Microsprinkler Irrigation Management - What's Your Application Rate?

Article reproduced with permission
Citrus Industry Magazine, March 2000

By L.R. Parsons, K.T. Morgan, and T.A. Wheaton

Note: Figures and tables mentioned in this article are not available at this time.

Microsprinkler irrigation was introduced into Florida citrus in the early 1970s. They were promoted as a low-volume form of irrigation that could save water and pumping energy. Initially, there were relatively few microsprinkler brands and spray patterns available. Most emitters consisted of one- or two-piece units that sprayed water in full-circle, half-circle, or quarter-circle patterns.

Demand for microsprinklers increased greatly when it was found they could provide some frost and freeze protection. New citrus plantings during and after the severe freezes of the 1980s made Florida one of the fastest growing markets for microsprinkler irrigation between 1985 and 1990. Current estimates are that microsprinkler irrigation is used on more than 450,000 acres of Florida citrus (Smajstrla, 1995).

Responding to increased demand, a number of manufacturers have introduced new emitters to the market. Today, growers have an extensive choice of emitters that vary widely in gallonage output, spray diameter, and spray patterns. Spray diameters now range from 3 to more than 20 feet. Deflectors or 'top hats' that concentrate water on a small area are available for young trees (Fig. 1). This large selection of emitters is beneficial, but growers may be unaware of the wide range of application rates produced by the various patterns and flow rates available. Accurate information on application or precipitation rates is essential for good irrigation management.

This article, among several to be presented on microsprinkler irrigation management, discusses the basis for determining application rates and irrigation duration. Earlier articles (Parsons, et al., 1993, 1995) considered irrigation management in relation to the potential leaching of agricultural chemicals. Future articles will discuss soil water holding capacity and ways to improve irrigation management.

Irrigation Application Rate
Knowing the microsprinkler application rate on the wetted area is essential to good irrigation management. When microsprinklers were first used in Florida, application rates varied little, because many full-circle microsprinklers had similar, relatively low, application rates of 0.10 to 0.15 inch/hour on the wetted area.

New developments have made many more brands and spray patterns available. Application rates now vary widely, from 0.05 to 3.00 inches/hour or more, a 60-fold range. Because there is a direct relation between application rate and irrigation duration, there can also be a 60-fold range in the duration to complete an irrigation. For example, 8 minutes is required to apply 0.4 inch of water on the wetted area with an application rate of 3 inches/hour, while 8 hours is needed to apply the same amount at a rate of 0.05 inch/hour. This range of rates is readily available with microsprinklers used in Florida citrus groves today.

With any application rate, operating the system for too long results in excessive amounts of water on the wetted area, which can drive most of the water below the main root zone. Excessive irrigation wastes water and energy, leaches fertilizers and chemicals below the root zone, causes an economic loss of these chemicals, and potentially contributes to groundwater contamination. To develop a proper irrigation schedule, the average application rate of the microsprinkler must be known. Most microsprinkler manufacturers list the gallonage output and spray diameter of their emitters. However, the application rate of each emitter may not be presented.

Application rates can be calculated from manufacturer's specifications of flow rate and wetted area diameter. The most accurate information on flow rate and wetted area can be obtained by measuring these factors in the grove with the irrigation system operating under normal conditions. For circular patterns, measure the wetted spray diameter of several microsprinklers and calculate an average.

Flow rate can be determined by measuring the length of time to fill a container of known capacity. Collect the flow from representative microsprinklers and record the time needed to fill a pint (16 oz.) or quart (32 oz.) measuring container. Divide 450 by the number of seconds to deliver one pint or divide 900 by the number of seconds needed to deliver one quart. The result is the emitter output in gallons/hour.

For example, if it takes 30 seconds to deliver one pint, that is equal to an output of 450/30 or 15 gal./hour. If it takes 50 seconds to deliver one quart, the emitter is delivering 900/50 or 18 gal./hour. Determining emitter flow for 30 seconds to 3 minutes provides adequate measurement accuracy.

