New, L. Leon

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Texas A&amp;M Repository

~_3-)"L-2219"f0~ lu-vJTexas Agricultural Extension Service~-PMJ~ H~iIt§PMJ~-------------'Center Pivot Irrigation SystemsL. Leon New*Water Application EfficiencyTable 1. Approximate friction loss (PSI) in center pivotsprinklers.B. Half-mile system:2,200 146 57 212,500 72 273,000 383,500 60The use of 8- or 10-inch mainlines allows the systemto operate at lower pressure and saves thousands ofdollars in reduced pumping cost during the usable lifeof the system. Table 1 shows approximate pressure lossdifferences for available mainline sizes. System pressureat the pivot point must be adequate to overcome fric-tion pressure loss and provide the desired pressure atthe sprinkler, sprayer or LEPA nozzle. As little as 9 psiis adequate at the pressure regulator just above a LEPAnozzle. For a quarter-mile system carrying 800 GPM,increase the system pressure at the pivot 18 psi for a6-inch mainline or 4 psi for an 8-inch mainline to over-come friction pressure loss. The added friction loss inthe smaller mainline adds 14 psi to the system pressurerequirement.Each additional 10 psi pivot pressure increases fuelcosts about $0.50 per hour or $0.22 per acre-inch foreach 1,000 GPM when natural gas costs $4.00 perMCF. With $0.08 per KWH electricity, the cost is $0.65per hour and $0.30 per acre-inch for each additional10 psi pressure.4578979111417202411141823283339Mainline pipe diameter, inches6 6-5/8 8 10Mainline pressure loss, psiFlow rate, GPMA. Quarter-mile system:6007008009001,0001,1001,200Center pivot irrigation systems, which move in a cir-cular path around a pivot point, can be effective toolsto improve irrigation water management and applica-tion. The self-propelled machine has removed much ofthe labor associated with sprinkler irrigation. However,a center pivot system has a high initial cost and.....majorbenefits in the form of reduced labor, improved waterdistribution and management convenience must berealized to justify the cost. Many improvements havebeen made in center pivot systems in recent years. Ma-jor improvements have been made in drivemechanisms, control devices and structure of themachines. The early machines were moved by watermotors operated from mainline pressures of 90 to 100pounds per square inch (psi) and sprinkled water highinto the air with large evaporation losses. Today, elec-tric or hydraulic motors at each tower drive the machine.Pressure as low as 12 to 15 psi is adequate for an ac-curately designed Low Energy Precision Application(LEPA) center pivot driven by electric motors. Watercan be placed at low pressure only 8 to 15 inches abovethe soil surface between circular crop rows with onlyone to three percent application loss.Options to consider before buying a center pivotsystem include: mainline size, length and outlet spac-ing; type of sprinklers, sprayers or nozzles; and, ·typeof drive mechanism. The choices made may affect theinvestment cost per acre, operating costs and flexibilityfor future changes. Larger systems usually cost less peracre than smaller systems. Large mainline sizes costmore than small ones, but have lower operating costs.An outlet spacing of 60 or 80 inches (a multiple of rowspacing) along the mainline is needed if LEPA nozzleswill be used.Plan new center pivots to operate at minimumpressure to minimize pumping cost. Each additional 10psi pivot pressure requires approximately 10horsepower for each 1,000 gallons per minute pumpedinto the machine. Horsepower is proportional to systemflow rates of more or less than 1,000 GPM. For exam-ple, seven horsepower is needed for each 10 psi pivotpressure when the system flow rate is 700 GPM.