Sprinkler System

International Training Workshop

Design and Evaluation of Pressurized

Irrigation Systems

March 3-7, 2009

Sponsored by

Islamic Development Bank

Prof. Dr. Muhammad Latif

Director

Centre of Excellence in Water Resources Engineering

University of Engineering and Technology

Lahore - Pakistan

Unit I

SPRINKLER IRRIGATION SYSTEM AND ITS TYPES INTRODUCTION

Sprinkler and trickle irrigations together represent the broad class of 'pressurized' irrigation methods. In these systems water is conveyed through a network of pipes to a point in the field where water is applied. The basic difference between surface irrigation and pressurized irrigation is that in surface irrigation methods, soil surface of upper parts of the field is used to transport water to lower parts of the field. Thus, surface irrigation methods are much more affected by soil topography, infiltration and soil type as compared to the pressurized irrigation method. Basic concepts about sprinkle irrigation and its type are discussed in this lecture.

Water is applied in droplets like rain by a sprinkler system. This method has been developed since the early part of 20th century primarily from irrigation of lawns, orchards and nurseries. Reduction in cost due to improvement in technology and development of light weight aluminum pipe led its extensive use in irrigation of field crops and vegetables.

A pump is normally used to lift water from the source and pressurize it to throw into the air for distribution by sprinklers. Other components of a sprinkle system include main pipeline, sub-mains, laterals, valves and sprinkler heads which distribute water across the surface of the field. A typical layout of a sprinkler system is shown in Fig. 1.1 whereas Fig. 1.2 shows different components of a trickle irrigation system. Sprinkle irrigation has some advantages over surface irrigation such as:

i. Sprinkler irrigation can be used on such undulating lands which are difficult or impossible to irrigate by surface methods.

ii. Sprinkler irrigation is suitable for light and frequent watering whenever needed, such as for germination and frost protection.

iii. Sprinkler irrigation may potentially provide better uniformity of application and high application efficiency.

iv. More effective use of a small, continuous stream of water such as from springs or dug wells.

Figure 1.2 Components of a drip irrigation system.

Irrigation of steep and rolling topography is possible by sprinkling without producing runoff or erosion.

v. Normally sprinkle irrigation is more efficient than surface irrigation which will result in water saving and thus more land can be irrigated with the same water supply.

vi. Much less labour is required in sprinkle method as compared to surface methods particularly with centre-pivot sprinkler system.

The sprinkle method has also some disadvantages which are mainly related to high cost, water quality, climatic and environmental constraints. To off-set the cost, high value crops, vegetables and young orchards are frequently irrigated by sprinkling. If it is unavoidable to use salty water by sprinkling, crop damage may be reduced if good quality water is applied at the end of irrigation to wash any salt deposited on the leaves. The use of sprinkler irrigation is less favorable under extremely dry and windy conditions due to increased evaporative water losses and poor distribution pattern. The efficiency may generally be improved by sprinkling during nights and less windy periods.

sprinklers are set at a particular position in the field, and 2) the continuous move systems that operate while the system is moving in the field based on portability, systems may be further classified as portable, semi-portable, semi-permanent and permanent systems. Centre-pivot system and linear move system are examples of continuous moving system.

Portable System

A portable system has portable main lines, sub-mains, laterals and a portable pumping plant. The entire system is designed to be moved from field to field or to different sites in the same field. This system may be moved manually or by mechanical power. In manually moved system called 'hand move system', the pipes are moved manually. This system has low initial cost but labour cost is high. In the mechanically moved system, the lateral is mounted on wheels and is moved as a unit instead of one pipe at a time. This system has high initial cost but labour requirement is less.

Semi-Portable System

This system is similar to a fully portable system except that the location of water source and pumping plant are fixed. Such a system may be used on more than one field where there is an extended main line, but may not be used on more than one farm unless there are additional pumping plants (see Fig. 1.3).

Figure1.3: Sprinkler with Portable Stand.

Semi-Permanent System

A semi-permanent system has permanent mains and sub-mains which are usually burried with risers at suitable locations for connecting the laterals, which are portable. Water source and pumping plants are stationary.

Permanent System

A fully permanent system commonly named as solid-set sprinkler system consists of permanently laid mains, sub-mains, laterals, stationary water source and pumping plant. Mains, sub-mains and laterals are usually burried. Sprinklers are permanently installed on each riser. Such systems are costly and are suited to automation. This type of system is usually installed in orchards, Golf courses and nurseries.