The application rate is the flow rate of water divided by the area on which it is applied. The square feet per acre and gallons per acre inch are shown in Equation 1. The wetted area of a 360¡ pattern is calculated as the area of a circle. Area may be calculated from the radius (A=¹R2) or from the diameter (A=¹D2/4). If radius is known, multiply it by 2 to convert to diameter before using the following equations. Equation 1 looks complicated because it includes all the terms needed to calculate application rate.

Equation 1

(FR) (43560 ft.2 acre-1)
AR = ¹(D2) (27154 gal. acre-1 inch-1)

where

AR = Application rate (inch/hour)
FR = Flow rate of emitter (gallons/hour)
D = Diameter of spray pattern (feet)
¹ = a mathematical constant = 3.14159

Equation 1 simplifies to Equation 2 which allows easy calculation of application rate.

Equation 2

(2.04) (FR)
AR = D2

Simply put, the application rate can be estimated by multiplying the flow rate by 2.04 and dividing that by the diameter squared.

Average application rates of microsprinklers with different flow rates and spray diameters are shown in Table 1. Figure 2 visually shows what typical 10, 16, and 24 gal./hour microsprinklers would apply on areas of different diameters. Note that the application rate on the wetted area increases rapidly as the spray diameter decreases. Because application rate responds to the square of the diameter, halving it results in a four-fold increase in application rate. This result can be seen in Fig. 2 where a 10-gph microsprinkler with a 16-foot diameter has an application rate of 0.08 inch/hr. If the same 10 gph is applied to an area with half the diameter (8 feet), the application rate increases fourfold to 0.32 inch/hr.

The increase in application rate becomes more dramatic if the spray diameter is cut to one-third the original size. In that case, the application rate increases ninefold (or 32). In Fig. 2, a 16-gph emitter with an 18-foot diameter pattern has an application rate of 0.10 inch/hr. When the diameter is reduced to one third Ñ to 6 feet Ñ the application rate jumps ninefold to 0.91 inch/hr. An extreme example would be decreasing the diameter to one-fifth the original size, which would increase the application rate 25-fold (or 52). When a 16-gph emitter with a 20-foot diameter is reduced to 4 feet Ñ or one-fifth the original size Ñ the application rate changes by 25-fold, from 0.08 to more than 2.0 inches/hr. (Fig. 2).

The dramatic increase in application rate is clearly shown in Fig. 3. When the diameter is below 8 feet, the application rate rises steeply. This fact becomes very important if top hats or other deflectors are used to concentrate water around a young tree. These deflectors are commonly about the size of a quarter and snap on to the top of the spray assembly. They do a good job of deflecting the water spray downward and focusing the water onto a smaller area. This confines the water to the root zone of a newly planted tree, but greatly increases the application rate on the wetted area (Figs. 1, 3).

Length of irrigation
The length of irrigation time depends on the application rate, the amount of water needed in the wetted zone, and the irrigation efficiency of the system. The application rate is the value calculated in Equation 2. The necessary water depends on a number of factors which will be discussed in future articles in this series. Irrigation efficiency means the fraction of water applied that reaches the root zone. Water can be lost by evaporation of spray droplets during application, or by runoff from the soil surface before it reaches the root zone. Efficiency depends on a number of factors, but values in the range of 0.75 to 0.85 are commonly used.

The length of time required to apply the desired inches to the wetted area is shown in
Equation 3:

LT = I (E) (AR)

where
LT = Length of time (duration) of irrigation (hours)
I = Inches to apply on the wetted area (inch)
AR = Application rate (inch/hour)
E = Irrigation efficiency (%)

Irrigation durations required to apply different amounts of water are shown in Table 2 and Fig. 4. This table uses an efficiency of 75 percent. For systems of greater or lesser efficiency, values can be adjusted according to Equation 3. This discussion of application rates and irrigation time has centered on microsprinklers. The concepts and equations, however, also apply to overhead sprinklers and drip systems. For drip systems, the wetted area for the drippers must be determined and multiplied by the number of drippers per tree. The wetted diameter of drippers generally balloons below the soil surface and must be established using the shovel method. Application rate and time to irrigate are based on the same principles used for microsprinklers. Because of overlap with overhead sprinklers, the wetted area is calculated from sprinkler spacing instead of wetted area diameter.

