Irrigation Chapter 12 - Sprinkler Irrigation Basics

The term “sprinkler irrigation” describes a variety­ of irrigation systems, all of which use sprinklers to distribute water. These systems can be stationary or mobile. For example, a towline is stationary and a center pivot or large volume guns are mobile. We can link sprinkler irrigation’s success to the system’s ability to work on many crops, to apply water uniformly and efficiently, and to deliver water under a wide range of climatic and field conditions.

When sprinkler irrigation first started, the term was synonymous with systems that used impact sprinklers to distribute the water. More recently, the term has been expanded to describe a broad range of impact sprinklers and spray nozzles. For the remainder of this chapter, nozzle will be used when describing spray nozzles and sprinklers will be used when referring to impact sprinklers. Most of the development in sprinkler technology has been directed at center pivot applications, however, in the process some new sprinklers have been developed for use with towlines and siderolls.

If some of the terms used in this chapter are unfamiliar,­ be sure to check the Glossary.

Author: Bill Kranz, University of Nebraska Lincoln Extension Irrigation Specialist, UNL Haskell Agricultural Laboratory, Concord, NE.

Center Pivot

Center pivots are the most widely used sprinkler system. Center pivots have been adapted to operate on many soils, to traverse extremely variable terrain, and to provide water to meet many management objectives­. In Nebraska, center pivots are used to irrigate about 6.1 million acres of cropland. The number of acres irrigated by center pivots is increasing each year largely because the system requires less labor and provides substantial improvements in the operator’s ability to manage irrigation water applications.

A center pivot is a water distribution pipeline anchored­ at one end and allowed to rotate or pivot about the stationary end. The system length can vary from 300 feet to more than 2600 feet. Water is supplied to the pivot point resulting in a circular irrigated area (Figure 12.1). Beginning at the pivot point, each additional foot of system length must irrigate an area that increases as the system length squared. For a 1300-foot-long center pivot making a complete circle, the first 130 feet of the system irrigates 1.2 acres, and the last 130 feet of the system irrigates 23 acres. To distribute the same amount of water to every­ portion of the field, the last 130 feet of the system must receive more than 19 times more water than the first 130 feet. This causes the pressure to be greater at the pivot point than at the end of the system.­

Figure 12.1.  Impact of position on travel distance for the first and last 130 foot segments of a 1,300 foot center pivot.

The second design consideration involves the water application rate. Because the system rotates about the pivot point, each tower must travel a different distance to make one revolution. For a 1300-foot system, the outside tower will normally travel about 8,100 feet while the first tower might travel less than 1200 feet. The outside tower must move nearly seven times as fast as the inside tower. Combining the travel speed and the area irrigated means that 19 times the area must receive water in one-seventh of the time. This causes the water application rate at the outside end of the system to be much greater than near the pivot point. For some sprinkler packages, finding a nozzle opening small enough for the first 200 feet of the system is difficult and multiple nozzles may be necessary at each position on the outside end.

Determining System Requirements

Selecting a sprinkler package for a center pivot can involve several conflicting issues. As the owner/manager, you can choose from an array of sprinkler types, many of which can perform adequately. Your selection should be based on accurate field-based information and careful consideration of the interaction among several factors. This section describes some of the factors that need to be addressed and how you can collect the field information needed to select the right system. First, determine the area to be irrigated by the system. Since most new sprinkler installations involve­ center pivots, examples for other installations are not provided. If you are interested in calculating the area irrigated for other systems contact your local UNL Extension or Natural Resources Conservation Service Office.

Irrigated Area

As you become familiar with more field installations, you will note that few systems are exactly alike. Figures 12.2 and 12.3 present formulas for estimating the irrigated area of fields with corner systems, end guns that run intermittently, systems making a part circle, and other field orientations. Note the requirement to estimate the arc lengths for the end gun and corner system operations. The acreage increases proportionally­ as the system wetted radius squared (Rs2).

Figure 12.2. Formulas for calculating the area irrigated by center pivots with and without endguns.  Note: Lengths and angles must be measured in the field or from maps, etc. The above dimensions and angles are only typical examples.

Figure 12.3. Formulas for calculating the area irrigated by center pivots with corner systems.  Note: Lengths and angles must be measured in the field or from maps, etc. The above dimensions and angles are only typical examples.

Though the sprinkler system typically applies water beyond the end of the pivot lateral, the effective wetted radius will vary as the system makes a revolution due to atmospheric conditions. The most significant atmospheric condition is the wind direction and speed. It is advisable to calculate the irrigated acreage using a system wetted radius equal to the pivot length plus approximately 80 percent of the wetted radius of the last sprinkler or end gun. Beyond­ that point, the water application pattern often­ varies greatly and the depth of application could be much less than the crop water use rate. The irrigated area for a full or part circle center pivot can be determined using Equation 12.1.