*Extension agricultural engineer-irrigation" The Texas A&MUniversity System.Manufacturers of center pivot water application com-ponents are improving equipment and refining systemTexas Agricultural Extension Service • Zerle L. Carpenter, Director • The Texas A&M University System • College Station, Texasdesign and nozzle selection techniques to reduce waterlosses. However, accurate measurement of availableflow rate (gallons per minute) is essential for correctsystem design. Nozzles must be sized precisely to applythe water uniformly. Performance is best when thesystem is operated as designed, especially at the cor-rect water pressure. Improved technology produces littleor no benefit if the design input information is inaccurateor the system is not operated properly.Field tests on the High Plains indicate that centerpivots improve water application efficiency enough toirrigate 20 to 25 percent more acreage than can becovered with furrow irrigation with the same water. Ir-rigation time of 16 to 17 hours per acre per year withfurrow irrigation was reduced to 12 to 13 hours per acreper year with center pivot units. Crop yields weresimilar. Also, center pivots equipped with LEPA are ex-pected to make 20 to 25 percent more water (100 to150 GPM in a typical quarter mile center pivot) availableto the crop than pivots equipped with impact sprinklersor with spray nozzles on drops. This comparison isshown in Table 2.speed of the center pivot, which is the basic means ofcontrolling the amount of water (inches) applied at eachirrigation. Runoff is less likely when small amounts ofwater are applied, but the amount applied must be ade-quate to meet crop needs and wet the desired soildepth.Water application amounts for a typical quarter-milecenter pivot that irrigates 120 acres are shown in Table3. One to 1.5 inches, normally an optimum irrigationamount, usually provides sufficient water for the cropuntil the machine can complete a circle, minimizesrunoff and avoids early season depletion of subsoilmoisture ideally reserved for high crop water useperiods. Application losses are normally less and cropresponse is better from a single 1.5 inch irrigation thanfrom two 0.75 inch irrigations. However, smaller andmore frequent irrigations may be appropriate in somecases with LEPA irrigation. Use formulas following Table3 to calculate water application for a center pivot systemof any length.TABLE 3. Inches water applied by 1,290-foot center pivot. *Table 2. Pumping rate needed to irrigate with center pivotsequipped with conventional impact or spray nozzles versusLEPA nozzles, gallons per minute.0.25 740 3,000 600 2,4500.30 880 3,620 725 2,9600.35 1,040 4,230 840 3,4200.40 1,175 4,800 965 3,9300.45 1,325 5,400 1,080 4,4400.50 1,475 6,000 1,200 4,900Applicationrate,inch/dayConventional(80% efficiency) LEPA(98% efficiency)Mainline length, miles Mainline length, milesQuarter Half Quarter HalfPivot Hours to complete 120-acre circleGPM 12 24 48 72 96 120400 ,09 .18 .36 .53 .71 .89500 .11 .22 .44 .67 .89 1.11600 .13 .27 .53 .80 1.06 1.33700 .16 .31 .62 .93 1.24 1.55800 .18 .36 .71 1.07 1.42 1.78900 .20 .40 .80 1.20 1.60 2.001,000 .22 .44 .89 1.33 1.78 2.221,100 .24 .49 .98 1.47 1.95 2.44Endtower 667 334 167 111 83 67ftIhrAc/hr 10 5.0 2.5 1.7 1.3 1.0* 1,275 feet from pivot to end tower + 15-foot end section.Runoff Losses Calculations for other length pivots can be made using the formulasbelow.Distance from pivot to outside wetting patternLEPA Irrigation4. Feet end of machine must move per acre =871201. Inches = Pivot GPM x hours to complete circle450 x acres in circleAcres in circleHours to complete circle2. Acres per hour =Low Energy Precision Application (LEPA) irrigationwith center pivots is one of the most efficient irrigationmethods available today. LEPA irrigation offers both3. End tower speed In feet per hour =Distance from pivot to end tower in feet x 2 x 3.14Hours to make circleA major problem when using center pivots is waterrunoff. Water intake rates of some clay loam, silty clayloam and clay soils are less than the irrigation applica-tion rate. Runoff is normally less on sandy and sandyloam soils. Developments in center pivot waterapplicators that permit lower mainline pressure, whilereducing the evaporation rate, also tend to concentratewater on a smaller area and thus incre"ase the water ap-plication rate (inches per hour) on the area covered.The wetted pattern is smaller using spray nozzles thanwith impact sprinklers. LEPA applicators apply waterover a smaller soil surface area than spray nozzles.Reductions in runoff have been achieved by furrowdiking