Hill, Robert W.

Winger, Marlon

Worwood, Dennis

English

DigitalCommons@USU

1SPRINKLERS, CROPWATER USE, ANDIRRIGATION TIMECARBON AND EMERY COUNTIES Robert W. Hill, Extension Specialist - Irrigation;Marlon Winger, Extension Agent - Carbon County;Dennis Worwood, Extension Agent - Emery CountyAugust 2000 ENGR/BIE/WM/07Sprinkler irrigation has been an important part of Utah’s agricultural production since the early 1950's. About 40% of Utah’s 1.3 million irrigated acres are watered with sprinklers,including hand move, wheel move, center pivot and other types. Sprinklers can be a goodinvestment when properly designed, installed, maintained and managed. For every acre-foot ofwater supplied to an efficient sprinkler system a farmer can expect to harvest about 1-3/4 ton ofalfalfa and 46 bushels of wheat. In contrast, the expected harvest with a typical surface irrigationsystem (flood or furrow) is less than 1-1/4 ton of alfalfa or about 30 bushels of wheat for eachacre-foot of water supply to the farm. Sprinklers produce more yield than typical surfaceirrigation systems per acre-foot because sprinklers apply water more efficiently.Not all water applied by an irrigation system is used by the crop. Some water is lost todeep percolation, evaporation, or runoff. Application efficiency (Ea) is a term that tells howmuch of the water applied by the system is actually stored in the root zone for crop use. In Utah atypical sprinkler system has an Ea of 70% which means that 70% of the water applied by thesprinkler heads is actually stored in the soil for crop use. The actual Ea depends upon how evenlythe sprinklers distribute water and other factors such as operating pressure, nozzle size andspacing, wind, air temperature and humidity (day versus night), irrigation scheduling andmaintenance condition. The average efficiency of surface irrigation in Utah is about 50% ascompared to the higher sprinkler efficiency of 70%.SPRINKLER IRRIGATION MANAGEMENTAn efficient sprinkler system is the result of good system design, proper irrigationscheduling and careful operation and timely maintenance.2Table 1. Sprinkler Pressure and Flowrate.Nozzle Nozzle Pressure, psi  size  30 40 50 60 70  inch Nozzle flow rate, gallons per minute (gpm)  5/32 3.9 4.5 5.0 5.4 5.811/64 4.7 5.4 6.0 6.6 7.1  3/16 5.5 6.3 7.0 7.7 8.313/64 6.4 7.4 8.2 9.0 9.7  7/32 7.4 8.6 9.6           10.5           11.3Note: Flowrates are for agricultural sprinkler heads with brass nozzles. Sprinkler nozzle flowrateis proportional to the square root of the water pressure at the base of the nozzle.DESIGNA well designed sprinkler system applies water uniformly to the soil surface, and iscapable of applying enough water to meet the peak demands of the crop without producingexcess runoff. Good design considers such factors as pressure; nozzle size and spacing; wind, airtemperature and humidity (day versus night); soil intake rate; crop rooting depth and water userates.The flow rate from a sprinkler nozzle depends upon nozzle size and water pressure.  Flowrates for selected nozzle sizes and pressures are given in Table 1. Typical sprinkler flow ratesmay vary from 4 gallons per minute (gpm) from a 5/32-inch nozzle at 30 pounds pressure to over11 gpm from a 7/32-inch nozzle at 70 pounds pressure. The nozzle size is usually stamped on thenozzle. Wheelmove systems typically have 3/16-inch nozzles.On sloping fields there may be considerable pressure differences between sprinkler headson high and low ends of the line. In this situation, flow control nozzles may be used to improvethe uniformity of water application. Flow control nozzles apply water at