SET-MOVE SYSTEMS

Set-move sprinkle systems are periodically moved from one position (irrigation) to another by hand or mechanically. These systems remain stationary as water is applied where they are set. When the desired amount of water has been applied, water is turned off: the pipes are drained and the system is moved to the next position. When the move is completed, water is turned on and irrigation is resumed at the next position. This process is repeated until the entire field has been irrigated. Common types of set-move systems include hand-move, tow-move, side-roll and gun-type.

Hand-move systems

Components of the system are moved by uncoupling the pipes to the next position by manual labour. This system became popular after the development of light weight aluminum pipes. Most hand-move laterals are 50 to 150 mm (2 to 6 in) in diameter. Length of pipe sections may be either 6, 9 or 12 m (20, 30 or 40 ft). These systems are least expensive but have high labour requirement. An example of hand-move system is shown in Fig. 1.4.

lateral after draining water. This system is the least expensive mechanically move system. Tow-move systems are not used extensively because moving the lateral is tedious and also damage crop. This system has been generally used for forage and row crops.

Side-roll system

This system is very popular type of mechanically moved system. Sections of long lateral pipes are supported on wheels with the lateral serving as axle of the wheels (see Fig. 1.5). Length of the lateral may be as long as 800 m (about one half of a mile) and diameter is normally 100 to 125 mm (4 to 5 in). A gasoline engine is usually fixed in the center of the lateral to drive it to the next position. Common wheel diameters used for side-roll systems vary from 1.2 to 1.9 m. The wheel diameter must be large enough to allow the lateral to pass over the crop without damaging it. Water is supplied either in the middle of the lateral or at the end of the system. The friction losses within the lateral pipe are reduced when water is supplied at the center of the lateral.

Figure 1.5: A Side-Roll Sprinkler System.

Gun and Boom Sprinklers

Gun or giant sprinklers have 16 mm (5/8 in) or larger nozzles attached to long discharge pipes. Boom sprinklers have 18 to 36 m long rotating arms and water is applied through nozzles on the arms (See Figures 1.6 and 1.7). The anns, or booms are supported by a cable suspension system mounted on a four-wheel trailer. Boom sprinklers apply more uniform water in smaller droplets than gun sprinklers. But gun sprinklers are simpler to operate than booms and also less

expensive for the farmer. Discharge of gun and boom sprinklers vary from 65 to 78 l/s (l00 gpm to 1250 gpm) operated at high pressures ranging from 480 to 896 kPa (70 to 130 psi). Wetting diameter may be as large as 180 m.

Raingun Sprinklers

A typical stationary or traveling raingun sprinkler system consists of a pumping plant, flexible hose, traveler assembly and gun sprinkler (see Figure 1.7). A stationary or fix gun irrigates at one setting which is shifted to the next field. Whereas the traveling gun is mounted on a cart and it moves continuously across the field while applying water. The assembly is pulled either by the flexible pipe itself or by an anchor cable stretched across the field. The unit is moved to the next irrigation position after irrigation is completed at the first setting. Both full and part-circle sprinklers of large size can be used equally but the part-circle sprinklers are preferred as the cart moves in relatively dry soil. The gun sprinklers have trajectory angles ranging between 18 to 32 degrees but for average conditions, trajectories between 23 to 25 degrees give satisfactory results. The traveling system can be used on all field sizes and shapes. These systems can be easily transported from field to field.

Gun and boom sprinklers can be used for most crops, but their application rates are high and large sized water drops may compact the soil surface and cause surface runoff. Therefore, these sprinklers are more suitable for coarse-textured soils.

Figure 1.7: Hose-Fed Traveling-Gun Sprinkler.

CONTINUOUS MOVE SYSTEMS

Laterals and sprinklers of the continuous move systems are constantly moving when applying water. These systems have become more common due to low labour cost. Main types of continuous moving sprinkler systems are center-pivot (CP), traveler and linear-move systems.