Equation 12.1     A = [ π x Rs2 x Pr ] / [ 43560 ]

where,   

A          =  irrigated area, acres

π           =  constant, Pi = 3.1416

 Rs         =  wetted length of the system, feet

 Pr          =  portion of a full circle, decimal

43560   =  conversion constant, square feet per acre

Example 12.1:

If the system does not make a full circle, determine the area irrigated by measuring the distance traveled by any tower using a measuring wheel. Compare that length of travel with the total distance the tower would travel during a complete revolution. The total travel distance per revolution is equal to the circumference of a circle that is C = [2 x p x R] or ( 2 x 3.14 x R) where R equals the distance from the pivot point to the tower being monitored. In this exampl­e, if you measured the distance to the second tower as 372 feet, the total travel distance per revolution­ would be ( 2 x 3.14 x 372) = 2337 feet. If you measured­ the actual travel distance at 2,000 feet, the portion of a full circle irrigated by the pivot would be 2000 / 2337 or Pr = 0.86. To calculate the irrigated­ area enter 0.86 for Pr in Equation 12.1 above or multiply the area for a complete circle by 0.86 and get 104 acres (0.86 x 122). If the measurements are recorded­ to get the travel distance of the outside tower, the accuracy of the irrigated acre calculation would improve.

Estimating Flow Rates

The gross pumping rate required to irrigate a given crop is determined largely by the type of sprinkler irrigation system installed and the crop to be irrigated. Stationary sprinkler systems such as towlines and siderolls require greater flow rates than center pivots to apply a set amount per acre due to lower water application efficiencies (Chapter 8).

Peak water use rates differ between eastern Nebraska­ and western Nebraska. At any location, the only difference among crops is how long the crop uses water at the peak rate and what time of year the peak rate occurs. For design purposes we could start with a system designed to replace 100 percent of peak crop use and which accounts for efficiency of water application.

When estimating the required system flow rate also consider: a) climatic factors; b) system down time for electric load management; c) system down time for repair and maintenance activities; and d) the soil type. The most important climatic factors are humidity­ and the likelihood of rainfall. The minimum net system capacity is determined by estimating how often­ the soil available water content might go below the 50 percent of available water level for a given flow rate, soil type, and location. Nebraska­ has been divided into two areas for estimating system capacity­ using the 22-inches-per-year annual precipitation as the dividing line (Figure 12.4).

Figure 12.4. Identification of two regions for use in determining the minimum net system capacity for center pivots. Regions correspond to data presented in Table 12.1.

During an irrigation season, most systems will be shut down by the operator for maintenance or due to electrical or mechanical failure. Internal combustion engines will require an oil change every 200-300 hours of operation. If the pump is powered by an electric motor, consider participating in the power company’s load control program to reduce power costs. This will mean that on high power demand days, the power company might shut down the system for up to twelve hours. Each time the system is shut down, it reduces the time available for water distribution. To meet crop water demands, the system flow rate must be increased­ so that the desired amount of water is distributed in the time available. The system flow rate in gallons per minute (gpm) needed to meet peak water use rates is determined using Equation 12.2.

Equation 12.2     Qpump  =  [ 18.9 x ETp x Area ] / [ Ea x Tirr ]

where,

Qpump   = gross pumping rate, gallons per minute

K           =  18.9 = conversion constant

ETp       =  peak water use rate, inches per day

Area      =  irrigated area, acres

Ea         =  application efficiency, decimal

Tirr        =  portion of day the system is operating, decimal­

Example 12.2

The center pivot in Example 12.1 (Irrigated area = 122 acres) is planted to corn with a peak water use rate of 0.35 in/day (ETp = 0.35 in/day). The system delivers water with an application efficiency of 85 percent (Ea = 0.85). The system operates an average of 23.5 hours per day (tr = 23.5 hrs / 24 hrs = 0.98).

Determine:  The pumping rate needed to meet crop water needs.

Using Equation 12.2:

          Qpump =    [18.9 x 0.35 in/day x 122 ac] / [0.85 x 0.98]

          Qpump =    969 gpm

This example was based on the knowledge that the system, if operated an average of 23.5 hours per day, could supply sufficient water for a crop using up to 0.35 inches per day during the entire growing season. However, if we assume that we will receive some rainfall, and we use the water stored in the soil profile to supplement the irrigation system during peak evapotranspiration periods, the required flow rate could be decreased. For example, Table 12.1 shows that a system operating 23.5 hours per day located­ in Region 2 with a silt loam w/silt loam subsoil as the major soil type will require a minimum net flow rate of 552 gpm (122 ac x 4.62 gpm/ac / 0.98). If we assume an 85 percent water­ application efficiency, the pumping rate would be 650 gpm. The potential savings derived by reducing the flow rate from 969 gpm to 650 gpm could be significant­. The pump, motor­, pipeline diameter and pumping lift would all be smaller and the main components could be downsized, lowering the installation cost.