the same rate, regardlessof water pressure.Precipitation Rate (How hard is it raining?):The Precipitation Rate (Pr) is the rate at which water is delivered from the nozzle,averaged as inches per hour, over the area covered by one nozzle. It is important to consider thePrecipitation Rate when designing a sprinkler system, since water will run off if applied fasterthan the soil can absorb it. Precipitation rate can be calculated using the following formula:Pr (inches/hr) = 96.3 x nozzle flow rate (gpm)/area covered (ft2) (1)Precipitation Rate can be calculated as follows: In a typical wheelmove system, each sprinkler covers 2400 square feet. This is based on a spacing of 40 feet between sprinklers on theline, and a 60 foot move (40' x 60' = 2400 square feet). If we have 3/16 inch nozzles that areoperating at 50 pounds pressure, the nozzle flowrate is 7.0 gpm (from Table 1). The Precipitationrate would be:3Pr = 96.3 (7.0 gpm)/2400 ft2 = 0.28 inches per hourApplication Rate (How much of the rain stays in the soil?): The Application Rate (Ar) is the average rate at which water is stored in the soil, ininches per hour.Ar = Application Efficiency (Ea) x Precipitation rate (Pr) (2)Typical sprinkler application efficiency values vary from 60% to 80%, 70% is areasonable average.Example:Ar = (70/100) x 0.28  0.20 inches per hourHow Long to Irrigate (Duration):The duration of irrigation needed to store the crop irrigation requirements in the root zone is:Irrigation Duration (hours) = Crop Irrigation requirements (inches)/Ar (3)Example:Determine how many hours to irrigate in July with a crop irrigation requirement of 8.5inches, 3/16 inch diameter nozzles at 50 psi and 40' x 60' spacing (use results of previousexamples).Hours to irrigate in July = 8.5 inches/ 0.20 inches/hour  43 hoursAssuming that the sprinklers were moved twice per day, about 11 ½ hour sets, then aboutfour irrigations (4  43/11.5) are needed in July. This is equivalent to one 11 ½ hour irrigationabout every 8 days [8  31/(43/11.5)].Calculated irrigation duration for nozzle sizes of 5/32 to 7/32 and pressures of 50 and 60psi are given in Table 2. The durations shown in Table 2 were obtained from the use of Table 1and Equations 1, 2, and 3, assuming sprinkler spacing of 40' by 60' and 70% applicationefficiency. The Table 2 duration value corresponding to the above examples is 43.2 hours, whichis found at the intersection under the 3/16, 50 psi column and the 8.5 inch row.IRRIGATION SCHEDULINGIrrigation scheduling is the process of determining when to irrigate and how much waterto apply. It depends upon design, maintenance, and operation of the irrigation system and theavailability of water. The objective of irrigation scheduling is to apply only the water that thecrop needs, taking into account seepage, runoff losses, and leaching requirements. Scheduling isespecially important to pump irrigators if power costs are high. Common irrigation schedulingapproaches include the following:1. Irrigation on fixed intervals or following a simple calendar, i.e., when a water turnoccurs or according to a predetermined schedule.2. Irrigating when the neighbor irrigates.43. Observation of visual plant stress indicators.4. Measuring (or estimating) soil water by use of instruments or sampling techniquessuch as probes.5. By following a soil-water budget based on weather data and/or pan evaporation.6. Some combination of the above.For irrigation scheduling to be most useful at a specific location, the following should be done:1. Evaluate the irrigation system. Determine application depth, efficiency, and operating capabilities and