rate is strongly
influenced by the area of leaf surface that is intercepting sunlight,
that is, the percent of soil surface shaded by the crop. For this
reason, ET for row crops during early growth stages and that of
many orchards and vineyards is considerably less than the ET that
would occur from a complete canopy. As growth increases, ET
reaches its maximum at nearly complete ground cover. ET measure-
ments indicate that when the percent of ground covered by the
canopy is above 60-70 percent, full ground cover and full ET rates
can be assumed.

Immediately after an irrigation, evaporation from the wet soil
occurs at approximately the same rate as full cover ET, but as the
soil dries, rates of evaporation are quickly reduced. Thus, frequency
of irrigation plays an important role in determining evaporation
losses from the soil, especially when the entire soil surface is
wetted. There are both positive and negative aspects to evaporation
from sandy soils -- the soils are self-mulching and evaporation rates
are quickly reduced when the soil surfaces dry, but, because of their
low water-holding capacities, the surfaces must be wetted more
frequently than those of heavier-textured soils because more
frequent irrigations are required.


Estimating Evapotranspiration

Because climatic conditions largely determine ET, various methods
based on meteorological factors have been developed to estimate ET
rates. A summary and discussion of several ET equations and their
modifications for Florida conditions were presented by a committee
of IFAS researchers (Jones et al., 1984). The ET estimation equations
which can be applied on a daily basis for irrigation scheduling
require inputs of measured or estimated solar radiation. The Penman
equation, which is believed to be the most accurate for Florida








conditions, is also mathematically complex and difficult to use
manually. For this reason, computer software which calculates ET
from climatic and crop factors was developed for IBM-compatible
microcomputers (Zazueta et al., 1987) and is available from the
University of Florida.

One of the simpler methods of estimating daily ET in the field is
by measuring evaporation from a standardized free-water surface,
since a correlation exists between crop ET and evaporation from
free water. The standard water surface commonly used is the
National Weather Service Class A evaporation pan located in an
irrigated grassed area. The ratio between potential ET (ET for a
well-watered short green grass crop) and evaporation from a
well-maintained evaporation pan is typically assumed to be about 0.7.
Then crop ET can be estimated by multiplying potential ET by water
use coefficients (Kc) for specific crops, growth stages, and manage-
ment factors. Kc values for many crops that are grown in Florida
have been published by Doorenbos and Pruitt (1977), Jones et al.
(1984), and SCS (1970).

When a complete crop canopy exists, the daily ET can be es-
timated by multiplying the measured pan evaporation by 0.7. This
procedure can be used as a "rule of thumb" if other specific crop
coefficient data are not available.


Soil-Water Storage

During irrigation, water infiltrates (penetrates) the soil surface.
It is then distributed in the soil by gravity and soil capillary forces
(attraction for water). As the soil becomes wetter, gravitational
forces dominate and water drains downward through the soil.
Drainage is rapid at first, but after one to several days (depending
on soil type, layering, etc.) it decreases to a very small rate, so
that for practical purposes it may be neglected. At this point, soil
moisture in the root zone may be considered to be in storage; it can
be depleted primarily by plant transpiration or evaporation from the
soil surface. This upper limit of water storage in soil is called
"field capacity" (FC). Field capacity in sandy soils in Florida
commonly occurs within one or two days after an intense rainfall or
maximum irrigation because of the rapid movement of water in sandy
soils.








A practical lower limit of soil water may be defined as the
soil-water content below which severe crop water stress and perma-
nent wilting occurs. This lower limit has been defined as the
permanent wilting point (PWP). While plants may remove some
water below this level, such extraction has little or no significance
in irrigated agriculture, although it may be crucial for plant sur-
vival. In fact, yield reduction typically occurs long before PWP is
reached.

