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contents pivot design choices .3 costs .3 types of drive systems .3 electric .3 hydraulic .4 which is better .4 wheel and drive options .4 design printout .5 system capacity .8 mainline pipe sizing .8 telescoping .8 pressure regulators .10 water applicators .10 pads .10 impact sprinklers .11 low-pressure applicators .11 mesa .11 lesa .12 lepa .12 converting existing pivots .12 required accessories .13 other considerations .14 pivot management .14 runoff control .14 irrigation scheduling .15 et-based .15 soil moisture-based .15 chemigation .16 fertigation .17 suggested reading .18[close]
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leon new professor and extension agricultural engineer irrigation guy fipps professor and extension agricultural engineer the texas a&m university system the center pivot is the system of choice for agricultural irrigation because of its low labor and maintenance requirements convenience flexibility performance and easy operation when properly designed and operated and equipped with high efficiency water applicators a center pivot system conserves three precious resources water energy and time manufacturers have recently improved center pivot drive mechanisms motors gears and shafts control devices optional mainline pipe sizes and outlet spacings span lengths and structural strength the first pivots produced in the 1950s were propelled by water motors they operated at high pressures of 80 to 100 psi and were equipped with impact sprinklers and end guns that sprayed water toward the sky resulting in significant evaporation losses and high energy use today pivots are driven by electric or oil hydraulic motors located at each tower and guided by a central control panel pressures as low as 10 to 15 psi at the pivot mainline are usually adequate for properly designed lesa low elevation spray application and lepa low energy precision application pivots that are 1/4 mile long operating on level to moderately sloping fields water application efficiency with such systems is 85 to 98 percent duced with 12 to 13 hours per acre per year lepa and lesa applicators further reduce irrigation to an average of 10 to 11 hours per acre per year costs a quarter-mile 1,300-foot system that irrigates about 120 acres typically costs $325 to $375 per acre excluding the cost of groundwater well construction turbine pumps and power units longer systems usually cost less on a per-acre basis for example half-mile systems 2,600 feet that irrigate approximately 500 acres cost about $200 to $250 per acre this relatively high cost is often offset by a number of advantages including reduced labor and tillage improved water distribution more efficient pumping lower water requirements more timely irrigation and convenience programmable control panels and remote control via phone lines or radio can start and stop irrigations identify location increase or decrease travel speed and reverse direction fertilizers and certain plant protection chemicals can be applied through the center pivot which increases the value and use of the system programmable injection unit control monitoring and safety are compatible with center pivot control systems towable pivot machines are a vailable so that additional tracts of land can be irrigated with the same machine when considering a towable machine remember that sufficient water is needed to irrigate all tracts plan the irrigated circle and position the pivot so that it can be moved to drier soil at the location from and in the path in which it is to be towed pivot design choices when purchasing a center pivot system one must select sssss mainline size and outlet spacing length including the number of towers drive mechanisms application rate of the pivot and the type of water applicator types of drive systems electric in electric drive pivots individual electric motors usually 1.0 or 1.5 horsepower power the two wheels at each tower fig 1 typically the outermost tower moves to its next position and stops then each succeeding tower moves into alignment thus at any time a tower can be in motion where electricity is provided by on-site generation the generator must operate continuously the rotation speed or travel time of the pivot depends on the speed of the outermost tower and determines the amount of water that is applied the operator selects the tower speed using the central power control panel normally located at the pivot point at the 100 percent setting the end tower moves 3 these choices affect investment and operating costs irrigation efficiency and crop production wise decisions will result in responsible water management and conservation flexibility for future changes and low operating costs switching from furrow to pivot irrigation can save water and money for example on the texas high plains field measurements show that corn is irrigated an average of 16 to 17 hours per acre per year with furrow irrigation with center pivot mesa irrigation over canopy applicators similar corn yields are pro-[close]