and farming in a circular pattern, both integralparts of the LEPA concept. (To farm in a circularpattern, run the machine around the circle without waterand use the wheel tracks as row gUides.) Other methodsof redUcing runoff include deep chiseling of clay sub-soils, adding organic matter to the soil and using tillagepractices that leave the soil open. All of these methodsare being used in conjunction with variations in the travelhigh water application efficiency and low operatingpressure. The combined benefits often warrant the pur-chase cost of LEPA equipment to convert existing centerpivots. The extra cost of LEPA on a new center pivotis even more easily justified and should be consideredcarefully by prospective buyers. Increased crop yield isa third potential benefit, especially where irrigation wateris limited.The precisely planned irrigation system uses specialLEPA applicators which release water 8 to 15 inchesabove the ground. This low release point greatly reduceswater evaporation losses. Water application losses fromLEPA are only two to three percent compared to 20to 25 percent from typical impact sprinklers and lowpressure spray nozzles. The lower application lossmeans about 20 percent more water is available for cropuse.System operating pressure can be lower since LEPAheads are located close to the ground and are design-ed for low pressure. Fuel consumption and cost willaverage 15 to 20 percent less than that of typical centerpivots equipped with low pressure nozzles but about 10percent more per acre inch of water pumped than fur-row irrigation. Actual percentage fuel saving dependsupon the reduction in total amount of water pumpedand in total pumping head, including pumping lift.To convert existing center pivots to LEPA, attach newclamp-on drops to the center pivot mainline to applywater between alternate crop rows planted in a circle.To do this, install extra plumbing from existing wateroutlets on the mainline (usually spaced 81j2 to 10 feetapart) to the clamp-on drops attached 60 or 80 inchesapart, depending upon crop row spacing. Somevegetable crops require irrigation in every furrow andthe drop spacing must match row spacing. Connectspecial LEPA heads to the drops with a flexible hoselong enough to release water 8 to 15 inches above theground. Remove drops for conventional spray nozzlesor impact sprinklers.Conversion to LEPA can also be accomplished bywelding threaded 3/4-inch female pipe couplings to theexisting mainline. Since welding destroys the galvaniz-ed coating, welded couplings are most applicable forungalvanized mainlines. Existing goose-necks and dropscan be used with the welded couplings.Center pivots can be purchased from most manufac-turers with outlets spaced 60 or 80 inches apart on themainline. Regular goose-necks and drops can then beused. The LEPA head is connected to the drop withflexible hose so that it is located at the desired height.A pivot span length that accommodates an evennumber of rows is desirable.A minimum pressure of 4 psi at the end of themainline, when it is located at the high point, is ade-quate when the pressure ~egulator is installed just abovethe LEPA head. A pressure regulator is normally in-stalled with each LEPA head but may not be necessaryfor very level fields. The design objective is 9 psi at theinlet of a six psi regulator. A ground level pivot pressureof 15 psi can be used for many quarter-mile systemson level land. LEPA heads normally apply water in abubble-shaped pattern 12 to 15 inches in diameter, butthey can be set to spray to germinate seed, pre-plantirrigate or apply chemicals.Combined savings in fuel costs resulting from lowerpivot pressure and reduced operating time (LEPA canapply the same amount of effective water in less time)usually will pay the cost of converting an existing centerpivot to LEPA in 1 to 3 years. The extra cost to equipa new center pivot is usually regained in 1 year or less.LEPA should be strongly considered when buying anew center pivot.Water application rates are higher with LEPA irriga-tion because water is applied over a smaller area of soil.Runoff will likely occur, especially on clay soils, withoutfurrow