Special Situations
Variations in Spray Patterns. The wetted area of circular patterns is easily calculated, and the application rates for 90¡ and 180¡ patterns may be estimated as four times or two times greater, respectively, than a 360¡ pattern. Application rate may also vary considerably within the irrigated zone, due to the design characteristics of the microsprinklers. In most cases, an average application rate is assumed.

The application rate of several microsprinklers is not very uniform. Some emitters put out a 'doughnut' pattern where more water is thrown to the outside and less remains near the center. With these type of units, an adjustment in the wetted area may be needed. Distribution patterns of a number of microsprinklers are shown in University of Florida Extension Bulletin 256 (Boman, 1989).

Chemigation and Fertigation. The impact of application rate on chemigation and fertigation practices must also be considered. Small diameter patterns or deflectors are of particular concern. The high application rates with such systems require very short duration irrigations to avoid leaching losses. In some cases, it may be difficult to inject and flush irrigation lines without exceeding the optimal length of irrigation.

Elevated Emitters. Emitters elevated 2 to 3 feet inside the tree canopy have improved microsprinkler freeze protection effectiveness. Because of labor costs, some growers have kept the microsprinklers elevated year-round. The leaves and branches that intercept water from the elevated emitter can distort the pattern and alter the application rate. A non-uniform application situation may result in some groves, but may not be a problem in other groves.

Resets in Mature Groves. Resets in a mature grove present a special problem. A deflector or top hat can usually cover most of a young tree's root system for the first one or two years. With the higher application rate of the hat, excessive irrigation will occur on the resets because of the longer durations required for mature trees. An emitter with the same area coverage as that used for a mature tree will put water outside or beyond the reset root system. Installing lower gallonage emitters at the resets may be a partial solution, but too many emitters of different outputs can upset the irrigation design and cause potential pressure problems. Furthermore, emitters delivering less than 10 gph may require improved filtration if the water is of fair or poor quality. There may be no one best solution for resets in a mature grove.

Growers must recognize the probability of overirrigation with resets, and may need to modify their management. For example, a controlled release fertilizer for young trees may be desirable where localized overirrigation is unavoidable.

Pulsation of System. Pulsators are devices that attach to the individual emitters and turn the water on and off rapidly in a matter of seconds. They help increase the wetted diameter of drip or very low volume systems. With pulsators, it is particularly important to calculate application rates based on measured flow rate and wetted diameter under field conditions. Because flow rates are in the 2- to 6-gallons-per-hour range and diameters can be large, the application rate of pulsators can be quite low. If the application rate becomes too low, system efficiency can decrease due to increased evaporation.

Irrigating for relatively short intervals more than once a day may also be referred to as pulsing the irrigation system, but should not be confused with individual pulsators. In fact, irrigation more than once daily is used on some crops to avoid stress. During periods of high water demand, the plant uses up part of the water from the first irrigation before the second one begins. This reduces the potential for movement of water below the root zone.

Summary
The first step in good irrigation management is establishing the application rate of the system and determining the duration of irrigation necessary to provide the desired amount of water. The large range of application rates provided by different irrigation systems is not widely recognized. Equations, tables and graphs provide the basic information needed. The amount of water required, movement of water, and other factors of importance in irrigation management will be discussed in future articles.

Literature Cited

Boman, B. J. 1989. Distribution patterns of selected emitters used for microirrigation of Florida citrus. Univ. of Fla. Coop. Ext. Serv. Bulletin 256. 21pp.

Parsons, L. R., K. T. Morgan, and T. A. Wheaton. 1993. Effects of microsprinkler precipitation rate, soil type, and water depletion on depth of soil setting. Proc. Fla. State Hort. Soc. 106:38-41.

Parsons, L. R., T. A. Wheaton, D. P. H. Tucker, and J. Noling. 1995. Management of soil applied agricultural chemicals on Ridge citrus. Citrus Industry. 76(2):10-11.

Smajstrla, A. G. 1995. Florida. In: 1994 Irrigation Survey Overall Growth a Continuing Trend. Irrigation Journal. 45(1):27-42.

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