constraints.2. Select an appropriate irrigation scheduling method.3. Monitor performance at intervals during the growing season.4. Perform a post-season evaluation and determine changes for next year.OPERATION AND MAINTENANCETo realize the full benefit of the sprinkler system, it must be operated according to designand properly maintained throughout the irrigation season. This may involve special operatingtechniques such as using an offset hose or alternating between day and night on successiveirrigation cycles to improve distribution uniformity. Where pressure differences within asprinkler system result in low uniformity of water application, special hardware such as flowcontrol nozzles or pressure regulators may be required.An audit or evaluation of the irrigation system is recommended if you suspect that thesystem is not as efficient as it might be. An audit determines application depth, distributionuniformity, and hydraulic performance of the supply system. If a pump is used, it is tested todetermine fuel or energy use efficiency. An audit may also identify steps to improve systemoperation and maintenance.Good operation also includes matching the set time (or rotation time with a center pivot)with the applied water depth and application rate to maximize the fraction of water stored in theroot zone. Field irrigation (application) efficiency is the water stored in the root zone divided bythe water delivered to the field. If a field is under-irrigated, a high irrigation efficiency couldresult with a low uniformity. Conversely, an over-irrigated field will have a low irrigationefficiency even with a high uniformity because of the deep percolation. Thus, a knowledge of thesoil moisture content prior to irrigation is essential to maintaining a high application efficiencywhile providing for optimum crop water use and growth.CROP WATER USEThe single most important factor that influences plant growth and crop yields from onelocation to another, or from one year to the next, is moisture availability. A better understandingof how water influences crop growth is essential for good water management practices. Water isthe most massive of the inputs to crop yield. It takes 120 pounds of water (evapotranspirationonly) to produce 1 pound of Idaho potatoes, 560 pounds of water for 1 pound of alfalfa hay and790 pounds of water for 1 pound of wheat. Soil water availability is affected by infiltrated irrigation water and rainfall, drainage andevapotranspiration. Evapotranspiration (Et) is the combination of transpiration from plant leavesplus evaporation from adjacent soil surfaces. While crop Et can be measured, it is most oftenestimated with equations from weather data collected locally. Estimated average monthly cropwater use (Et) for alfalfa, spring grain, pasture, turf, and garden at Castle Dale and Ferron are5given in Table 3. Seasonal Et is higher at Castle Dale than at Ferron for all crops. However, Etfor all crops in July is higher at Ferron. Assuming that the soil water depletion is completely replenished with each irrigation, theirrigation requirement is equal to Et minus effective rainfall. As a general rule, field crops shouldbe irrigated whenever the soil water depletion approaches 50% of the available water in the rootzone (see Appendix). In the peak crop water use period in an arid area, the occurrence of rain isoften neglected in determining an irrigation schedule as in the example following Table 3.Table 2. Required Irrigation Duration for Selected Irrigation Water Requirement ValuesIrrigation.    