Center-pivot irrigation systems

The center-pivot sprinkler irrigation systems are the most popular sprinkle systems used in many countries as shown in Fig. 1.8. A center-pivot system consists of a pipeline supported on towers usually spaced about 61 m apart. The length of the lateral pipeline varies from approximately 200 to 800 m but 400 m is the most common length. A center-pivot system of 400 m long lateral covers an area of 65 ha (160 acres). Water is introduced at the pivot and flows outward through the pipeline, supplying to each of the individual sprinkler heads. Some of the main advantages of center-pivot sprinklers are (Keller and Bliesner, 1990, p. 307)

i. Water delivery is simplified through the use of a stationary pivot point. ii. Guidance and alignment are controlled at a fixed pivot point.

iii. Relatively high water application uniformities are easily achieved under the continuously moving sprinklers.

iv. After completing irrigation cycle, the system is at the starting point for the next irrigation. v. Achieving good irrigation management is simplified because accurate and timely

vi. More accurate and timely applications of fertilizer and other chemicals are possible by applying them through the irrigation water which is also possible with other sprinkler systems as well.

vii. Flexibility of operation makes it feasible to develop electric-load-management schemes.

Figure 1.8: A Center-Pivot Irrigation System

Because this system irrigates a circular area, only about 79 percent of the area of a square field is effectively irrigated by the basic unit (Figure 1.9). With an end-gun modification, an additional five percent of the total area can be irrigated, leaving 16 percent of the area un-irrigated. Not irrigating 16 percent of the area increases the cost per unit area of the irrigated land. This problem can be overcome, to a large extent, by the addition of a 'corner-pivot' system to the conventional center-pivot machine as shown in the above figure. The corner-pivot assembly pivots about the outer tower and applies water at the corners of the field. This increases the irrigated area to approximately 96 percent and reduces the investment per unit irrigated area appreciably (Callies 1978). The corner-pivot unit is folded with the last tower of the main system when not in use.

Center-pivot systems vary a great deal in their design. Some systems use only one type of sprinkler head and vary the spacings between the sprinkler heads on the lateral to obtain

proportional to the radial distance and the sprinkler heads at different distances from the pivot point travel at different speeds, the time during which water is applied to a point is also different along the lateral. To compensate for this and to achieve reasonably uniform depth of water, the application rate is low near the pivot and increases toward the end tower. Such a distribution of application rates may cause undesirable runoff near the end of the system on soils of low water intake rate.

Figure 1.9 Area irrigated by the main system, corner-pivot assembly and endgun of a center-pivot system (only one-fourth of field is shown).

Linear Move Systems

Linear-move systems have been developed in response to the problems of irrigating the corners and runoff associated with the center-pivot system. Linear move systems have towers with electric motors and alignment systems, just as do center-pivot systems. The lateral moves continuously in a linear fashion across the field rather than rotating about a central pivot point. Water is supplied to the lateral through a flexible hose hooked to a mainline or by a traveling pumping plant that pumps water from an open ditch. Recently, a linear-move system that moves continuously and obtains water from risers connected to a buried mainline by automatically connecting and disconnecting itself from riser valves has been developed.

Linear moving systems may travel forth and back, i.e. they travel twice the length of the field to complete each irrigation cycle. Hence, the sprinklers may operate continuously forth or back, or for only half of the total distance traveled on both sides. Each method has its own advantages and disadvantages.

If the sprinklers are operated continuously, the total operating time will be longer. This will reduce system flow and application rate requirements. However, the area that is most recently watered near the end of the field will be immediately re-watered each time on the move of lateral in reverse direction. On the other hand, if the system is returned with the water shut off (or empty) the operating time will be decreased accordingly.

The continuous versus part-time watering and the possibility for backtracking over relatively dry rather than fully irrigated soil leads to following irrigation strategies.

1. Irrigate in only one direction and return empty (with the water off) as quickly as possible to the starting point.

2. Irrigate while traveling at the same speed in both directions,

3. Irrigate half the field and quickly continue to the other end with the water off, then on the return irrigate the other half of the field and quickly continue back to the starting point with the water off, and

4. Proceed as in strategy 3 but continue as quickly as possible with the water on (instead of off) during the last half of travel in either direction.

The purpose of strategy 3 is to eliminate returning across soil that has just been fully irrigated. Otherwise it is like strategy 1. The purpose of strategy 4 is to reduce the hazard associated with watering back across the area at the end that has just been fully irrigated. However, both strategies 3 and 4 require more management and labour than 1 and 2.