The difference between FC and PWP is called the available water
(AW). Table 1 presents typical values of AW for various soil types.
Most of the major irrigated soils in Florida are in the top category
(Sands and fine sands) in Table 1. Local soil surveys and irrigation
guides available from the Soil Conservation Service (SCS) provide
information on specific Florida soil types. Available water may also
be estimated in the field by applying a known limited amount of
water to the soil when the profile water content is near PWP,
observing the volume of soil wetted, and calculating the volume of
water stored per unit volume of soil.


Table 1. Available Water for Various Soil Types


Available Water (AW)
range average
Type of Soil (inches/ft) (inches/ft)


Sands and fine sands 0.4 to 1.00 0.75

Moderately coarse-textured-
sandy loams and fine 1.00 to 1.50 1.25
sandy loams

Medium texture-
very fine sandy loams 1.25 to 1.75 1.50
to silty clay loam

Fine and very fine texture- 1.50 to 2.50 2.00
silty clay to clay

Peats and mucks 2.00 to 3.00 2.50








Once AW is known, the total depth of water available, TAW (and
thus the capacity of the soil-water reservoir), can be obtained by
multiplying AW by the crop effective root zone depth. For layered
soils, TAW is calculated by adding the multiples of AW and depths
of all soil layers contained in the crop root zone.

The effective root depths of Florida agricultural crops can be
estimated from SCS irrigation guides, but local conditions may
affect root depths. The best way to determine effective root zone
depths is by digging and observing where most of the roots are
located. The effective root zone is that where most of the roots
actively involved in water uptake are located -- this is normally the
upper 1 to 3 ft of the soil profile, depending on the crop being
grown. In a humid area such as Florida, irrigations should be
concentrated in this upper portion of the crop root zone where the
great majority of the crop roots are located.


Allowable Soil Water Depletion

The allowable soil water depletion is the fraction of the available
soil water that will be'used to meet ET demands. As ET occurs, the
soil water reservoir begins to be depleted. As the soil dries, the
remaining water is bound more tightly to the soil, making it more
difficult for the plant to extract it. For this reason ET will start to
decrease long before the PWP is reached. This lower ET generally
does not increase water-use efficiency because it also reduces yield.
For this reason, growers should irrigate before the root zone water
content reaches a level that restricts ET.

The critical soil water depletion level depends on several factors:
crop factors (rooting density and developmental stage), soil factors
(AWC and effective root depth), and atmospheric factors (current ET
rate). Therefore, no single level can be recommended for all
situations.

Allowable depletions of 1/3 to 2/3 of the available soil water are
commonly used in scheduling irrigations. The smaller allowable
depletions are commonly used for sensitive crops at critical stages
of crop growth. The greater depletions are allowed for less sensitive
crops and at less-critical growth stages. As a "rule of thumb", an
allowable water depletion of 1/2 of AW should be used if other
specific data are not available.








The Water Budget


The water-budget procedure is also called a water balance or
bookkeeping procedure. It is similar to keeping a bank account
balance. If the balance on a starting date and the dates and
amounts of deposits and withdrawals are known, the balance can be
calculated at any time. Most importantly, the time when all funds
(or water) would be withdrawn can be determined so that an over-
draft is avoided (or an irrigation can be scheduled).

The water budget equation for irrigation scheduling on a daily
basis can be written as follows:


AS = R + I-ET-(D + RO) (1)
where
A S = change in available soil water (inches),
R = rainfall measured at the field site (inches),
I = irrigation applied (inches),
ET evapotranspirationn estimated from pan evaporation or
other method (inches),

D + RO = drainage and runoff, calculated as rainfall in excess of
that which can be stored in the soil profile to field
capacity (inches).

The soil water content on any day (i) can be calculated in terms
of the water storage on the previous day (i-1), plus the rain and
irrigation, and minus the ET, drainage, and runoff that occurred
since the previous day as:

S(i) = S(i-1) + R + I ET (D + RO) (2)

The starting point for irrigation scheduling is often after a
thorough wetting of the soil by irrigation or rainfall. This brings
the soil reservoir to full capacity and S(i) to TAW. If this does not
occur, the initial available soil water must be determined by direct
observation (measurement or estimation).