Nozzle size, inchesWater        5/32      11/64       3/16      13/64      7/32Req’d,                                                          Pressure psi inches   50   60   50   60   50   60   50   60   50   60Irrigation Duration, Hours0.5   3.6   3.3   3.0   2.7   2.5   2.3   2.2   2.0   1.9   1.71.0             7.1   6.6   5.9   5.4   5.1   4.6   4.3   4.0   3.7   3.41.5 10.7   9.9       8.9   8.1   7.6   6.9   6.5   5.9   5.6   5.12.0 14.2 13.2 11.9 10.8 10.2  9.2   8.7   7.9   7.4   6.82.5 17.8 16.5 14.8 13.5 12.7 11.6 10.9   9.9   9.3   8.53.0 21.4 19.8 17.8 16.2 15.3 13.9 13.0 11.9 11.1 10.23.5 24.9 23.1 20.8 18.9 17.8 16.2 15.2 13.8 13.0 11.94.0 28.5 26.4 23.7 21.6 20.3 18.5 17.4 15.8 14.8 13.64.5 32.0 29.7 26.7 24.3 22.9 20.8 19.5 17.8 16.7 15.35.0 35.6 33.0 29.7 27.0 25.4 23.1 21.7 19.8 18.5 17.05.5 39.2 36.3 32.6 29.7 28.0 25.4 23.9 21.8 20.4 18.66.0 42.7 39.6 35.6 32.4 30.5 27.7 26.1 23.7 22.3 20.36.5 46.3 42.9 38.6 35.1 33.1 30.1 28.2 25.7 24.1 22.07.0 49.8 46.2 41.5 37.8 35.6 32.4 30.4 27.7 26.0 23.77.5 53.4 49.4 44.5 40.5 38.1 34.7 32.6 29.7 27.8 25.48.0 57.0 52.7 47.5 43.2 40.7 37.0 34.7 31.6 29.7 27.18.5 60.5 56.0 50.4 45.9 43.2 39.3 36.9 33.6 31.5 28.89.0 64.1 59.3 53.4 48.5 45.8 41.6 39.1 35.6 33.4 30.59.5 67.6 62.6 56.4 51.2 48.3 43.9 41.2 37.6 35.2 32.2Note: Irrigation duration, hours, calculated from flowrate in Table 1 and from Equations (1), (2),and (3) assuming sprinkler spacing of 40' by 60' and 70% application efficiency.6Table 3.  Monthly Crop Evapotranspiration for Castle Dale and Ferron          Season Site Apr May Jun Jul Aug Sep Oct Total   Alfalfa Water Use, InchesCastle Dale 0.48 5.49 6.95 8.47 7.04 4.52 1.30 34.24Ferron 0.44 5.45 6.57 8.66 6.89 4.21 1.31 33.54Pasture Water Use, InchesCastle Dale 1.44 3.35 6.04 6.53 5.23 3.99 1.44 28.01Ferron 1.34 3.33 5.72 6.72 5.10 3.75 1.48 27.43Sp Grain Water Use, InchesCastle Dale 0.58 3.83 9.29 8.78 0.60 23.08Ferron 0.55 3.82 8.79 9.02 0.55 22.74Turf Water Use, InchesCastle Dale 1.77 3.42 5.21 5.62 4.50 3.44 1.23 25.21Ferron 1.66 3.39 4.94 5.78 4.39 3.23 1.28 24.67Garden Water Use, InchesCastle Dale 1.38 3.62 6.62 6.00 1.82 0.26 19.71Ferron 1.38 3.42 6.84 5.84 1.74 0.26 19.48Adapted from: Consumptive Use of Irrigated Crops in Utah, Utah Agricultural ExperimentStation Research Report No. 145. Oct. 1994.Example: Simple Irrigation Calendar for Alfalfa at Castle Dale, July Et = 8.47 in.     A. Irrigate at 50% management allowed depletion (MAD).Soil water holding capacity (sandy loam) is 1.5 inches/ft (from Appendix)     Root zone available water = 5 ft x 1.5 in/ft = 7.5 inches     MAD of 50%, irrigation amount  = 7.5 x .5 = 3.8 inches depletion betweenirrigations..     Irrigation interval = Amount/daily Et rate  = 3.8 inches/(8.47 inches/31 days)   = 3.8 inches/0.27 in. per day  = 14 daysB. Alternate schedule from sprinkler net irrigation.      Net irrigation of 2.3 inches stored in the soil (2.3 = 0.20 inches per hour x 11.5 hoursper set).  Irrigation Interval = 2.3 inches/0.27 inches per day = about 8 ½  days.7SUMMARYGood sprinkle irrigation requires:• Understanding of Soil-Water-Plant Relationships• Irrigation timing and amount depends on soil water holding capacity, weather, and crop growth progress.