COMPONENTS OF A SPRINKLE IRRIGATION SYSTEM

Primary components of all sprinkle systems are similar in many respects. They consist of sprinkler with or without risers, laterals, main pipeline and pumping plant as shown in Fig. 1.1. Different types of valves and couplings also form integral parts of any system. More detail of the

Types of Sprinklers

The sprinklers also called sprinkler heads form an important component of any sprinkle system. Water is sprayed like raindrops through these sprinklers. There are many types of sprinklers available to suit different farming and climatic conditions. The sprinklers may be either rotating or fixed type. Rotating sprinklers have three types: impact, reaction and gear-driven sprinklers. Fixed-head sprinklers include spray-type and perforated pipes.

a) Rotating Head Sprinklers

i) Impact Sprinklers: Impact sprinklers have one or more nozzles that discharge jets of

water into the air. These jets are rotated in a start and stop manner by a spring loaded arm which strikes (impacts) and then is bounced out of one of the jets. The spring returns the arm to strike the jet again and the process is repeated. Several different nozzle types have been developed for impact sprinklers including constant-diameter, constant discharge, and diffuse-jet nozzles. Constant-diameter nozzles are the most commonly used with impact sprinklers. The discharge from these nozzles is proportional to the square root of the operating pressure. Small single nozzle sprinklers are designed at low pressures, while large multiple-nozzles require high operating pressure. Usually, low cover larger areas and also have high application rates per nozzle.

Constant-discharge nozzles are also used with impact sprinklers. These nozzle pressures are associated with a small diameter nozzle, small wetted area and low water application rates. On the other hand, large size nozzles require high operating pressures, are constructed so that as long as the operating pressure exceeds a threshold value, changes in pressure do not affect sprinkler discharge significantly. For example, constant discharge nozzles can be used to minimize the variation in sprinkler discharge along laterals with fluctuating pressure caused by undulating terrains.

Diffuse-jet nozzles are designed so that droplets are formed at a lower pressure than with other impact nozzles. This is accomplished by using noncircular-shaped nozzle openings or turbulence inducer at the orifice to spread (diffuse) the jet as it leaves the nozzle. Diffuse-nozzles do not wet as large an area as do constant-diameter and constant discharge nozzles.

ii) Gear-Driven Sprinklers: Some rotating sprinklers are driven by a small water turbine located in the base of the head sprinkler. These sprinklers are called gear-driven sprinklers because the high rotational speed of the turbine is reduced through a series of gears. Like impact sprinklers, gear-driven sprinklers have one or more jets that rotate around the vertical axis of the sprinkler. Unlike the start and stop rotation of impact sprinklers, gear-driven sprinklers rotate smoothly without the splash that occurs each time the arm of an impact sprinkler strikes the jet.

iii) Reaction Drive Sprinklers: Small reaction drive sprinklers are normally rotated by the

torque produced by the reaction of water leaving the sprinkler. These sprinklers usually do not wet as large an area as do impact or gear-driven sprinklers and usually operate at much lower pressures (70 to 210 kPa or 10 to 30 psi). Some pertinent data about different sprinklers is presented in Table 8.1 (Keller and Bliesner, 1990).

b) Fixed-Head Sprinklers

Fixed-head sprinklers depend on smooth and grooved cones, deflector plates, and slots to produce full-or nearly full-circle sprays or several small holes that spray around the circumference of the sprinkler. An example of a fixed-head spray-type sprinkler is a multi streamlet-type fixed-head sprinkler. Many fixed-nozzle sprinklers that produce small droplets and that operate at low pressures are currently available for center-pivot and linear-move sprinkle systems.

Risers

Riser pipe connects the rotating or fixed sprinkler head to the lateral pipe. It may be a fixed length of pipe depending upon the height of crop to be irrigated, or it may be collapsible pipe. Pipes from 10 to 75 mm in diameter are usually used. A minimum length of about 75 mm on small sprinklers and 1 m on large sprinklers generally provide the best flow pattern through the sprinkler nozzle. For over-crop sprinkling in orchards or other tall crops, the riser length may extend from 4 to 5m. Where high risers are used, quick-couplings are often provided to enable uncoupling during the moving of the lateral. A tripod is sometimes used to keep the uncoupled riser in upright position.

Laterals

The lateral allows transport of water from the main pipeline or from a sub-main to the sprinklers through the risers. The lateral pipes usually available in portable lengths of 5, 6 or 12 m are regularly spaced on the mains or sub-mains. Buried permanent laterals are however, used for orchards, tree nurseries, and for other special sites. Quick-coupled aluminum pipe is best for most portable laterals. A rubber gasket in the female portion of these couplings has a U-shape. The water pressure forces the outside of the 'U' to form a water tight seal. When the water is turned off, the seal is broken and water drains from the pipe, making it easier to uncouple and move the lateral line to next irrigation position.