If you decide to participate in a load control program­, each 12 hours of control time (or system shutdown) per week requires an increased flow rate of about 8 percent. If the operator agreed to be controlled­ two days per week, the flow rate should be increased­ from 650 gpm to 760 gpm. Table 12.2 provides­ multipliers for each 12 hours of downtime. The same adjustments must be made for planned maintenance breaks.

 

Table 12.1 Minimum net system capacities for the major soil texture classifications and regions of Nebraska.*
Soil Texture Available soil water capacity (inches/foot) Region 1 (gpm/acre)** Region 2 (gpm/acre)**
Loam, silt loam very fine sandy loam w/silt loam subsoil 2.5 3.85 4.62
Sandy clay loam loam, silt loam very fine sandy loam, w/silty clay subsoil 2.0 4.13 4.89
Silty clay loam Clay loam Fine sandy loam 2.0 4.24 5.07
Silty clay 1.6 4.36 5.13
Clay Sandy loam 1.4 4.48 5.19
Loamy sand 1.1 4.83 5.42
Find sand 1.0 4.95 5.89
Peak evapotranspiration***   5.65 6.60
*Taken from Transactions of the American Society of Agricultural Engineers, 27(2):419-428.**Net system capacity required to replace average peak water use rate nine out of ten years. Values must be multiplied by the acres irrigated and adjusted for irrigation efficiency to estimate gross pumping rates.***Estimated net system capacity to replace average crop water use 100 percent of the time.

In the end, the operator/owner of the equipment must decide how much water is enough. Extra system flow rate adds flexibility to system management decisions; however, larger flow rates could limit the types of sprinkler packages that might be feasible. Management flexibility should be weighed against the higher investment and operating costs in order to come to a final decision. The local dealer can provide cost estimates for different system designs.

Table 12.2. Power interruption and repair and maintenance multiplication factors for different system down times.
  Shutdown time (hours) per week
Hours 0 12 24 36 48 60
Multiplier 1.00 1.08 1.17 1.27 1.4 1.55

Peak Water Application Rate

One of the most important criteria for selecting a sprinkler package involves the peak water application rate for the system. Three factors affect the peak application rate — system length, system flow rate, and the sprinkler/nozzle wetted radius. Since in our example we established an irrigated area of 122 acres, the system wetted length at 1300 feet, and the flow rate was 969 gpm, the only factor remaining is the wetted radius of the sprinkler. The key is to select a sprinkler package in which the peak water application rate does not exceed the soil infiltration rate. Selecting a sprinkler package with too large a peak application rate could cause runoff. Equation 12.3 is used to calculate the peak water application rate in inches per hour (in/hr) at the outside end of a center pivot.

Equation 12.3         Ipeak =    [ 122.5 x Qpump ] / [ Rsystem x Rsprinkler ]

where,

Ipeak        =   peak water application rate, inches/hour

122.5       =   conversion constant

Qpump   =   system flow rate, gallons per minute

Rsystem   =   wetted length of the center pivot, feet

Rsprinkler     =         wetted radius of sprinklers at the end of the system, feet

Example 12.3

A center pivot has a system flow rate of 969 gpm (Qpump = 969 gpm), a wetted length of 1300 ft. (Rsystem = 1300 ft.), and a sprinkler package with a wetted diameter­ of 60 ft. (Rsprinkler = 60/2 = 30 ft.).

Determine:  The peak water application rate for the system.

Using Equation 12.3:

Ipeak  =  [ 122.5 x 969 gpm ] / [ 1300 ft x 30 ft ]

Ipeak  =  3.04 in/hr

The peak application rate of 3.04 inches per hour is compared to the soil intake rate to determine if runoff might occur during an irrigation event. This peak water application rate is the same regardless of how much water is applied; however, the soil infiltration rate decreases with water application time. Therefore, large applications can increase the potential for runoff (Figure­ 12.5).

Figure 12.5.  Increase in potential runoff from a low pressure spray nozzle due to an increase in application from 0.5 to 1.0 inches using a 1,340-foot system supplied by an 800 gpm well.