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figure 1a electric drive figure 1b electric drive continuously at the 50 percent setting the outer tower moves 30 seconds and stops 30 seconds each minute etc the speed options on most central power control panels range from approximately 2 to 100 percent hydraulic with oil hydraulic drive systems all towers remain in continuous motion the outermost tower speed is the greatest and each succeeding tower moves continuously at proportionally reduced speeds as with electric drive machines the center pivot travel speed is selected at a central control it is a master control valve that increases or decreases oil flow to the hydraulic motor/s on the last tower two motors per tower are used with the planetary drive one for each wheel fig 2 one motor per tower powers the optional worm drive assembly not shown the required hydraulic oil pressure 1,500 to 1,800 psi is maintained by a central pump usually located near the pivot pad the central pump may be powered by natural gas diesel or electricity the number of towers and maximum travel speed determine the hydraulic oil flow and the central pump power requirement which usually ranges from 7.5 to 25 horsepower for quartermile systems additional site specific travel speed options are available contact your local dealer for more information which is better theoretically continuous move systems provide greater irrigation uniformity however other factors influence uniformity including travel speed and thus the amount of water applied system design type of water applicator and operator management in combination with the amount of water gallons per minute or gpm the machine is nozzled to deliver in field tests both electric and hydraulic drive systems work well the choice is often guided by available power sources personal preference in servicing and maintaining the system the service history of local dealers what is being sold in the local market and why purchase price and dependability wheel and drive options the travel speed is determined by the wheel size in combination with the power drive mechanism and is set at the central control panel the speed of the pivot determines the amount of water applied as specified on the corresponding system design precipitation chart see the following discussions on the system design precipitation chart and system management as related to travel speed gear drives should be checked 4 figure 2 hydraulic move.[close]
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for proper oil levels and any water in the gear boxes removed at least once each year electric power drive has two gear reductions one gear reduction is in the drive shafts connecting the electric motor to a gear box located at each of the two tower wheels the second gear reduction is the gear box driving each wheel the maximum center pivot travel speed depends on the s design printout the design computer printout provides required information about the center pivot and how it will perform on a particular tract of land a portion of a typical design printout is shown in figure 3 it includes s the pivot design flow rate or system capacity in gpm irrigated acreage under the pivot elevation changes in the field as measured from the pivot point operating pressure and mainline friction losses the pressure regulator rating in psi if used the type of water applicator spacing and position from the mainline nozzle size for each applicator water applicator nozzle pressure maximum travel speed and the precipitation chart electric motor speed or rotation in revolutions per minute rpm speed reduction ratios in both the center drive shafts and gear boxes and wheel size sssssss table 1 gives examples of electric center drive and gear box reductions wheel circumference travel distance for each revolution and representative maximum travel speed in feet per hour hydraulic drive pivots have one gear reduction two configurations are used a hydraulic motor in each wheel hub or a single motor located at one wheel coupled to a right angle gear drive with a connecting drive shaft that also po wers the second wheel a hydraulic valve meters oil flow to each set of drives at each tower to maintain system alignment total oil flow is determined by the travel speed number of drive units towers gear reduction and tire size table 1 lists typical hydraulic drive center pivot oil pump horsepower tire size and end tower travel speed ssssa sample precipitation chart is shown in figure 4 it identifies irrigation amounts in inches of water applied for optional travel speed settings gear reduction ratios and tire size it corresponds with figure 3 table 1 typical gear reduction wheel drive rpm and maximum end tower travel speed motor rpm 1740 1740 3450 center drive ratio 58:1 40:1 40:1 gear box ratio 52:1 50:1 52:1 wheel diam.inch rim 24 24 38 rim tire 40 40 54 rim tire circum ft 10.47 10.47 14.13 last wheel drive rpm .5769 .8700 1.6586 end tower feet per hour 362 546 1406 drive electric electric electric hi-speed no towers hydraulic hydraulic hydraulic hi-speed hydraulic hi-speed 8 8 8 18 hydraulic pump drive hp 10 15 25 25 tire size 16.9 x 24 14.9 x 24 11.2 x 38 11.2 x 38 rim tire circum ft 10.47 10.47 14.13 14.13 last wheel drive rpm .5730 .9312 1.5723 .6286 end tower feet per hour 360 585 1333 533 5[close]
