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  • Irrigation Training ToolboxIrrigation System Design

    Selecting Sprinkler Packages For Center Pivots

    National Employee Development CenterNatural Resources Conservation Service

    Fort Worth, TexasOctober 1996










    William Kranz, Extension Irrigation Specialist

    Participants will:Understand the operating characteristics of center pivot irrigation systemsIdentify the important features of different sprinkler packagesUtilize CPNOZZLE computer program to match sprinkler package withsoils and field conditionsConduct a center pivot water application uniformity test

    Nebraska Irrigation Guide, UNL NebGuides, CPNOZZLE Users manual

    Course outline, CPNOZZLE Program, Sprinkler demonstration model,Figures referenced it the text are copied on the reverse side of the page.

    Field trip to conduct uniformity test and view sprinkler model,and computer access.

    2.5-3 hours






    Application Rate--is the rate water is applied to a given fieldarea expressed in units of depth per time (inchesfhour).

    Canopy Evaporation Loss-- is that water that evaporates directlyoff the crop leaves during and immediately following anirrigation event.

    Center Pivot--is an automated irrigation system consisting of aline of sprinklers rotating about a pivot point and supported bya series of self-propelled towers. Water is supplied at the pivotpoint and flows outward toward individual sprinklers.

    Christiansen's Uniformity Coefticient--is a measure of theuniformity of irrigation water application. The average depth Ofirrigation water infiltrated minus the average absolute deviationfrom the average depth is divided by the average depthinfiltrated.

    Depression Storage--is water stored in surface depressions anddoes not contribute to runoff.

    Detention Storage--is water applied in excess of the soilinfiltration rate and depression storage. Most eventually runs offbut some may infiltrate or evaporate.

    Distribution Uniformity--is a measure of the uniformity ofirrigation water application to a field area.

    Water Distribution Pattern--is the water application depth todistance from the sprinkler or center pivot relationship(see Figure 1).

    Elevation Head--is the relative energy possessed by a fluid dueto its position when compared to a datum. Elevation head ispositive for sprinklers below the datum and negative forsprinklers above the datum.

    Electric Load Control-- occurs when the electrical supplierinterrupts power to selected systems so that the peak powerrequirement can be controlled. In trade for the powerinterruptions, operators receive a cut in power cost commensuratewith the level of control. Each power supplier has somewhatdifferent methods of determining which systems are to be

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    controlled on a given day. The types of control fit into threemain categories:

    Anytime control-authorizes the power supplier tointerrupt the power supply for up to six 12-hour periodsduring a week. Field data show that shut down timeseldom is above 42 hours per week though 72 hours ofcontrol are possible.

    Two day control-allows power to be interrupted for two12-hour periods during a week.

    One day control-allows power to be interrupted for one12-hour period during a week.

    Flow Control Nozzle--is a device used to maintain constant flowrate out of a sprinkler regardless of sprinkler elevation changes(See NebGuide G88-888).

    Gross System Capacity--is the amount of water that must bepumped to insure that crop water use requirements are satisfied.Gross system capacity is the net system capacity divided by thewater application efficiency.

    Interception--is the portion of the irrigation water applied by thesystem that is caught by vegetation and thus prevented fromreaching the soil surface.

    Irrigation Efficiency (IE)-- is the ratio of the average depth ofirrigation water that is beneficially used to the average depth ofirrigation water applied.

    IE =(Irrigation - Runoff - Deep Percolation - Evaporation Loss). Irrigation

    Irrigation Frequency--is the average time between the starttimes for successive irrigation events.

    Low Energy-Precision Application (LEPA)--is a completesprinkler irrigation concept that includes mounting sprinklerheads 12-18" from the soil surface, planting the crop in a circle,and creating and maintaining storage basins to store water appliedwith the system.

    Nozzle--is a discharge opening of a sprinkler head used to controlvolume, distribution pattern and droplet size. Impact sprinklers

  • (I)-ctIa:(I)





    Figure 1. Typical water distribution patterns for sprinkler systemsrelative to the center pivot structure.

    Figure 2. Sprinkler arrangement for constant spacing package.