Hours per Revolution/Irrigation Time

When a center pivot is purchased, the manufacturer provides a chart showing the depth of irrigation and the time required to make a revolution at different percent timer settings. This information is based on the flow rate used to design the sprinkler package and the travel speed of the outer most tower; however, the travel speed is for a system with zero wheel slip and will likely need to be adjusted for each field installation. The actual revolution time could be substantially different than presented in the chart.

There are several reasons for accurately recording how long it takes to make a revolution with a center pivot or to complete an irrigation event. Small amounts of water may be needed to activate herbicides applied with ground rigs if rainfall is not timely. If chemigation is used, some chemicals require that the material be applied with a precise amount of water­. As the growing season progresses, system management may require a gradually increasing depth of water application per event. In each case, you must set the percent timer to apply a specific amount of water. This requires knowledge of how the system length, travel speed, and flow rate combine to distribute water to the field.

Each revolution of a center pivot is likely to be slightly different even if the system percent timer remains­ unchanged. The speed of travel (for a given setting) also may change with position in the field. Each time the system makes a revolution it will encounter­ different wheel track conditions. The drive wheels may not slip while going uphill during the first revolution, but may begin to slip during subsequent revolutions. The system will travel faster when going downhill because gravity is forcing the system down the hill. For these reasons, the travel speed should be determined as frequently and for as many field conditions as possible. Determining the speed of travel for a series of percent timer settings is desirable. Construct a table showing the speed of travel at different percent timer settings and mount it in the pivot control box.

If the timer is set to 100 percent, the end tower drive wheels should almost turn continuously; however, a timer setting less than 100 percent causes the end tower to turn on and off (cycle). To determine the approximate­ ‘on time’, take the timer setting, divide by 100 and multiply by 60 seconds per minute. In other words, the timer setting is the percent of one minute that the end drive wheels should turn/move.

The procedure for measuring system travel speed is quite simple. First, measure the distance from the pivot point to the center of the outside wheel track. Start the system and let the travel speed become consistent (approximately one hour). This is particularly important for lower timer settings. Equipped with a 100-foot measuring tape, a stop watch, and two flags, proceed to the outside wheel track. From here two procedures should be followed, depending on the percent timer setting.

When the timer is set to 100 percent, set a flag adjacent to the axle of the rear drive wheel of the outer most tower and start the stop watch as you place the flag. Let the system run for at least 10 minutes. Then set a second flag adjacent to the axle of the same drive wheel and stop timing when you place the flag. In this procedure we are assuming that the wheel is moving continuously. If the percent timer is set to less than 100 percent, a different procedure is more accurate.

When the timer is set to less than 100 percent,the end tower will cycle on and off. Thus, the flags should be set at the same point in the on/off cycle. For example, set the first flag adjacent to the axle of the rear drive wheel while the system is not moving. Start your stopwatch when the tower begins to move. Let the system run for at least 10 minutes. Set a second flag adjacent to the axle of the same drive wheel while the system is not moving. Stop the stopwatch when the tower begins to move.

Record the time of travel in your record book. Be sure to convert seconds to a decimal portion of a minute ( 20 seconds = 20/60 = 0.33 minutes). Then measure the distance between the flags to determine how far the system has moved. Record the distance traveled in feet in your record book. The speed of travel is calculated using Equation 12.4:

Equation 12.4     Stravel = Dtraveled / Ttravel

where,

Stravel         =   system speed of travel, feet/min

Dtraveled      =   distance traveled, feet

Ttravel          =   travel time, minutes

 

The accuracy of the speed of travel measurement is critical when estimating the revolution time of the system. Consequently, the more locations in the field a travel speed measurement is recorded, the more accurate the revolution time estimate will be.Equation 12.5 is used to calculate the system revolution time in hours.

 

Equation 12.5     Trevolution      =      [ 2 x π x Lend ] ¸  [ Stravel x 60 ]

where,       Trevolution  =   system revolution time, hours       π               =   constant, Pi = 3.14       Lend           =   distance from pivot point to the end tower, feet       Stravel        =   system speed of travel, feet/minute

Example 12.4

A center pivot is 1270 feet long (Lend = 1270 ft.)to the outside tower. A speed of travel test found that the system traveled 30.3 feet in 10.4 minutes.

Determine:      The time required to make one full revolution.

Using Equation 12.4:

Stravel        =     30.3 feet / 10.4 minutes

Stravel        =     2.91 feet/min

Using Equation 12.5:

Trevolution   =   [2 x 3.14 x 1270 feet] / [2.91 ft/min x 60 min/hr]

Trevolution   =   45.7 hours per revolution

 

Another means of determining system revolution time is to record the stop and start time for each revolution each year along with the percent timer setting.  By recording these times, a more accurate revolution time can be determined that integrates all the factors that could impact revolution time as discussed above.