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it is essential that correct information about available water supply in gpm and changes in field elevation are used in designing the pivot so that accurate irrigation amounts operating pressure requirements and the need for pressure regulators can be determined give this information to your dealer and then inspect the resulting computer design printout before placing your order to ensure that the system is designed to accommodate your site conditions and will perform as expected always look at the design mainline operating pressure at the pad to determine if it is what you want if not inquire about ways to lower it minimize pumping cost for a pivot nozzled at 1,000 gpm rules of thumb are as follows s each additional 10 psi pivot pressure requires approximately 10 horsepower each additional 10 psi pivot pressure increases fuel costs about $0.35 per hour or $0.16 per acre-inch at natural gas costs of $3.00 per mcf at $0.07 per kwh for electricity the cost is $0.60 per hour 0.27 per acre-inch for each additional 10 psi pressure it costs $0.48 per hour 0.22 per acre-inch for each additional 10 psi pressure for diesel priced at $0.80 per gallon s s system capacity system irrigation capacity is determined by the gallons per minute gpm and the number of acres irrigated system capacity is expressed in terms of either the total flow rate in gpm or the application rate in gpm per acre knowing the capacity in gpm per acre helps in irrigation water management table 2 shows the relationship between gpm per acre and irrigation amounts these irrigation amounts apply for all irrigation systems with the same capacity in gpm per acre the amounts do not include application losses and are for systems operating 24 hours a day to determine your system s capacity select the desired irrigation amounts in inches and multiply the corresponding gpm per acre by the number of acres you are irrigating for example if you irrigate 120 acres with 4 gpm per acre 480 gpm 120 acres x 4 gpm per acre are required to apply 0.21 inches per day 1.50 inches per week and 6.40 inches in 30 days s note horsepower is proportional to system flow rates of 1,000 gpm for example when the system flow rate is 700 gpm 7 horsepower is needed for each 10 psi pivot pressure table 3 lists friction pressure losses for different mainline sizes and flow rates total friction pressure in the pivot mainline for quarter-mile systems table 3 section a on flat to moderately sloping fields should not exceed 10 psi therefore s for flows up to approximately 750 gpm 6 5/8-inch diameter mainline can be used friction pressure loss exceeds 10 psi when more than 575 gpm is distributed through 6-inch mainlines some 8-inch spans should be used when 800 gpm or more are delivered by a quarter-mile system s s mainline pipe sizing mainline pipe size influences the total operating cost smaller pipe sizes while less expensive to purchase may have higher water flow friction pressure loss resulting in higher energy costs plan new center pivots to operate at minimum operating pressure to for center pivots 1,500 feet long table 3 section b 6 5/8-inch mainline can be used for 700 gpm while keeping friction pressure loss under 10 psi some dealers may undersize the mainline in order to reduce their bids especially when table 2 daily and seasonal irrigation capacity pushed to give the best price check the proposed design printout if operating gpm inches in irrigation days pressure appears high ask the dealer to acre inch/day inch/week 30 45 60 80 100 provide another design using propor 1.5 .08 .55 2.4 3.8 4.8 6.4 8.0 tional lengths usually in spans of larger pipe or to telescope pipe see below to 2.0 .11 .75 3.2 4.8 6.4 8.5 10.6 reduce operating pressure table 3 sec3.0 .16 1.10 4.8 7.2 9.5 12.7 15.9 tion c shows how friction and operating pressure for half-mile systems can be 4.0 .21 1.50 6.4 9.5 12.7 17.0 21.2 reduced with size 8 and 10-inch mainline pipe saving money on the initial 5.0 .27 1.85 8.0 11.9 15.9 21.2 26.5 purchase price often means paying 6.0 .32 2.25 9.5 14.3 19.1 25.4 31.8 more in energy costs over the life of the system 7.0 .37 2.60 11.1 16.7 22.6 29.7 37.1 8.0 .42 2.97 12.7 19.1 25.4 33.9 42.4 telescoping telescoping involves using larger mainline pipe at the beginning and then 8[close]
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table 3 approximate friction loss psi in center pivot mainlines mainline pipe diameter inches 6 6 5/8 8 10 mainline pressure loss psi flow rate gpm a quarter-mile system 500 600 700 800 900 1000 1100 1200 b 1500-foot system 600 700 800 900 c half-mile system 1600 2000 2400 2800 8 11 14 18 23 28 33 39 5 7 9 11 14 17 20 24 4 smaller sizes as the water flow rate gpm decreases away from the pivot point typical mainline sizes are 10 8 1/2 8 6 5/8 and 6 inches mainline pipe size governs options in span length the distance between adjoining towers span length options are usually s s 100 to 130 feet for 10-inch 130 to 160 feet for 8 1/2 and 8-inch and 160 to 200 feet for 6 5/8 and 6-inch s 5 7 8 9 13 16 21 26 8 10 13 16 3 4 5 6 134 83 125 31 48 67 10 15 22 29 telescoping mainline pipe size is a method of planning a center pivot