Daily measurements or estimates of ET are subtracted from the
available soil water until the soil water has been reduced to the
allowable depletion level. At that point an irrigation should be








applied with a net amount equivalent to the accumulated ET losses
since the last irrigation. The soil reservoir is thus recharged to full
capacity, and the depletion cycle begins again. Fig. 2 shows a
sample of a water budget for a Florida sandy soil with a total
available water of 1.5 inches in the plant root zone. It was assumed
that a management decision was made to irrigate when 2/3 of the
available soil water (1.00 inch) was depleted. In this example, that
level of depletion occurred after 4 days. At that time, an irrigation
should be scheduled to replenish the 1-inch of soil water depleted.

The water budget procedure also accounts for rainfall. Rainfall
is entered into Fig. 2 in the same way that an irrigation application
would be. That is, it refills the soil profile and raises the soil
water content. If large rainfalls occur, only that portion required
to restore the soil water content to field capacity will be effective.
Greater amounts of rain will either run off of the soil surface or
drain below the plant root zone.

The management decision concerning the level of allowable water
depletion (AWD) is one that will need to be made by each irrigation
manager. It will vary depending upon soil, crop, and climatic
factors. Commonly it will vary during the growing season. For
example, AWD may be set at 2/3 during non-critical crop growth
stages, but it may be decreased to 1/3 during critical growth stages
such as during fruit set. Decreasing AWD increases the frequency
of irrigation (but decreases the amount per irrigation) to provide a
more favorable crop root environment to reduce water stress during
critical growth stages. Decreasing AWD will generally result in
greater irrigation requirements because the soil will be maintained
wetter and thus rainfall will be less effective. More frequent
irrigations will also promote increased evaporation from the soil
surface.

The capacity of the root zone reservoir and allowable depletion
levels can be estimated before the start of a growing season. For
annual crops the capacity will change as the season progresses and
as their roots develop. For mature perennial crops such as citrus,
the root zone may be considered to be a constant for a given set of
soil conditions.

The soil depth to be managed for irrigation must be refined by
field experience. For example, experience in many parts of the
world has shown that the citrus root zone to be irrigated should be
much less than the 5 to 8 ft depths where some plant roots exist.







THE WATER BUDGET METHOD OF IRRIGATION SCHEDULING


ET I
THE Al


SOIL WATER
RESERVOIRr



ALLOWABLE
DEPLETION*
AVAILABLE = 1.0 IN.
SOIL WATER
= 1.5 IN.



1___ ________ ___


.OSS TO
ATMOSPHERE



E
INCHES
0.2


T
i/DAY DAYS
5 1


-- 0.30

- 0.17


- 0.28 4
1.00 4


IRRIGATE I. WHEN- -
2. HOW MUCH *


* AFTER 4 DAYS
* APPLY 1.0 INCHES OF WATER
PLUS LOSSES
(EFFICIENCY CONSIDERATION)


*ALLOWABLE DEPLETION= 2/ AVAILABLE SOIL WATER (MANAGEMENT DECISION)


2. Illustration of the water budget method of irrigation scheduling.








Rather, the irrigated zone should be the upper 2 to 3 ft of the root
zone where the majority of the roots are located. This practice
also has the advantage of allowing some soil capacity for rainfall
when it occurs.

Daily ET values for specific water use periods should be es-
timated from pan evaporation or ET equations. If current daily ET
estimates are not available, the use of soil moisture measurement
instrumentation or the installation of evaporation pans should be
considered. The use of long-term average ET data (Smajstrla et al.,
1984) will result in scheduling errors because day-to-day ET rates
are highly variable. Long-term average ET data can be used as a
guide for daily ET estimates, but they will need to be modified for
climatic variabilities. That is, they will need to be increased during
long-term hot, dry periods, and decreased during mild weather
periods.