Water Depth Applied per Revolution

Once the irrigated acres, revolution time, and flow rate have been determined for the system, the calculation of the water application depth per revolution is easy. The water applied at different travel speeds can be calculated by changing the irrigation­ time per event or revolution time. Use Equation 12.6 to determine the depth of water applied in inches per revolution.

Equation 12.6     Idepth  =   [ Qpump x Trevolution ] / [ 450 x Area ]

where,

Idepth           =   depth applied per revolution, inches

Qpump         =   irrigation system flow rate, gallons per  minute

Trevolution    =   revolution time, hours

Area            =   irrigated area, acres

450              =   conversion constant

Example 12.5

A center pivot has a pumping rate of 969 gpm (Qpump = 969 gpm) and require­s 72 hours to make a revolution ( Trevolution = 72 hours). The area irrigated by the center pivot is 122 acres (Area = 122 acres).

Determine:  The depth of water applied per revolution.­

Using Equation 12.6:

Idepth = [969 gpm x 72 hours] / [450 x 122 acres]

Idepth = 1.27 inches

To check our math let’s determine if the water applied­ per revolution would meet crop water demands­ of 0.35 inch per day. Dividing 1.27 inches per revolution by 72 hours (72 / 24 = 3 days) per revolution we get 0.42 inches applied per day (1.27 / 3). But remember that we assumed­ that we were going to operate the system 23.5 hours out of each day (98%) and that the water application efficiency was 85 percent. Considering those two items gives a net depth applied of 0.35 inches per day [ 0.42 x 0.98 x 0.85 = 0.35 inches per day ].

Measuring Depth of Application

The depth of water applied per irrigation is one of the most important pieces of information needed to manage an irrigation system. Not knowing how much water is being applied could result­ in reduced yields due to under irrigation or excessive­ production costs and nitrogen leaching due to over irrigation. It is imperative than an accurate measurement of the system flow rate be acquired.

Rain gauges provide an inexpensive means of determinin­g if a system is applying the depth of water­ it is supposed to be based on the calculations in the previous sections. The problem is: How large should the rain gauge be?  And, How many rain gauges does it take to get an accurate estimate of the depth of water applied? In general, larger rain gauges provide a more accurate estimate of applied water.  Gauges should be at least 4-inches in diameter and be capable of recording the range of irrigation and rainfall depths that might occur during the growing season. Gauges used for the Nebraska Rainfall Assessment and Information Network (NeRAIN) will work well (http://dnrdata.dnr.ne.gov/NeRAIN/docs/about.asp).

Rain Gauges

Rain gauges can provide an estimate of the water applied if the depth applied is recorded immediately after an irrigation event. If you read your rain gauge daily, evaporation will reduce how much water remains in the gauge so you will underestimate your water application depth. To eliminate evaporation from the rain gauge, pour a little vegetable oil into the gauge. Because oil is less dense than water, it will float on the water surface effectively­ eliminating evaporation from the rain gauge. Each type rain gauge will need a different amount of vegetable oil to get the job done because of its size and shape. When you read the rain gauge ignore­ the oil floating on the water surface.

A minimum of one rain gauge can be used for each 160 acres; however, a much better estimate can be achieved by using at least one rain gauge per 40 acres. With only one rain gauge, you might be asking yourself: “Where do I place the rain gauge to get the most accurate results?”, and “Did the small rain cell move across the middle, east, west, north or south portions of the field?” Other problems could develop if you want to use the same rain gauge to record the sprinkler’s water application. As the center pivot makes a revolution, it goes through the stop-start cycle determined by the percent timer setting. Without doubt, the system will not pass over a point in the field in the same manner each time it makes a revolution. In addition, the wind speed and direction will rarely be the same. One should expect that the depth of water caught in the rain gauge would be different for each revolution. So a single rain gauge may provide a very good estimate of the water applied during one irrigation event and a horrible estimate the next irrigation event. That is why more rain gauges provide a better estimate. Each rain gauge position will integrate the environmental, topographic, and system conditions prevalent during that irrigation event. By averaging the different positions, a more accurate estimate can be determined for the entire field.

Water Meters

Another means of determining how much water was applied is to directly measure it using a water meter. Most water meters have a totalizer that will be accurate to within 1-3 percent for the irrigation event and the season. That kind of accuracy is nearly impossible­ with rain gauges. Different types of water meters are available. Each type of meter uses a different­ means of measuring the flow of water through a pipeline. In addition, the accuracy of the meter and its initial cost vary among meter types. Propeller meters have received the most widespread use with irrigation.  UNL NebGuide G1426 Using Ultrasonic Flow Meters in Irrigation Applications describes use of an ultrasonic flow meter to determine system flow rate.  The NebGuide includes recommendations on how to select a good location for collecting flow rate measurements.  Recommendations for other meter technologies is the same as described in that publication.

