Chapter 9 Irrigation Water Management - USDA

Chapter 9Irrigation Water ManagementPart 652Irrigation Guide(1) Gravimetric or Oven Dry Method: Soilsamples are collected using a coresampler. Samples must be protected fromdrying before they are weighed. Samplesare taken to the office work room,weighed (wet weight), oven dried, andweighed again (dry weight). An electricoven takes 24 hours at 105 degrees Celsiusto adequately remove soil water.Percentage of total soil-water content on adry weight basis is computed. To convertto a volumetric basis, the percentage watercontent is multiplied by the soil bulkdensity. Available soil water is calculatedby subtracting percent total soil water atwilting point. This procedure is the mostaccurate method for determining the soilwater content, but is time consuming andmay be impractical for most farmers.The following equipment is required:(1)Moisture proof seamlessaluminum or tin sample boxes(cans) with a capacity of 3 ouncesor more to contain sample fordrying.(2)Collect a representative soilsample of known volume using acore sampler.Determine the wet weight of thesample (WW).(3)Dry the sample in the oven at1050C to 1150C until it attains aconstant weight. This will takeabout 24 hours. Check the weightat one hour intervals near the endof the 24 hour period. When thereis no weight change, the sample isdry.(4)Determine the dry weight of thesample (DW). Remember todeduct the tare weight of thecontainer when determining wetweight and dry weight.(5)Compute the bulk density (BD)and total soil water content(TSWC):BD DW(g)(dry weight basis)Sample Volume (cc)Weight of water lost,(2)(3)(4)Beam balance with a minimum of500-gram capacity, accurate to 0.1gram.Drying oven, with thermometer,capable of maintaining atemperature of 2200 F to 2400F.(105ºC to 115ºC).A core soil sampler to take bulkdensity samples. Alternatively,the bulk density can bedetermined by the sand conemethod or water balloon method.WAT WW - DWPercent Water Content (dry weightbasis),WC WAT x 100DWTSWC BD x WC (inches water per inch soil100depth)(6)Procedure:(210-vi-NEH 652, IG Amend. NJ1, 06/2005)The total soil water content(TSWC) includes moisture that isnot available to the plant at thepermanent wilting point. Thepermanent wilting point (Pw), theNJ9-5

Chapter 9Irrigation Water ManagementPart 652Irrigation GuideFine sandy loamSandy loamLoamy fine sandLoamy sandFine sandSandpoint at which a plant can nolonger obtain enough soil water tomeet transpiration needs, occurs at15 atmospheres of tension and isnormally determined in thelaboratory. When laboratory dataare not available, one of thefollowing procedures can be usedto estimate the wilting point.(a)This procedure requiresknowledge of the percent clay,less any clay size carbonateparticles, of the soil beingmeasured. For many soils, thisprocedure will provide a closeestimate of the wilting point.The wilting point is calculatedwith use of the followingequation:Pw is expressed as percent wateron a dry weight basis. Thevalues given do not apply tosoils having soil fragments largerthan 2.0 millimeters.(7)To determine the soil watercontent (SWC), Pw percentage isfirst converted to inches of water(WP). WP is then subtracted fromTSWC to obtain SWC.WP BD x Pw (inches water per inch soil depth)100Pw 0.4 x clay (%)SWC TSWC-WP (inches/inch soildepth)Where:Pw is the wilting point,using the clay content,expressed as percent ofwater on a dry weight basis.(b)443322This procedure can be usedwhere only the soil texture isknown. The values givenrepresent and average wiltingpoint for the given texture.TexturePwClaySilty claySandy claySilty clay loamClay loamSandy clay loamSilt loamLoamVery fine sandy loam2519171313115.574To determine the SWC for a givenincrement of depth, multiply SWC by thedepth, in inches, being evaluated.To determine the available water capacity(AWC) for soils not listed in Chapter 2,or for a special case where the data inChapter 2 are not satisfactory, it isnecessary to make soil water contentmeasurements at field capacity. Thesemeasurements are made after an irrigationor effective rainfall. Before making themeasurements, allow about 24 hours forsand and about 48 hours for clay for thegravitational water to drain. DetermineAWC using the above procedure, whereSWC becomes AWC.(210-vi-NEH 652, IG Amend. NJ1, 06/2005)NJ9-6

Irrigation Water ManagementPart 652Irrigation GuideFigure NJ 9.2Soil-water content worksheet (gravimetric method)Chapter 9(210-vi-NEH 652, IG Amend. NJ1, 06/2005)NJ9-7