for minimum water flow friction loss and low operating pressure and thus lower pumping costs telescoping uses a combination of pipe sizes based on the amount of water gpm flowing through telescoping is usually accomplished in whole span lengths its importance increases with both higher flow rates gpm and longer center pivot lengths dealers use computer telescoping programs to select mainline pipe size for lowest purchase price and operating costs if your dealer does not offer this technology request it table 4 shows examples of telescoping mainline size to manage friction pressure loss example 1 shows that for a center pivot 1,316 feet long fric table 4 telescoping to reduce mainline friction pressure with outlets spaced at 60 inches gpm example 1 1100 1100 example 2 2500 2500 2500 2500 2500 0 0 897 1057 1697 0 897 0 640 0 1697 800 800 540 540 927 927 927 387 387 2624 2624 2624 2624 2624 73 63 48 32 25 0 0 0 0 0 640 1316 676 1316 1316 19 10 10-inch feet of mainline size 8 1/2-inch 8-inch 6 5/8-inch total feet friction pressure psi 9[close]
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tion pressure loss is reduced from 19 to10 psi by using 640 feet of 8-inch mainline rather than all 6 5/8-inch to deliver 1,100 gpm example 2 lists friction pressure losses for various lengths and combinations of mainline pipe size for the delivery of 2,500 gpm by a 2,624-foot system irrigating 496 acres friction pressure loss is reduced from 73 to 25 psi by using more 10 and 8 and less 6 5/8-inch mainline pipe when designing your system compare the higher cost of larger mainline pipe to the increased pumping costs associated with smaller pipe higher pumping costs are caused by higher operating pressure requirements total operation pressure is the sum of friction and system design pressures and terrain elevation pressure gauges located at the pivot pad and on the last applicator drop will identify system operating pressure shows how variations in terrain elevations influence mainline operating pressure elevation changes in the field have the largest impact with lower design pressures from the first to last drop on a pivot the operating pressure at the nozzle should not vary more than 20 percent from the design operating pressure without regulators operating pressure and pumping cost usually will not increase significantly if the elevation does not change more than 5 feet from the pad to the end of the pivot where elevation changes are greater than 5 feet the choice is to increase operating pressure and probably pumping costs or to use pressure regulators this decision is site specific and should be made by comparing the extra costs of pressure regulators to the increased pumping costs without them note as shown in table 5 every additional 2.3 feet of elevation requires an additional 1 psi of operating pressure where the water flow rate and thus the operating pressure vary significantly during the growing season perhaps from seasonal variations in groundwater pumping levels the design flow rate or system capacity and the use of pressure regulators should be evaluated carefully if water pressure drops below that required to operate the regulators then poor water application and uniformity will result in contrast if the design operating pressure is high pumping costs will be unnecessarily high when operating pressure decreases to less than required the solution is to renozzle for the reduced gallons per minute the amount of water flow in the mainline decreases or increases operating pressure for the nozzles installed pressure regulators pressure regulators are pressure killers the y reduce pressure at the water delivery nozzle so that the appropriate amount of water is applied by each applicator selection of nozzle size is based on the rated delivery psi of the pressure regulators nozzles used with 10 psi regulators are smaller than those used with 6 psi regulators when the same amount of water is applied low rated psi pressure regulators if used allow center pivot design to be appropriate at minimum operating pressure pressure regulators require energy to function properly water pressure losses within the regulator can be 3 psi or more so entrance or inlet water pressure should be 3 psi more than the regulator rating six-psi regulators should have 9 psi at the inlet 10-psi regulators 13 psi 15-psi regulators 18 psi and 20-psi regulators 23 psi regulators do not function properly when operating pressure is less than their rating plus 3 psi pressure regulator operating inlet pressure should be monitored with a table 5 percent variation in system operating pressure created gauge installed upstream adjacent to by changes in land elevation for a quarter-mile pivot maintain the regulator in the last drop at the less than 20 percent variation outer end and should be checked system design pressure psi when the machine is upslope another 6 10 20 30 40 gauge located in the first drop in span elevation change one will monitor operating pressure feet psi variation when the center pivot is located on 2.3 1 16.5 10.0 5.0 3.3 2.5 downslope terrain 4.6 2 33.0 20.0 10.0 6.6 5.0 pressure regulator psi rating influ6.9 3 50.0 30.0 15.0 10.0 7.5 ences system design appropriate operating pressure the total energy 9.2 4 40.0 20.0 13.3 10.0 requirements and the costs of pivot 11.5 5 50.0 25.0 16.6 12.5 irrigation see the discussion of water applicator arrangement for more infor13.9 6 30.0 20.0 15.0 mation on pressure regulators 16.2 7 23.3 17.5 as with other spray and sprinkler 18.5 8 26.6 20.0 systems pressure regulators are not necessarily needed for all sites table 5 pressure at the nozzle 10[close]