Site Selection

With every sprinkler system, site selection begins with answers to questions like: Are the field conditions appropriate for irrigation? Will I be irrigating any areas that sometimes contain surface water? Is there an adequate water supply to irrigate with? Does the field prohibit the use of the type of irrigation I am proposing? Is there a source of electricity close by? Will the crop I plant respond to irrigation at that location? Will the irrigation system be able to deliver­ water uniformly? and, Will irrigation produce enough extra income to pay for the investment and operating costs? If the answers to these questions are favorable, some detailed investigations can begin.

Field Data Collection

Soil survey mapsare an excellent source of estimates for soil drainage classes, field slopes and soil water holding capacities.  Figure 12.6 shows a copy of a quarter section in Pierce County, NE.  Determine the area of each mapping unit by contacting your local Natural Resources­ Conservation Service­ Office. Record the total number of acres of each mapping unit in a table (Table 12.3). Obtain the soil intake­ family, average field slope, and the soil water holding capacity information for each mapping unit, and record them in the table.  If soil moving has occurred on the field, develop­ a map showing the locations where surface soil has been removed and store it with the permanent field records.

Figure 12.6.Soil survey for a 160-acre tract of land located in Pierce County. Summarized information is provided in Table 12.3.

Begin by looking at the mapping units with substantial­ areas (20 acres or more). Pay particular attention­ to mapping units with steep slopes (greater than 7 percent) and with low infiltration rates (less than the 0.5 intake family). Also look for the soil water­ holding capacity. If sufficient area is involved, the system may need to be designed and managed according­ to areas with low water holding capacity or steep slopes. You probably should not select a system to match soils that comprise less than 10 percent of the irrigated area; however, field areas with 25 to 50 acres cannot be ignored. Tabulating soil information in this manner will make it easier to make your selection. Many field sites will have more soil mapping units than this one; however, the process is educational because it clearly identifies which field areas could present problems for water application and crop production. You will begin to understand how the particular sprinkler package you select could impact­ irrigation efficiency.

Table 12.3 Summary of soil mapping unit information for a quarter section of land in Pierce Counth, NE and presented in Figure 12.4.
Soil mapping unit Field slope Intake family number (%) Water holding capacity (inches/foot) Field area (acres)
Co 0 to 1 0.3 0.20 to 0.23 42
He 0 to 1 1.0 0.21 to 0.23 24
CsC2 1 to 7 1.0 0.20 to 0.23 11
HhC 1 to 7 1.0 0.21 to 0.23 5
MoC 1 to 7 0.5 0..19 to 0.22 37
CsD2 7 to 11 1.0 0.20 to 0.23 28
MpD 7 to 11 1.0 0.20 to 0.23 2
CsE2 11 to 17 1.0 0.20 to 0.23 11

Construct a surface topography map. Many sprinkler packages are designed without a field site visit. The field visit, however, is one of the most importan­t aspects of selecting a sprinkler package because­ the dealer rarely has all the information needed. For example, one of the most significant reasons for not selecting a particular sprinkler package is field slope. Though soil mapping units show average field slope conditions, the data is seldom sufficiently accurate to allow an educated decision. A rough topography map, for example on a 400' x 400' grid, will catch major field elevation differences. However, to determine if areas mapped as 7 to 11 percent slopes are closer to 7 percent or 11 percent may take more intensive measurements. Take readings in other areas if the highest or lowest position in the field occurs between grid points. This information can be plotted on graph paper and contour lines added to delineate steep slopes from flat areas.

Two other pieces of information are minimum requirements­ for selecting and designing a sprinkler package — the pump output pressure at the desired flow rate, and the elevation difference between the pivot point and the highest point in the field. Without accurate estimates of these data, the sprinkler package may not operate as designed.

A field farmed on the contour can safely use a sprinkler package that would otherwise generate small amounts of runoff. Crop residues left on the soil surface absorb the impact energy of rainfall and irrigation. Thus, the soil infiltration rate would be more consistent throughout the season. Soil residues also reflect incoming radiation resulting in less soil evaporation. Each of these factors may contribute to a slightly different selection.

Sprinkler Types

Pressure ratings for center pivot sprinkler packages span a broad range. Within each major category the range in pressure could be more than 20 pounds per square inch (psi). The exact break off point between­ categories is debatable. Operating a sprinkler/nozzle package at or above the upper pressure limit produces small water droplets that will be easily transported by wind. A package operated below the low pressure limit will produce large water droplets and a reduced wetted diameter. Large water droplets can encourage a soil crust to be developed on the surface of some soils that could greatly reduce soil infiltration rates. Below is a general list of sprinkler package categories.