Chapter 9 Irrigation Water ManagementChemical Drying Method (SpeedyMoisture Tester) This method ofdrying soil samples is based on theprinciple that a given quantity ofmoisture, when combined withcalcium carbide, will react to producea specific volume of gas (acetylene).By applying this principle, a deviceknown as the Speedy Moisture Testerwas developed in England and iscommercially available. It confines ina cylindrical pressure chamber the gasproduced from this reaction. One endof the pressure chamber is equippedwith a cap for inserting the carbide andsoil sample and a clampingarrangement to confine the gas duringthe test. The gas pressure is read on agage located on the other end of thepressure chamber. The gage iscalibrated for a 26-gram wet weightsample of soil and the reading can beconverted readily to a dry weightmoisture percentage by use of acalibration curve supplied with theinstrument, or by use of the conversionchart in Chapter 16.The calcium carbide gas pressure methodis a quick and reasonably accurate way ofdetermining the moisture content of asoil. The time required to dry a sample isabout 3 minutes. Its simplicity of use issuch that anyone can become proficientenough in a short time to make its use aroutine operation.Care must be taken in measuring the 26gram soil sample. When the test isfinished, examine the sample for lumps.If the soil was not completely brokendown by the steel balls, retest andincrease the shake-and-rest time by oneminute. The calcium carbide gas pressuremethod should not be used on saturatedhighly organic soils.Part 652Irrigation Guide Tensiometers: Soil-water potential(tension) is a measure of the amount ofenergy with which water is held in the soil.Tensiometers work on the principle that apartial vacuum is created in a closed chamberwhen water moves out through a porousceramic tip to the surrounding soil. Tensionis measured by a water manometer, amercury manometer, or a vacuum gage. Thescales are generally calibrated in eitherhundredths of an atmosphere or incentimeters of water. Tensiometers thatutilize a mercury manometer are usuallypreferred as research tools because theyafford great precision. Because of theirsimplicity, tensiometers equipped withBourdon vacuum gages are better suited topractical use and to irrigation control.After the cup is placed in the soil at thedesired depth, the instrument must be filledwith water. Water moves through the porouscup until the water in the cup and the water inthe soil reach equilibrium.Any increase in tension that occurs as the soildries causes the vacuum-gage reading, whichcan be read above ground, to increase.Conversely, an increase in soil-water contentreduces tension and lowers the gage reading.The tensiometer continues to recordfluctuations in soil-water content unless thetension exceeds 0.85 atmosphere, at whichpoint air enters the system and the instrumentceases to function. Then after an irrigation orrain, the instrument must again be filled withwater before it can operate.Some experience is required to use atensiometer. If air enters the unit through anyleaks at the rubber connections, measurementsare not reliable. Air leaks can result fromfaulty cups. They may occur also at thecontact points of the setscrews used to securethe porous cup to the metal support. Somemanufacturers provide a test pump that can be(210-vi-NEH 652, IG Amend. NJ1, 06/2005)NJ9-8

Chapter 9Irrigation Water Managementused to test the gage and to remove air fromthe instrument.Tensiometers readings reflect soil-moisturetension only; that is, they indicate the relativewetness of the soil surrounding the porous tip.They do not provide direct information on theamount of water held in the soil. Tensionmeasurements are useful in deciding when toirrigate, but they do not indicate how muchwater should be applied. A special moisturecharacteristic curve for the particular soil isneeded to convert moisture-tensionmeasurements into available-moisturepercentages. Typical curves for soils areshown in Figure NJ 9.3.Guidelines for using soil moisturetension data to schedule irrigationevents.25% MAD 50% MAD(cb)(cb)Coarse Sand1220Sand1220Fine Sand1220Loamy Sand1525Loamy Coarse Sand1525Loamy Fine Sand1525Loamy Very Fine Sand2040Sandy Loam2040Fine Sandy Loam2550Very Fine Loam2550Loam3060Silt Loam4085Sandy Clay Loam4085Clay Loam4590Silty Clay Loam4590Sandy Clay5095Silty Clay5095Clay5095Part 652Irrigation Guideconditions in the wet range. They are bestsuited to use in sandy soils, since in these soilsa large part of the moisture available to plantsis held at a tension of less than I atmosphere.Tensiometers are less well suited to use infine-textured soils, which hold only a smallpart of the available moisture at a tension ofless than 1atmosphere.Tensiometers installed at different rootingdepths have different gauge readings becauseof soil water potential change in rootingdepths. With uniform deep soil, about 70 –80% of soil moisture withdrawal by plantroots is in the upper half of the rooting depth.Recommended depths for setting tensiometersare given in Table NJ 9.4.Table NJ 9.3cb Centi barsTensiometers do not satisfactorily measure theentire range of available moisture in all soiltypes. But they probably are the best fieldinstruments to use to determine moistureTable NJ 9.4Plant rootzone depth(in)Recommended depths for settingtensiometersShallowDeeptensiometer Tensiometer(in)(in)18812241218361224 481836Installing tensiometers must be done carefullyand good maintenance is required for accurateand reliable results. They also must beprotected against freeze damage.Maintenance kits that include a hand vacuumpump are required for servicing tensiometers.The hand pump is used to draw out air bubblesfrom the tensiometer and provide anequilibrium in tension. Tensiometers shouldbe installed in pairs at each site, at one-thirdand two-thirds of the crop rooting depth. Asmall diameter auger (or 1/2” steel water pipe)is required for making a hole to insert thetensiometer. Figure NJ 9.4 shows atensiometer and gauge and illustratesinstallation and vacuum pump servicing.(210-vi-NEH 652, IG Amend. NJ1, 06/2005)NJ9-9