Soil-Moisture Indicators for Irrigation Scheduling

Devices for monitoring soil moisture have been available for more
than 20 years. Among them, tensiometers are the instruments most
commonly used for scheduling irrigations. Gypsum blocks are also
being used on a limited basis. These devices register the status of
water in the soil, in terms of soil-water tension, at the depth at
which the device is placed. They have the advantage of providing a
measurement of the soil water status rather than relying upon
estimates of ET to calculate the soil water content. When placed in
the plant root zone they indicate the soil water status that the
plants are experiencing. Disadvantages of soil moisture sensors
include their cost, labor requirements for reading and servicing, and
need for periodic calibration. They also make point rather than
field scale measurements, thus many instruments may need to be
installed to accurately represent a given field.

Details of the use, cost, advantages, and disadvantages of these
and other devices which can be used for soil moisture measurement
are given in IFAS Extension Circular 532, "Measurement of Soil
Water for Irrigation Management". Details of the use of tensio-
meters are given in IFAS Extension Circular 487, "Tensiometers for
Soil Moisture Measurement and Irrigation Scheduling", available from
IFAS County Extension Offices.








No single soil-water tension level can be recommended as indica-
ting the need for irrigation when using tensiometers. For the same
reasons that allowable soil water depletion is not constant for all
crops and conditions, critical soil water tension also varies with soil
and crop conditions and management objectives. The level also
varies with depth of placement of the tensiometer. However, crop
water stress is normally avoided when irrigations are scheduled in
the range of 15-30 centibars (cb) in the upper portion of the crop
root zone where most of the roots actively involved in soil water
extraction are located. Lower readings should be used for crops
that are more sensitive to water stress. Field experience is required
to refine the interpretation of instrument readings for a given crop
and management system.

Tensiometers or any other soil-moisture monitoring device are
most effectively used in combination with ET data. The device is
read to determine when to irrigate, and the ET data are used to
calculate the volume of water lost since the last irrigation. From
this, the volume to be replaced can be determined.


Irrigation Water Management

Good on-farm water management practices include not only
precise irrigation scheduling, but also knowing (or being able to
accurately measure) the volume of water applied to each field. For
example, if the field associated with the irrigation scheduling
example in Fig. 2 was 40 acres of citrus, irrigated with an overhead
sprinkler system in 4 sets of 10 acres each, and if the application
efficiency for the overhead system was 75% (25% of the water
applied is assumed to be lost to evaporation, wind drift, and
nonuniform application during sprinkling), the depth of water to be
pumped at each irrigation would be 1.0"/0.75 = 1.33 inches. The
volume of water required for each 10 acre set would be (1.33 in.)
(10 acres) = 13.3 acre-inches or approximately 362,000 gal.

Flow meters can accurately measure irrigation water to verify
that the correct amount is applied. They are available with registers
in units of either gallons or acre-inches. Flow meters can easily
pay for themselves with savings in fuel costs for irrigation pumping.
More information on irrigation flow measurement is available in
IFAS Extension Bulletin 207, "Agricultural Water Measurement",
available through IFAS County Extension Offices.








Good farm irrigation management requires that an irrigation
system be capable of applying water in sufficient quantities to meet
the crop's water requirements and with high uniformity to minimize
waste. Nonuniform irrigation will cause excess water to be applied
in some areas while other areas will not get enough.

Irrigation systems are more expensive if they are designed for a
high degree of uniformity. Thus, there is a temptation to sacrifice
uniformity when systems are purchased on the basis of competitive
bids. The system manager should recognize that operating costs will
be greater or yield losses will result when systems which apply
water and chemicals nonuniformly are operated. A lower initial
system cost which sacrifices uniformity of water application may be
false economy. A technique for field evaluation of the uniformity of
water application by trickle irrigation systems is available as IFAS
Extension Bulletin 197, "Field Evaluation of Trickle Irrigation
Systems: Uniformity of Water Application", and as computer software
from the University of Florida.