• Adequate Design and Installation• Proper Operation and Maintenance• Dedication and Commitment of Resources to Manage (i.e., the WILL to manage)WHERE CAN YOU GET HELP?Utah State University - Extension ServiceUSU Extension, Biological Carbon County Emery Countyand Irrigation Engineering 120 East Main P.O. Box 8474105 Old Main Hill Price, UT 84501 Castle Dale, UT 84513Logan, UT 84322-4105 marlonw@ext.usu.edu dennisw@ext.usu.edubobh@ext.usu.edu (435) 636-3233 (435) 381-2381(435) 797-2791Robert W. Hill, Extension Irrigation Specialist,Biological and Irrigation Engineering Department,Utah State University, Logan, UT  84322-4105Marlon Winger, Extension Agent - Carbon CountyUtah State University, Logan, UT  84322Dennis Worwood, Extension Agent - Emery CountyUtah State University, Logan, UT  84322BIBLIOGRAPHYHill, R.W. 1991. Irrigation Scheduling. Chapter 12 in Modeling Plant and Soil Systems, JohnHangs and J.T. Ritchie (eds.), American Society of Agronomy, Inc., Crop Science Societyof America, Inc., Soil Science Society of America, Inc., Madison, WI. 491-509.Hill, R.W. 1994. Consumptive Use of Irrigated Crops in Utah. Utah Agr. Exp. Stn. Res. ReportNo. 145, Utah State University, Logan, UT. Oct. 370 pp. Revised and reprinted Feb.1998.Hill, R.W. and R.G. Allen. 1996. Simple Irrigation Scheduling Calendars. Journal Irrigation andDrainage Division. ASCE. 122:2, March/April.Hill, R.W. , R.J. Hanks, and J.L. Wright. 1984. Crop Yield Models Adapted to IrrigationScheduling Programs. Utah Agr. Exp. Stn. Res. Report 99.8Utah State University Extension is an affirmative action/equal employment opportunity employer andeducational organization. We offer our programs to persons regardless of race, color, national origin, sex,religion, age or disability.Issued in furtherance of Cooperative Extension work, Acts of May 8 and June 30, 1914, in cooperationwith the U.S. Department of Agriculture, Robert L. Gilliland, Vice-President and Director, CooperativeExtension Service, Utah State University, Logan, Utah.(EP/08-2000/DF)For additional reading/resource material see:Larsen, D.C. and T.S. Longley. “Nozzle Management and Leak Prevention for Sprinkler Irrigation.”Current Info. Series No. 569. University of Idaho Coop. Extension Service, Moscow, ID 83843.MSU Agronomy Notes Series numbers 44, 47, 49, 53, 102, and 122. J.W. Bauder. Soil and WaterSpecialist; Plant, Soils and Environmental Science Department, Montana State University,Bozeman, MT 59717. Telephone (406) 944-5685; email: jbauder@montana.edu.Pacific Northwest Extension Publication, PNW Series numbers 286-292. Jan. 1986. Available fromUniversity of Idaho Cooperative Extension Service, Moscow, ID 83843. Titles in this seriesinclude: Pumping Plant Efficiencies, Offsets for Stationary Sprinkler Systems, Irrigation RunoffControl Strategies, Irrigation Scheduling, Converting Sprinkler Systems to Lower Pressure,Sizing Irrigation Mainlines and Fittings, Electrical Demand Charges-How to Keep them Low,Extending Electric Motor Life.APPENDIXAvailable Water-holding Capacity of Soils     Typical Crop Rooting DepthsInches of available water Typical active rootSoil Texture per foot of moist soil Crop Zone depth, feetSands and fine sands 0.5 - 0.75 Alfalfa 5Very fine sands, loamy sand 0.8 - 1.0 Corn 4 - 5Sandy Loam 1.2 - 1.5 Small Grains 3 - 4Loam 1.9 - 2.0 Dry Beans 3Silt loam, silt 2.0 Pasture 1.5 - 2.5Silty clay loam 1.9 - 2.0Potatoes 2 - 3Sandy clay loam, Clay loam 1.7 - 2.0 Turf 1 - 2Vegetables 1.5 - 3Allowable depletion to avoid crop water stress is usually about 50% of available water holdingcapacity for most field crops.The web site address for UAES Research Report #145 data tables used in Table 3 hereinis found by going to the Utah Division of Water Rights home page at:http://nrwrtl.nr.state.ut.us/Then select “Publications” and then select “Consumptive Use Tables”The direct URL is:http://nrwrt1.nr.state.ut.us/techinfo/consumpt/default.htm

Sprinklers, Crop, Water Use, and Irrigation Time

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Sprinklers, Crop, Water Use, and Irrigation Time

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