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water applicators pads there are various types of spray applicators available each with pad options low-pressure spray applicators can be used with flat concave or convex pads that direct the water spray pattern horizontally upwards and downwards at minimum angles spray applicator pads also vary in the number and depth of grooves they have and thus in the size of water droplets they produce fine droplets may reduce erosion and runoff but are less efficient because of their susceptibility to evaporation and wind drift some growers prefer to use coarse pads that produce large droplets and control runoff and erosion with agronomic and management practices there is little published data on the performance of various pad arrangements in the absence of personal experience and local information following the manufacturer s recommen dations is likely the best strategy in choosing pad configuration pads are very inexpensive some growers purchase several groove configurations and experiment to determine which works best in their operation impact sprinklers high-pressure impact sprinklers mounted on the center pivot mainline were prevalent in the 1960s when energy prices were low and water conservation did not seem so important now high-pressure impacts are recommended only for special situations such as the land application of wastewater where large nozzles and high evaporation can be beneficial impact sprinklers are usually installed directly on the mainline and release water upward at 15 to 27 degrees undistorted water pattern diameters normally range from 50 to more than 100 feet water application losses average 25 to 35 percent or more lowangle 7-degree sprinklers reduce water loss and pattern diameter somewhat but do not significantly decrease operating pressure end guns are not recommended because they are higher volume gpm impact sprinklers with lower application and distribution efficiencies and high energy requirements low-pressure applicators very few center pivots in texas are now equipped with impact sprinklers there are improved applicators and design technology for more responsible irrigation water management these new applicators operate with low water pressure and work well with current center pivot designs low-pressure applicators require less energy and when appropriately positioned ensure that most of the water pumped gets to the crop the choice is which low-pressure applicator to use and how close to ground level the nozzles can be generally the lower the operating pressure requirements the better when applicators are spaced 60 to 80 inches apart nozzle operating pressure can be as low as 6 psi but more applicators are required than with wider spacings 15 to 30 feet water application is most efficient when applicators are positioned 16 to 18 inches above ground level so that water is applied within the crop canopy spray bubble or direct soil discharge modes can be used field testing has shown that when there is no wind low-pressure applicators positioned 5 to 7 feet above ground can apply water with up to 90 percent efficiency however as the wind speed increases the amount of water lost to evaporation increases rapidly in one study wind speeds of 15 and 20 miles per hour created evaporative losses of 17 and 30 percent respectively in another study on the southern high plains of texas water loss from a linear-move system was as high as 94 percent when wind speed averaged 22 miles per hour with gusts of 34 miles per hour evaporation loss is significantly influenced by wind speed relative humidity and temperature the following sections describe three types of lowpressure application systems that can significantly reduce operating pressure and deliver most of the water pumped for crop production mesa with mid-elevation spray application mesa water applicators are located approximately midway between the mainline and ground level water is applied above the crop canopy even on tall crops such as corn and sugar cane rigid drops or flexible drop hoses are attached to the mainline gooseneck or furrow arm and extend down to the water applicator fig 5 weights should be used in combination with flexible drop hose nozzle pressure varies depending on the type of water applicator figure 5 drop arrangement and pad arrange 11[close]
