Chapter 11: Sprinkle Irrigation - NRCS Irrigation ToolBox Home Page

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For center-pivot systems and fully automatic fixed systems, it is best to let d equal the gross depth required per day and f = 1.0. To allow for some breakdown or moving of systems, T can be reduced by 5 to 10 percent from the potential value of 24 hr. Of major importance are f and T because they have a direct bearing on the capital investment per acre required for equipment. From equation 1 it is obvious that the longer the operating time (ff) the smaller the required system capacity and, therefore, the cost for irrigating a given acreage. Conversely, where the farmer wishes to irrigate an acreage in a minimum number of days and has labor available only for operation during daylight hours, the equipment costs per acre will be high. With center-pivot and automated field systems, light, frequent irrigations are practical because labor requirements are minimal. With these systems irrigation frequency should be based on maintaining optimum soil-plantwater conditions rather than on allowing soil moisture depletion levels that are a compromise between labor requirements, capital costs, and growing conditions (as recommended in Chapter 1). Before a sprinkle system is planned, the designer should thoroughly acquaint the owner with these facts and the number of operating hours that can be allowed for completing one irrigation. Also the farmer should understand the amount of labor reh quired to run the sprinkle system so that this operation interferes minimally with other farming operations. Areas that have several soil zones that vary widely in water-holding capacity and infiltration rate can be subdivided on the basis of the water needed at each irrigation (fig. 11-14) for all systems except center pivots. It is easier to operate center-pivot sprinklers as though the entire field has the soil with the lowest water-holding capacity and infiltrae tion rate. Sample calculation 11-1 has been prepared as an example of the use of the formula where a single crop is irrigated in the design area. The design moisture use rate and irrigation frequency can be obtained from irrigation guides where available. Otherwise, they may be computed from Technical Release No. 21, Irrigation Water Requirements and Chapter 1, Soil-Plant- Water Relationships, Figure Il-14.-Subdivision zones. of design areas having different soil
Sample calculation 11-1,-Computing capacity requirements for a single crop in the design area. Given: 40 acres of corn (A) Design moisture use rate: 0.20 inlday Moisture replaced in soil at each irrigation: 2.4 in Irrigation efficiency: 70% Gross depth of water applied (d): 2.410.70 or 3.4 in a 70% efficiency Irrigation period (f): 10 days in a 12-day interval System to be operated 20 hrlday (T) 3.0 in required at each irrigation 1.5 in required at each irrigation A The design area can be served by a mainline as indicated by the dotted line. Laterals can operate on both sides, but must run twice as long on the 3.0-in zone and twice as often on the 1.5-in zone, or else separate laterals must be designed for each zone with different water application rates. In either case the frequency of irrigation would be two times on the 1.5-in zone for each time on the 3.0-in zone. B The system is designed for a uniform soil area using the 1.5-in water-application rate. Once during the early growing season, the lateral or laterals could be operated twice as long on the 3.0-in zone, but the entire area would be irrigated at the frequency required for the 1.5-in zone during peak-moisture-use periods. C Again the system would be designed for the 1.5-in zone. For deep-rooted crops, the entire area might be given a 3.0411 application for the first irrigation in the spring. However, this would mean some sacrifice in water-application efficiency. Calculation using equation 1: Where two or more areas with different crops are being irrigated by the same system and peak designwe rates for the crops occur at about the same time of the year, the capacity for each area is computed as shown in sample calculation 11-1 and capacities for each area are summed to obtain the required capacity of the system. The time allotted for completing one irrigation over all areas (f) must not exceed the shortest irrigation-frequency period as shown in the local irrigation guide or determined by the procedure in Chapter 1. System-capacity requirements for an area in a crop rotation are calculated to satisfy the period of water use. Therefore, allowances must be made for the differences in time when the peak-use requirements for each crop occur (sample calculation 11-2). Sample calculation 11-2.-Computing capacity requirements for a crop rotation. Given: Design area of 90 acres with crop acreages as fola lows: 10 acres Irish potatoes, last irrigation May 31; 2.6-inch application lasts 12 days in May (peak period); 30 acres corn, last irrigation August 20; 2.9-inch application lasts 12 days in May; 3.4-inch application lasts 12 days in July (peak period); 50 acres alfalfa, irrigated through frost-free period; 3.6-inch application lasts 12 days in May; 4.3-inch application lasts 12 days in July (peak period); Irrigation period is 10 days in 12-day irrigation interval; System is to be operated 16 hr per day. Calculations using equation 1: Capacity requirements for May when all three crops are being irrigated. Irish potatoes Q = 453 X 10 X 2.8 = 74 mm 10 X 16 453 X 30 X 2.9 = 246 Corn Q - 10 X 16 Alfalfa -453 X 50 X 3.6- 510 Q - - 10 X 16 Total for May = 830 gpm Capacity requirements for July when potatoes have been harvested but corn and alfalfa are using moisture at the peak rate Corn
Page 1 and 2: United States Department of Agricul
Page 3 and 4: Application intensity .............