    Figure 3. Sprinkler arrangement for variable spacing package.

    Figure 4. Sprinkler arrangement for narrow or spray nozzle package.

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    are equipped with a nozzle(s) and an impact arm. Spray headsare equipped only with nozzles.

    Peak Application Rate--is the maximum water application rateapplied by the sprinkler package (See Figure 1). For impactsprinklers the peak application rate is located at the pivot lateral.For some spray packages, the peak application rate occurs somedistance (60-70% of the wetted radius) away from the lateral.

    Potential Runoff--is the water applied in excess of the soilinfiltration rate. Therefore, it is the water that could off the field if soil depression storage is unavailable. Therunoff potential is typically greatest at the o.uter end of the pivatlateral and increases with decreasing sprinkler wetted radius orincreasing system capacity.

    Pressure Regulating Device--is a pressure decreasing deviceused to maintain constant pressure at the base of the sprinklerregardless of elevation changes (see NebOuide 088-888). Forthe device to function properly, the pipeline pressure mustbe 5to 10 psi greater than the desired sprinkler pressure or pressurerating of the regulator.

    Pump Output Curve--is a graphical representation of the changein pumping head resulting from a change in pumping rate. Usethe pump curve for retrofitting a system with reduced pressuresprinklers or when determining the water applicationcharacteristics of the pivot.

    Sprinkler Package-- the group of sprinklers installed on theirrigation system. Sprinklers may be mounted above the pivotpipeline, on the side of the pipeline or suspended on drop tubesbelow the pipeline. The package can have the followingdistributions among individual sprinklers:

    Constant Spacing (Figure 2)--sprinklers a placed atconstant spacings along the system length but the size ofthe sprinkler is increased to account for the change inarea irrigated. Thus, the wetted radius of the sprinklertends to increase with distance from the pivot point.

    Variable Spacing (Figure 3)--Sprinklers are all the samesize but the spacing between the sprinklers decreaseswith distance from the pivot point to account for thechange in area irrigated. The wetted radius is similar forthe entire system.

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    Narrow Spacing (Figure 4)--is used for spray nozzles.Different size nozzle openings are needed to comperisatefor differences in irrigated area but the spacing isnarrower along the system due to their lower wettedradius.

    . Net System Capacity--is the amount of water that must besupplied to the crop or soil to replace crop water use or ET. Theamount of water supplied should consider the estimateddowntime, rainfall probability, water application efficiency, andsoil water h0lding capacity (see NebGuide G89-932).

    Water Application Efficiency (WAE)-- is the ratio of the.average depth of irrigation water that infiltrates and is stored inthe rootzone to the average depth of water applied. The averageapplication efficiency of a center pivot is often assumed to be80% in lieu of accurate field estimates.

    System Wetted Radius (RJ-- Wetted radius of the irrigationsystem taken as the distance from the pivot point toapproximately 80% of the throw radius of the end sprinkler.

    Sprinkler Wetted Radius (R.p)--Wetted radius of the sprinklerpackage. If the wetted radius of the sprinkler changes with thesystem length, the specific position on the system must beevaluated.

    Wind Drift Loss-- is water transported away from the targetlocation by wind. Drift loss often includes evaporation losseswhile the water droplet is in the air. Drift losses are affected bywater .droplet size, trajectory of the water stream, andenvir()nrnentalconditions.

    ==================================================II. BACKGROUND

    Sprinkler irrigation systems and specifically center pivots havebeen adapted to operate on many different soils, to traverseextremely variable terrain, and to provide water to meet a numberof different management objectives. A center pivot is merely anirrigation pipeline that rotates about the pivot point. Water issupplied to the system at the pivot point. Thus, the circularrotation results in two very unique and important designparameters.

  • Figure 5. Acres irrigated by each 10% of the system length.

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    A center pivot making a complete circle irrigates an area equalto the area of a circle with a radius equal to the length of thesystem pipeline. Thus, the irrigated area increases as the systemlength squared. Figure 5 presents the area irrigated by each 10%of the system length. For a 1300 foot system, the first 130 feetof system length would irrigate 1.2 acres and the


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