—High pressure impact (HPI)          —50 to 70 psi

—Medium pressure impact (MPI)          —40 to 55 psi

—Low pressure impact (LPI)          —25 to 45 psi

—Low pressure spray (LPS)          — 6 to 25 psi

As you look for suitable options, remember the existing pipeline and system. For example, a water drive system requires high pressure to operate the drive mechanism.  Thus, reduced pressure packages may not be practical­. Likewise, older electric drive systems typically had wide spacings between sprinkler outlets. More outlets would need to be added to install low pressure spray nozzles. More information about how to deal with converting systems from one operating pressure to another can be found in UNL NebGuide G1124 Converting Center Pivot Sprinkler Packages: System Considerations.

Nozzle orientation represents the largest change in sprinkler package options. The trend has been toward­ more narrow spacings but at largely constant spacings between nozzles. This results in a larger number of nozzles, but limits the size of individual nozzles at the outer end of the system. A range in spacings of 7.5 to 19 feet is common for systems with nozzles mounted above the lateral or just below the truss rods.

Low pressure spray nozzles can also be mounted on drop tubes below the truss, at canopy level, or at various levels within the canopy. The most extreme case is to mount the nozzle about 12-18 inches above the soil surface. Each arrangement alters­ a number of water application factors, most notably,­ the peak water application rate. For example, some nozzles are capable of operating in the bubble mode where the water bubbles out of the nozzle at less than 6 psi. These nozzles are used as part of the Low Energy Precision Application (LEPA) system and can have a peak application rate of over 60 inches per hour when installed on a 1300-foot center pivot supplied by an 800 gpm well. No soil can infiltrate water that rapidly, so soil surface storage is required­ to prevent runoff.

A typical spray nozzle mounted at truss level may have a peak application rate of 3-5 inches per hour. Depending on the depth of water applied, this water application rate could be used successfully under­ many field conditions without the need for soil surface storage. Spray nozzles mounted at truss level could produce runoff when field slopes are greater than 5 percent on silty clay soils. Care should be taken to ensure that the sprinkler package is well suited to field conditions.

Positioning the sprinkler near or in the plant canopy reduces the impact of wind drift and canopy evaporation, making water conservation a motivation. However, it may cause two negative outcomes. In general, the closer the nozzle is to the ground the greater the water application rate. If the nozzle is positi­oned well into the canopy in a field with a very sandy soil, plant stems and leaves will intercept the water pattern causing poor water distribution. Extremely­ poor uniformity may result in decreased production. For such installations, nozzle spacings must be reduced to 5-7.5 feet to provide uniform water­ application. Increasing the number of nozzles increases the total cost of the installation. Positioning the nozzles within the canopy also may affect the use of insecticides since water may not contact all areas that need treatment.

Package Selection

The main goal for water application systems is to apply water uniformly in sufficient quantities to meet crop water needs without generating runoff. In addition, the system must work well with your management plan. This chapter has covered many factors influencing these goals. Let’s separate these goals into four parts.

1. To apply water uniformly requires that the correct sprinklers/nozzles be installed at the various positions along the pivot lateral, that the pumping plant deliver water at the appropriate pressure and flow rate, and that the system is not operated under high wind conditions. Uniformity of infiltration is another­ factor. Water can be applied to the soil surface with a high degree of uniformity and yet the infiltration­ uniformity can be quite poor due to surface­ runoff. The goal of the system designer must be to consider how the sprinkler package will match up with the field soil and slope conditions.

Some of this may seem trivial but it is not uncommon to find sprinklers incorrectly located on the pivot. Worn or damaged sprinklers are sometimes replaced­ by whatever is available at the time. Likewise, the spacing between sprinklers may not be correct­. Older systems may have enough nozzle wear to alter the amount of water applied by the system.

Problems due to sprinkler placement can be identified in two ways: a) get up on the pivot support truss and compare the sprinklers in each position with the system design computer printout; and b) perform a water uniformity check. Either choice takes some time and effort.

It is safe to say that the uniformity of water application­ generally improves with a decrease in sprinkler/nozzle spacing. This statement assumes that the operating characteristics of the sprinkler do not change. Narrowing the spacing results in more overlap­ among the water application patterns of individual­ sprinklers.