Page 5 and 6: Advantages Some of the most importa
Page 7 and 8: End-Tow Lateral An end-tow lateral
Page 9 and 10: Figure 11-4.-Solid-set sprinkler la
Page 11 and 12: or for dual cropping, it is practic
Page 13 and 14: Planning Concepts A complete farm s
Page 15: I. Crop (Type) (a) Root depth (ft)
Page 19 and 20: 11-rfi Such systems are capable of
Page 21 and 22: Table 11-2. Effective crop root dep
Page 23 and 24: Impact sprinklers produce a circula
Page 25 and 26: n n = mean depth of observations (i
Page 27 and 28: 11-24 I. Increases of 20 to 40 perc
Page 29 and 30: plants is similar for all systems.
Page 31 and 32: 1. At the lower side of the specifi
Page 33 and 34: diameter based on average wind spee
Page 35 and 36: LATERAL SET A L 50 FEET LATERAL SET
Page 37 and 38: such as forage crops, According to
Page 39 and 40: Table 11-10.-A guide to recommended
Page 41 and 42: Table 1 l-12.-A guide to recommende
Page 43 and 44: I. Crop (type) (a) Root depth (ft)
Page 45 and 46: F F I k% TabIe 11-13.-Nozzie discha
Page 47 and 48: 1‘ HOSE-LINE ’ A-Fully portable
Page 49 and 50: A-Layout on moderate, unifbrm Slope
Page 51 and 52: such cases, lines are laid on the c
Page 53 and 54: .I* en factor (F) based on the numb
Page 55 and 56: Figure Il-33.-lateral laid downhill
Page 57 and 58: This is obviously outside the 20 pe
Page 59 and 60: Table 11-17.-Typical as % 3 -. " 3
Page 61 and 62: Table 11-19a.-Friction-loas J value
Page 63: one of selecting mainline pipe size
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ft; for D3 and Dz, friction loss in
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The standard capital recovery facto
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Step 4.-The WHP savings needed to o
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1. Economic method-Selection of the
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Table Il-24.-Comparison of total an
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ial, and then determine the total a
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Table 11-28.-Values of resistance c
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can be solved very simply from equa
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sure is representative of the entir
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turer at the given pressure. This o
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Table 11-30.-Typical discharges and
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This allows for a 1-hr set time, 20
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ftlmin. The required sprinkler disc
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R *= maximum radius irrigated when
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sprinkler pattern. High instantaneo
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Where soil sealing and infiltration
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Thus, to compute the (Hfk, with two
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And the net depth of application as
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Machine Selection Ultimately the ty
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Span I Container Span Position Weig
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Sample calculation 11-20.-Using fie
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Span Container span No. Position We
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4. Increasing system pressure and r
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Figure 11-60.-A method for adding f
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Table Il-35.-Steps for estimating f
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Frost Control Operation For complet
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Installation and Operation of Sprin
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T actual operating time (hrlday) t
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