Chapter 9Figure NJ 9.3Irrigation Water ManagementPart 652Irrigation GuideWater retention curves for several soils plotted in terms of percent available water removedAVAILABLE WATER DEPLETION, PERCENT00.1102030405060708090100SOIL SUCTION, BARS (atmospheres)0.51.05.01015(210-vi-NEH 652, IG Amend. NJ1, 06/2005)NJ9-10

Chapter 9Irrigation Water ManagementFigure NJ 9.4Part 652Irrigation GuideTensiometer, installation, guage, and servicingVacuumgaugeSoil lineWater-filledtubeDrive shaped rodto exact depth ofceramic tip, or augerhole aand use soilpaste.PorousceramictipInstallation procedureServicing tensiometer usinga vacuum pumpVacuum guage(210-vi-NEH 652, IG Amend. NJ1, 06/2005)NJ9-11

Chapter 9Irrigation Water ManagementElectrical-Resistance Instruments:These instruments use the principle that achange in moisture content produces a changein some electrical property of the soil or of aninstrument in the soil. They consist of twoelectrodes permanently mounted inconductivity units, usually blocks of plaster ofparis, nylon, fiberglass, gypsum, orcombinations of these materials. Electrodes inthe blocks are attached by wires to a resistanceor conductance meter that measures changesin electrical resistance in the blocks. Whenthe units are buried in the soil, they are inclose contact with soil particles and respond tochanges in soil moisture content. Since theamount of moisture in the blocks determineselectrical resistance, measurement of anychange in resistance is an indirect measure ofsoil moisture if the block is calibrated for aparticular soil.Nylon and fiberglass units are more sensitivein the higher ranges of soil moisture thanplaster of paris blocks, but often their contactwith soil that is alternately wet and dry. is notvery good. Nylon units are most sensitive at atension of less than 2 atmospheres. Plaster ofparis blocks function most effectively at atension between I and 15 atmospheres, andfiberglass units operate satisfactorily over theentire range of available moisture. Acombination of fiber glass and plaster of parisprovides sensitivity in both the wet and dryrange and provides good contact between thesoil and the unit.Electrical-resistance instruments are sensitiveto salts in the soil; fiberglass units are moresensitive than plaster of paris. Their readingsare also affected by concentrations offertilizer. Where fertilizer is spread in bands,the unit should be placed well to one side ofthe bands. Temperature also affects reading inall units, but much less than other sources ofvariation. In some units,Part 652Irrigation Guidecalibration drift has caused changes of asmuch as I atmosphere of tension in a singleseason. The magnitude of a change dependson the number of drying intervals and thenumber of days between each. Readings alsovary with soil type. Since the same readingmay indicate different amounts of availablemoisture for different soil textures, theinstrument must be calibrated for the soil inwhich it is to used. For good accuracy, eachinstrument site shall be calibrated. Due to thepossibility of calibration drift, particularly iffertilizer is applied with irrigation, thecalibration should be checked once or twiceeach growing season. Calibration should bedone with an accurate method, such as thegravimetric or the chemical drying methods.If readings are to be representative of an area,the blocks must be properly installed.Individual blocks must be placed in a hole,which disturbs the soil. If the soil is notreplaced in the hole at the same density and inthe same way as in the rest of the profile, theroot-development and moisture pattern maynot be representative. A good method is toforce the block into undisturbed soil along thesides of the hole dug for placement of theblocks. In one type, the blocks are cast in atapered stake. A tapered hole, the same sizeas the stake, is bored into the ground with aspecial auger. The stake is saturated withwater and then pushed into the hole so thatclose contact is made between the stake andthe soil.Most of the commercial instruments give goodindications of moisture content if they are usedaccording to the manufacturer's instructions. For goodresults, however, the blocks need to be calibrated in thefield for each job. Experience and careful interpretationof instrument readings are needed to get a goodestimate of soil moisture conditions. Electricalconductivity sensors are becoming a popular tool, andare(210-vi-NEH 652, IG Amend. NJ1, 06/2005)NJ9-12