Summary

Proper irrigation scheduling will help to assure efficient use of
water and energy in crop production. Irrigation scheduling methods
that are currently applicable in Florida are 1) a water budget
method requiring estimation of daily ET and soil water content, and
2) the use of soil moisture measurement instrumentation. Techniques
for estimating ET, determining soil water storage, determining
allowable water depletions, and water budgeting were described.
When properly used and combined with efficient methods of water
application, these techniques should also result in increased produc-
tion and profits.








References


Doorenbos, J. and W.O. Pruitt. 1977. Crop water requirements.
FAO Irrigation and Drainage Paper No. 24. Food and Agric.
Organiz. of the U.N. Rome.

Fereres, E., D.W. Henderson, W.O. Pruitt, W.F. Richardson and R.S.
Ayres. 1981. Basic irrigation scheduling. Leaflet 21199. Div. of
Agric. Sci., Univ. Calif.

Izuno, F.T. 1987. Water budgeting for high water table soils. Ext.
Cir. 769. IFAS, Univ. Fla.

Izuno, F.T. and D.Z. Haman. 1987. Basic irrigation terminology.
Agric. Engr. Dept. Fact Sheet AE-66. IFAS, Univ. Fla.

Jones, J.W., L.H. Allen, S.F. Shih, J.S. Rogers, L.C. Hammond, A.G.
Smajstrla and J.D. Martsolf. 1984. Estimated and measured
evapotranspiration for Florida climate, crops, and soils. Bul. 840
(Tech.) IFAS, Univ. Fla.

SCS Technical Staff. 1970. Irrigation water requirements. Tech.
Release 21. U.S. Dept. of Agric., Soil Conservation Service.
Washington, D.C.

Smajstrla, A.G., G.A. Clark, S.F. Shih, F.S. Zazueta and D.S.
Harrison. 1984. Potential evapotranspiration probabilities and
distributions in Florida. Ext. Bul. 205. IFAS, Univ. Fla.

Smajstrla, A.G. and D.S. Harrison. 1984. Measurement of soil water
for irrigation management. Ext. Cir. 532. IFAS, Univ. Fla.

Smajstrla, A.G., D.S. Harrison and F.X. Duran. 1985. Tensiometers
for soil moisture measurement and irrigation scheduling. Ext. Cir.
437. IFAS, Univ. Fla.

Smajstrla, A.G., D.S. Harrison and F.S. Zazueta. 1985. Field
evaluation of trickle irrigation systems: Uniformity of water
application. Ext. Bul. 195. IFAS, Univ. Fla.

Smajstrla, A.G., D.S. Harrison and F.S. Zazueta. 1985. Agricultural
water measurement. Ext. Bul. 207. IFAS, Univ. Fla.








Zazueta, F.S., A.G. Smajstrla and D.Z. Haman. 1987. Evapotran-
spiration estimation utilities. Cir. 744, Computer Series. IFAS,
Univ. Fla.

Zazueta, F.S., A.G. Smajstrla and D.S. Harrison. 1984. Glossary of
trickle irrigation terms. Agric. Engr. Dept. Fact Sheet AE-45.
IFAS, Univ. Fla.






MARSTON SCIENCE LIBRARY


COOPERATIVE EXTENSION SERVICE, UNIVERSITY OF FLORIDA, INSTITUTE -
OF FOOD AND AGRICULTURAL SCIENCES, John T. Woeste, Director, in coopera-
tion with the United States Department of Agriculture, publishes this information to
further the purpose of the May 8 and June 30,1914 Acts of Congress; and is authorized
to provide research, educational information and other services only to individuals and
institutions that function without regard to race, color, sex, age, handicap or national
origin. Single copies of extension publications (excluding 4-H and youth publications)
are available free to Florida residents from county extension offices. Information on
bulk rates or copies for out-of-state purchasers is available from C.M. Hinton, Publications Distribution
Center, IFAS Building 664, University of Florida, Gainesville, Florida 32611. Before publicizing this
publication, editors should contact this address to determine availability. Printed 2/93.