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ment selected while some applicators require 20 to 30 psi operating pressure improved designs require only 6 to10 psi for conventional 8 1/2 to 10-foot mainline outlet and drop spacing operating pressures can be lowered to 6 psi or less when spray applicators are positioned 60 to 80 inches apart with wider spacings such as for wobbler and rotator applicators manufacturers recommended nozzle operating pressure is greater research has shown that in corn production 10 to 12 percent of the water applied by above-canopy irrigation is lost by wetting the foliage more is lost to evaporation field comparisons indicate that there is 20 to 25 percent more water loss from mesa abovecrop-canopy irrigation than from lesa and lepa within-crop-canopy center pivot systems lesa low elevation spray application lesa applicators are positioned 12 to 18 inches above ground level or high enough to allow space for wheel tracking less crop foliage is wet especially when planted in a circle and less water is lost to evaporation lesa applicators are usually spaced 60 to 80 inches apart corresponding to two crop rows the usual arrangement is illustrated in figure 6 each applicator is attached to a flexible drop hose which is connected to a gooseneck or furrow arm on the mainline fig 7 weights help stabilize the applicator in wind and allow it to work through plants in straight crop rows nozzle pressure as low as 6 psi is best with the correct choice of water applicator water application efficiency usually averages 85 to 90 percent but may be less in more open lower profile crops such as cotton lesa center pivots can be converted easily to lepa with an applicator adapter that includes a connection to attach a drag sock or hose the optimal spacing for lesa drops is no wider figure 6 drops with lesa applica than 80 inches tors with appropriate installation and management lesa drops spaced on earlier conventional 8 1/2 to 10-foot spacing can be successful corn should be planted in circle rows and water sprayed figure 7 lesa applicator underneath the 12 primary foliage some growers have been successful using lesa irrigation in straight corn rows at conventional outlet spacing when using a flat coarse pad that sprays water horizontally grain sorghum and soybeans also can be planted in straight rows in wheat when plant foliage causes significantly uneven water distribution swing the applicator over the truss rod to raise it note when buying a new center pivot choose a mainline outlet spacing of 60 to 80 inches corresponding to two row widths lepa low energy precision application lepa irrigation discharges water between alternate crop rows planted in a circle water is applied with s applicators located 12 to 18 inches above ground level which apply water in a bubble pattern or drag socks or hoses that release water on the ground s socks help reduce furrow erosion double-ended socks are designed to protect and maintain furrow dikes fig 8 drag sock and hose adapters can be removed from the applicator and a spray or chemigation pad attached in its place when needed another product the lepa quad applicator delivers a bubble water pattern fig 9 that can be reset to optional spray for germination chemigation and other in-field adjustments fig 10 lepa applicators typically are placed 60 to 80 inch es apart corresponding to twice the row spacing thus one row middle is wet and one is dry dry middles allow more rainfall to be stored applicators are arranged to maintain a dry row for the pivot wheels when the crop is planted in a circle research and field tests show that crop production is the same whether water is applied in every furrow or in alternate furrows applicator nozzle operating pressure is typically 6 psi field tests show that with lepa 95 to 98 percent of the irrigation water pumped gets to the crop water application is precise and concentrated which requires a higher degree of planning and management especially with clay soil center pivots equipped with lepa applicators provide maximum water application efficiency at minimum operating pressure lepa can be used successfully in circles or in straight rows it is especially beneficial for low profile crops such as cotton and peanuts and even more beneficial where water is limited converting existing pivots to lepa water outlets on older center pivot mainlines are typically spaced 8 1/2 to 10 feet apart because lepa drops are placed between every other crop row additional outlets are needed for example for row spac-[close]
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ings of 30 inches drops are needed every 60 inches 5 feet likewise for 36-inch row spacings drops are placed every 72 inches 6 feet two methods can be used to install additional drops and applicators 1 converting the existing outlets with tees pipe and clamps or 2 adding additional mainline outlets installation is quicker if a platform is placed underneath the pivot mainline the platform can be planks placed across the truss rods or the side boards of a truck a tractor equipped with a front end loader provides an even better platform using existing outlets first the existing gooseneck is removed and crosses tees or elbows are connected to the mainline outlets as needed galvanized or plastic pipe is cut to extend from the outlet point to the figure 9 lepa bubble pattern drop location a galvanized elbow is used to connect the drop to the extension pipe this elbow should be clamped to the mainline to maintain the drop position fig 11 adding outlets it is less costly to convert to lepa by adding outlets than to