In the absence of some sort of flow control, the topographic features of the field can greatly alter the water distribution uniformity of a center pivot. This is particularly true for low pressure packages. Since each sprinkler has an orifice through which water is metered, altering the pressure supplied to that orifice changes the sprinkler output. The water distribution is inversely proportional to the field elevation unless pressure regulating devices are installed at each sprinkler/nozzle location. If the field is sloped uphill from the pivot point, the sprinklers at the highest elevation will be distributing less water and those close to the pivot will be distributing more water than is indicated on the design sheet. For this reason, it is recommended that flow control devices be installed if the elevation difference results in a change of flow greater than about 10 percent.

2.  Another factor that is generally ignored is the effect­ of the start-stop cycle of the drive towers.This factor will have little influence on uniformity of a medium­ or high pressure sprinkler package (wetted radius>35 feet); however, with low pressure spray nozzles and all in-canopy packages, the water uniformity can be reduced at low timer settings. This is because­ the tower, while moving, advances 8 to 14 feet per minute, but may stand stationary for 10-40 seconds­ of each minute.

Considerations for meeting crop water needs should include whether the system will be available to run continuously or will it be shut down due to electric load control or maintenance. The main factor is how long will the crop use water at peak rates and what time of year will water be needed. These factors are used in conjunction with rainfall probabilities and the soil type to determine the minimum system capacity­. Extra capacity will provide the ability to apply­ water more rapidly but may require greater investm­ent in the pumping plant and sprinkler system.­

3. The zero runoff goal requires that the sprinkler package selected for the system be carefully matched to the field conditions and to the operator’s management plan. Too often the desire to reduce pumping cost clouds the issue of water application uniformity. Try to avoid sprinkler/nozzle selections that could result in runoff. This requires comparing the sprinkler’s water application pattern to the soil infiltration rate. If an accurate estimate of soil surface storage is available, it should be considered in the analysis.

Each sprinkler will deliver water to the soil with a particular range of water droplet sizes and distribution of water droplets within the water application pattern. In general, larger water droplets are concentrated toward the outside of the wetted radius and smaller droplets fall closer to the sprinkler or nozzle. It is the large water droplets that tend to be a concern. Large water droplets carry a substantial amount of energy that is transferred to the soil upon impact. The impact will tend to break down the soil clods on the soil surface causing the soil to consolidate. Eventually a thin crust will be formed on the surface that can reduce­ soil infiltration by up to 80 percent compared to protected conditions.

The distribution of water droplet sizes delivered by a sprinkler may be altered by selecting a fine or medium grooved deflection plate for spray nozzles or installing controlled droplet size (or spreader) orifices in impact sprinklers. Either alternative reduces the average size of water droplets.

4. Irrigation management plans include such things as whether you start the system as soon as the soil will hold the water you plan to apply (no-sooner-than date) or if you are more prone to wait until the plant has used much of the allowable deficit (no-later-than date). If you wait until the plant has removed­ close to 50 percent of the available soil water­, you will need a system flow rate that is greater than if you irrigate as soon as possible. This is because­ you will have used up the buffer provided by the soil water holding capacity. The procedures for employing these two management strategies is presented in Chapter 10.

Management plans also include whether you leave room for rainfall and if you feel the need to have enough capacity to supply crop water needs 100 percent of the time. Leaving room for rainfall increases­ the water use efficiency within your field. However, like using the no-later-than irrigation timing approach, it will require slightly greater system flow rates to ensure that adequate water is available. If you are willing to risk not meeting all crop water needs, the system flow rate and some components could be smaller.

Chemigation is an appropriate means of applying both fertilizers and pesticides. By scouting your field for insects, you may be able to eliminate the application­ of insecticides. If needed, the center pivot can apply the material safely and uniformly when necessary safety equipment is present and you monitor the system during application. However, if you install a nozzle package with nozzles 3 feet above the soil surface on corn, efficacy of the insecticide applied with center pivot may be inadequate. Chemigation would not be appropriate for a field with surface water­ present. If you plan to chemigate, select your sprinkler package carefully.

Part circle center pivots require extra management to minimize wheel tracks, and deep percolation losses. A center pivot with computerized controls would help provide the necessary adjustments to system­ operation. Computerized controllers can also be useful if you plan to employ some site-specific­ irrigation­ practices.

Summary

Selecting a sprinkler system that is capable of applying­ the correct amount of water at the appropriate time requires careful analysis of field conditions and the management scheme you plan to employ. The selection process should begin with collecting field information and then selecting a system that will operate effectively and efficiently.  Information about the soil mapping units present in the irrigated area along with soil water infiltration, and soil water holding capacity are key factors involved in irrigation management.  Field topography must be considered when selecting a sprinkler/nozzle package.  System flow rate, and sprinkler/nozzle package are keys to applying water efficiently.  Should questions arise, specialists with the University of Nebraska Lincoln Extension, the Natural Resources Conservation Service, and your local Natural Resources Districts can help develop answers.