dikes, deep chiseling in the furrow or other tillagepractices to improve the water infiltration rate of the soil.Farming in a circle helps control runoff by holding waterin the furrow and is strongly recommended for all centerpivots. Circular farming is essential when growing cornirrigated with LEPA because the heavy growth causesthe LEPA bubblers to ride up in the corn stalks whendragged across straight rows.Inches of water applied to the smaller wetted soil sur-face areas using LEPA irrigation are described in Table4. These rates are representative of quarter mile centerpivots that deliver 800 GPM with LEPA drops spaced80 inches apart. The high rate of water application,especially with LEPA applicators in bubble position, in-dicates the need to use tillage practices that improvesoil water intake rates.Table 4. Inches of water applied to wetted soli surface andequivalent irrigation (inches) applied to total area as a func-tion of center pivot speed with maximum single nozzle waterdelivery of 8 GPM. 800 GPM delivered to mainline 1,300 feetlong (10-foot end section). 122 acres. LEPA nozzle spacing80 inches.Time to Diameter of wettedComplete circle soil surfaceEnd tower· Bubble Spray EquivalentDays Hours Speed-ftIhr 15" 80" irrigation1 24 338 1.9 0.35 0.3536 225 2.8 0.5 0.52 48 169 3.7 0.7 0.760 135 4.7 0.9 0.93 72 113 5.6 1.1 1.184 96 6.5 1.2 1.24 96 84 7.5 1.4 1.4108 75 8.4 1.6 1.65 120 68 9.3 1.7 1.7132 61 10.3 1.9 1.96 144 56 11.2 2.1 2.1* 1,290 feet from pivot to end tower.The rate of water application to the wetted soil sur-face and the equivalent irrigation amount in inches fora pivot system of any length and any LEPA nozzle spac-ing can be determined with the follOWing formulas.Acres in circle =(length of mainline in feet)2 X 3.1443.560Inches water applied =GPM delivered to pivot x hours to complete circle450 x acres in circleInches applied to wetted soil =Inches water applied x LEPA nozzle spacing in inchesDiameter of soil area wetted by LEPA nozzle in inchesInches applied to wetted soil =1.73 inches x 60 inches = 6.94 inches15 inchesNote that the "inches of water applied," or equivalentirrigation, is the same as the "inches applied to wettedsoil" when the diameter of the soil area wetted by anindividual LEPA nozzle is the same as the LEPA nozzlespacing.Acres in circle = (1200)2 x 3.14 = 103.8 acres43,560To illustrate the use of the formulas, assume the pivotmainline is 1,200 feet long, LEPA nozzle spacing is 60inches, nozzles are set to apply water in a IS-inchdiameter bubble, 675 gallons per minute are deliveredto the pivot and a circle is completed in 120 hours.Inches water applied =675 GPM x 120 hours450 x 103.8 acres1.73 inchesSummaryCenter pivot systems offer improved water distribu-tion and application efficiency and flexibility in manag-ing irrigation water. Several options are available andshould be considered carefully before purchasing asystem. The LEPA system is a new, attractive optionfor improved irrigation efficiency. Correctly designedand operated center pivot systems are being used suc-cessfully by many farmers. However, a center pivot ir-rigation system is not necessarily the best system forevery farm situation.ENG 9-3Published by the Texas Agricultural Extension Service (TAEX)and the Texas Agricultural Experiment Station (TAES) incooperation with the Texas Water Resources Institute (TWRI).Educational programs conducted by the Texas Agricultural Extension Service serve people of all ages regardless of socioeconomic level racecolor, sex, religion, handicap or national origin. ' ,Issued in furtherance. of Cooper~tive ~xtension .Work in Agriculture and Home Economics, Acts of Congress of May 8, 1914, as amended,a.nd June.30, 1914, In cooperation WIth the Untted States Department of Agriculture. Zerle L. Carpenter, Director, Texas Agricultural Exten-sIon ServIce, The Texas A&M University System.40M-8-86, New

Center Pivot Irrigation Systems.

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Center Pivot Irrigation Systems.

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Center Pivot Irrigation Systems.

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