purchase the tees plumbing clamps and labor required to convert existing outlets new mainline outlets can be installed quickly using a swedge coupler made of metal alloy an appropriate size hole is drilled into the pivot mainline at the correct spacing fig 12 the swedge coupler is then inserted into the hole the manufacturer recommends that a small amount of sealant be used with the coupler to ensure a leak-proof connection a standard hydraulic press body hydraulic punch equipped with a pull-type cylinder is attached to the coupler with a special fitting that screws into the coupler the press is used to compress the coupler against the inside of the mainline pipe to make a water-tight seal fig 13 the swedge coupler compresses quite easily be careful not to over-compress the coupler regular goosenecks or furrow arms are then screwed into the coupler fig 14 figure 8 double-ended sock bubble spray chemigate figure 10 multi-functional lepa head clamp figure 11 adding drops figure 14 swedge coupler installed figure 12 drilling for swedge coupler figure 13 installing swedge coupler 13[close]
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outlets also can be added by welding threaded 3/4inch female couplings into the existing mainline since welding destroys the galvanized coating welded couplings should be used only on ungalvanized main lines as with the swedge coupler goosenecks and drops can be used with the welded couplings other conversion tips when water is pumped into a center pivot it fills the mainline and drops the weight of the water causes the pivot to squat with 160-foot spans the pivot mainline will be lowered approximately 5 inches at the center of the span likewise a 185-foot span will be about 7 inches lower at the center when filled with water the length of the hose drops should be cut to account for this change so that all lepa heads are about the same height above the ground when the system is running center pivot manufacturers can provide appropriate drop hose cut lengths goosenecks or furrow arms and drops are installed alternately on each side of the mainline to help equalize stresses on the pivot structure for high profile crops also when crops are not planted in a circle having drops on both sides of the mainline helps prevent all the water from being dumped into the same furrows when the system parallels crop rows as with any other crop production investment a center pivot should be purchased only after careful analysis compare past crop production per acre-inch of irrigation applied to the projected production with center pivot use table 2 and consider the reduced cost of labor and tillage also consider how much water is available then answer the question will a center pivot cost or make money in my operation remember personal preference is one of the most important considerations pivot management pivot management is centered around knowing how much water is applied in inches the system design printout includes a precipitation chart that lists total inches applied for various speed settings on the central control panel if a precipitation chart is not provided fig 4 contact the dealer who first sold the pivot to obtain a copy dealers usually keep copies of the computer design printout indefinitely when a precipitation chart is not available use table 6 to identify the irrigation amount based on flow rate and time required to complete a circle for other sizes of pivots or travel speeds irrigation inches can be calculated using the first equation below keep in mind that the equations assume 100 percent water application efficiency reduce the amounts by 2 to 5 percent for lepa 5 to 10 percent for lesa 20 percent for mesa and 35 to 40 percent for impact sprinklers calculations for other length pivots can be made using the formulas below 1 inches applied pivot gpm x hours to complete circle 450 x acres in circle 2 acres per hour acres in circle hours to complete circle 3 end tower speed in feet per hour distance from pivot to end tower in feet x 2 x 3.14 hours to make circle 4 number of feet the end of machine must move per acre 87,120 distance feet from pivot to outside wetting pattern required accessories a permanently installed continuously functioning flow meter measures the actual amount of irrigation water applied and is highly recommended it is used in conjunction with the design printout for irrigation water management in addition properly located pressure gauges monitor system performance and in combination with the flow meter provide immediate warning of water deficiency and other system failures two pressure gauges are needed on the center pivot one at the end of the system usually in the last drop upstream from the applicator or regulator and one at the pivot point a third one in the first drop of span one will monitor operating pressure when the machine is downslope in relation to the pivot point other considerations on older equipment conventional mainline outlet spacings were 8 1/2 to 10 feet new center pivots should have 60 or 80-inch mainline outlet spacings even if this reduced spacing is not required by the water applicator initially selected manufacturers continue to develop more efficient applicators designed to be spaced closer together to achieve maximum irrigation efficiency and pumping economy ordering your pivot with a closer mainline outlet spacing will ensure that it can be quickly and inexpensively equipped with a new applicator design in the future retrofitting mainline outlet spacing typically costs $5,000 to $7,000 more than when the spacing is specified with the initial purchase 14 runoff control runoff from center pivot irrigation can be controlled by changing the optional speed control setting to match water application to soil infiltration agronomic methods of runoff control include furrow diking or chain diking for pastures farming in a circular pattern deep chiseling of clay sub-soils main-[close]