Chapter 9Table NJ 9.5Irrigation Water ManagementPart 652Irrigation GuideInterpretations of readings on typical electrical resistance meterSoil waterMeter readings 1/condition(0—200 scale)Nearly saturated180 — 200InterpretationNear saturated soil often occurs for a few hours following anirrigation. Danger of water logged soils, a high water table, or poorsoil aeration if readings persist for several days.Field capacity170 — 180Excess water has mostly drained out. No need to irrigate. Anyirrigation would move nutrients below irrigation depth (root zone).Irrigation range80 — 120Usual range for starting irrigations. Soil aeration is assured in thisrange. Starting irrigations in this range generally ensures maintainingreadily available soil water at all times.Dry 80This is the stress range; however, crop may not be necessarilydamaged or yield reduced. Some soil water is available for plant use,but is getting dangerously low.1/ Indicative of soil-water condition where the block is located. Judgment should be used to correlate these readings togeneral crop conditions throughout the field. It should be noted, the more sites measured, the more area represented by themeasurements.recommended on finer textured soils with ahigh water holding capacity. Theseinstruments are calibrated to read centibars ofsoil water tension correlated to tensiometerreadings. Since they are actually measuringelectrical conductivity which is converted tocentibars of tension, they can operate in amuch higher tension range resulting in moreaccurate readings. However in sandy soilsthey are not as sensitive as the tensiometerwith a longer response time to soil moisturechanges.Diaelectric Constant Method:The diaelectric constant of material is ameasure of the capacity of a nonconductingmaterial to transmit high frequencyelectromagnetic waves or pulses. Thediaelectric constant of a dry soil is between 2and 5. The diaelectric constant of water is 80at frequency range of 30 MHz — 1 GHz.Relatively small changes in the quantity offree water in the soil have large effects on theelectromagnetic properties of the soil-watermedia. Two approaches developed formeasuring the diaelectric constant of the soilwater media (water content by volume) aretime domain reflectometry (TDR) andfrequency domain reflectometry (FDR).For TDR technology used in measuring soilwater content, the device propagates a highfrequency transverse electromagnetic wavealong a cable attached to parallel conductingprobes inserted into the soil. A TDR soilmeasurement system measures the averagevolumetric soil-water percentage along thelength of a wave guide. Wave guides (parallelpair) must be carefully installed in the soilwith complete soil contact along their entirelength, and the guides must remain parallel.Minimum soil disturbance is required wheninserting probes. This is difficult when usingthe device as a portable device. The devicemust be properly installed and calibrated.Differing soil texture, bulk density, andsalinity do not appear to affect the diaelectricconstant.(210-vi-NEH 652, IG Amend. NJ1, 06/2005)NJ9-13

Chapter 9Irrigation Water ManagementFDR approaches to measurement of soil-watercontent are also known as radio frequency(RF) capacitance technique. This techniqueactually measures soil capacitance. A pair ofelectrodes is inserted into the soil. The soilacts as the diaelectric completing acapacitance circuit, which is part of afeedback loop of a high frequency transistoroscillator. The soil capacitance is related tothe diaelectric constant by the geometry of theelectric field established around theelectrodes. Changes in soil-water contentcause a shift in frequency. University andARS comparison tests have indicated that, assoil salinity increases, sensor moisture valueswere positively skewed, which suggestsreadings were wetter than actual condition.FDR devices commercially available include:Portable hand-push probes—These probesallow rapid, ea

Chapter 9 Irrigation Water Management Part 652 Irrigation Guide (210-vi-NEH 652, IG Amend. NJ1, 06/2005) NJ9-1 NJ652.09 Irrigation Water Management (a) General Irrigation water management is the act of timing and regulating irrigation water applications in a way that will satisfy the