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table 6 inches of water applied by a 1,290-foot center pivot with 100 percent water application efficiency pivot gpm 400 500 600 700 800 900 12 0.09 0.11 0.13 0.16 0.18 0.20 hours to complete 120-acre circle 24 48 72 96 0.18 0.22 0.27 0.31 0.36 0.40 0.36 0.44 0.53 0.62 0.71 0.80 0.53 0.67 0.80 0.93 1.07 1.20 0.71 0.89 1.06 1.24 1.42 1.60 120 0.89 1.11 1.33 1.55 1.78 2.00 weather station and crop water use reporting networks located at amarillo college station and lubbock they report daily crop water use based on research one strategy used by growers is to sum the daily crop water use et reported during the previous 3 to 4 days and then set the pivot central control panel to apply that amount of water for more information on pet networks contact your county extension office the daily crop water use reported by the pet networks is for full irriga1000 0.22 0.44 0.89 1.33 1.78 2.22 tion most center pivots operating on the texas south and high plains are 1100 0.24 0.49 0.98 1.47 1.95 2.44 planned and designed for insufficient end tower capacity gpm to supply full daily feet/hour 667 334 167 111 83 67 crop water use growers with insufficient capacity should use a high water acres/hour 10 5 2.5 1.7 1.3 1 management strategy that ensures that 1,275 feet from pivot to end tower 15-foot end section the soil root zone is filled with water by either rainfall pre-watering or taining crop residue adding organic matter and using early-season irrigation before daily crop water use tillage practices that leave the soil open exceeds the irrigation capacity most soils such as farming in the round is one of the best methods of pullman sherm olton and acuff series soils can store controlling runoff and improving water distribution approximately 2 inches of available water per foot of when crops are planted in a circle the pivot never topsoil sandy loam soils typically store 1 inch or more dumps all the water in a few furrows as it can when it of available water per foot of topsoil sandy soils store parallels straight rows circle farming begins by markless the county soil survey available from the ing the circular path of the pivot wheels as they make natural resources conservation service contains the a revolution without water the tower tire tracks are available water storage capacity for most soils be sure then a guide for laying out rows and planting if the to use the value for the soil at the actual center pivot mainline span length distance between towers does site not accommodate an even number of crop rows adjust soil moisture-based the guide marker so that the tower wheels travel between crop rows soil moisture monitoring is highly recommended furrow diking is a mechanical tillage operation that places mounds of soil at selected intervals across the furrow between crop rows to form small water storage basins rainfall or irrigation water is trapped and stored in the basins until it soaks into the soil rather than running off fig 8 furrow diking reduces runoff and increases yields in both dryland and irrigated crops a similar practice for permanent pastures called chain diking involves dragging a chain-like implement that lea ves depressions to collect water and complements et-based scheduling particularly when there is rainfall during the irrigation season soil moisture monitoring devices such as tensiometers and watermark and gypsum block sensors can identify existing soil moisture monitor moisture changes locate the depth of water penetration and indicate crop rooting depths these three types of sensors absorb and lose moisture similar to the surrounding soil gypsum block and watermark sensors are read with resistance-type meters tensiometers have gauges that indicate soil moisture by measuring soil moisture pressures in units of centibars tensiometers are very accurate but are most useful in lighter soils that are irrigated frequently watermark sensors respond more quickly and are more accurate than gypsum blocks but cost more readings may be taken weekly during the early growing season during the crop s primary water use 15 irrigation scheduling et-based maximum crop production and quality are achieved when crops are irrigated frequently with amounts that match their water use or et evapotranspiration irrigating twice weekly with center pivots is common texas has three pet potential evapotranspiration[close]
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