Mains and Sub-mains

Main pipelines and sub-mains convey water from the pumping unit to the laterals. In small systems, laterals may be directly connected to the main line without any sub-mains. Main lines may be portable or permanent. Permanent mains are used where crops require full season

system is to be used on a number of fields. Steel pipes are used for most permanent main lines and for center-pivot laterals.

With water being supplied by the pipelines at desired pressures to each lateral and sprinkler, the pipes must be strong enough to withstand expected operating and surge pressures in case of water hammering. Buried pipes must resist overburden and dynamic surface loads, while portable pipes must be tight and durable. PVC pipes are resistant to rust and corrosion while steel pipes are immune to both of these. Exposure to saline or acid conditions can corrode aluminum pipes.

Economical pipe selection: The selection of pipeline diameter is critical to the cost of

any sprinkler system. Pipeline cost consists of annual fixed cost of pipes (main, sub-mains and laterals) and the annual operating cost of pumping water through them. The total cost further depends on annual operation hours, costs of fuel, anticipated life and friction characteristics of pipe material and annual interest rate. Head loss due to the friction is a function of pipe diameter. If a small diameter pipe is used, operating cost will be more due to more friction loss but fixed cost will be low. As the pipe diameter is increased, the operating cost will decrease but the fixed cost will increase. The optimum pipe size is the one that minimizes the sum of the fixed- and operating costs as shown in Fig. 1.10.

Pumps

A pump is required for any sprinkle system. Centrifugal or deep-well turbine pumps are commonly used. A pump may be stationary or it may be mounted on a movable assembly. Pumps may be powered by electric motors or internal combustion engines using diesel, gasoline or natural gas.

Valves and Accessories

Different types of valves, joints, couplings, pressure regulators and pressure gauges are used in a properly designed sprinkle system. Main types of valves include pressure relief valves, check valves, foot valves, drain valves, Tee and Y valves, etc. Pressure relief valves are used to relieve excessive pressure surges. Check valves are used on the discharge side of the pump so that if the pump is shut off it should maintain water in the pipeline above the pump. Foot-valve on the bottom of the suction pipe maintains water in the pump when it is not in operation and thus keep the pump in primed condition.

PERTINENT DATA OF SPRINKLER IRRIGATION SYSTEM

Tables 1 and 2 shows water saving by using sprinkler as compared to surface irrigation method in Pakistan and India respectively. It is apparent from these tables that more than 50% water may be saved by using sprinkler as compared to surface irrigation. Tables 3 and 4 show cost of the piped irrigation system. It is apparent from Table 4 that almost half of the cost is for the pipe including mains, sub-mains and laterals.

Tables 5 and 6 present results of evaluation of a sprinkler system at different pressures and the same results are plotted in Figures 1.11 and 1.12. It is apparent from this data that uniformity of the system depends on operating pressure of the system and fuel consumption increases when the system is operated at higher pressure. A center-pivot system in maize crop and drip system on different crops and vegetable are shown in Figures given at the end.

Table 1 Saving in irrigation water using sprinkler irrigation in Pakistan

Crop Water applied (cm) Water saving (%) Furrow/Basin Sprinkler

Table 2 Saving in Irrigation Water Using Sprinkler Irrigation in India

Table 3. Comparative Costs of Piped Irrigation Systems Piped Surface Method Sprinkler Conventional

hand-move Micro-irrigation solid installation Area (ha) 1 1-2 2-3 1 1-2 2-3 1 1-2 2-3 Installation cost (US$/ha) 1700 1600 1400 2800 2700 2100 3950 3300 3000 Annual maintenance cost (US$/ha) 85 80 70 140 135 105 200 165 150 Note: Average 1997 prices in Europe.

Source: (Phocaides, 2000)

Crop

Water Applied (cm)

Water

Saving (%)

Surface

Sprinkler

Bajra

17.78

7.82

56

Jowar

25.40

11.27

56

Cotton

40.64

29.05

29

Wheat

33.02

14.52

56

Barley

17.78

7.82

56

Gram

17.78

7.82

56

Potato

60.00

30.00

50

Average

51.30

Table 4. Cost breakdown for piped irrigation systems Component Parts Sophisticated

Installation

Simple Installation Control station

Mains, submains and manifolds Fittings and other accessories Laterals (pipes and emitters)

> 23% 10% 22% 45% 13% 21% 24% 42% Source: (Phocaides, 2000)

Some Sprinkler Manufacturer Web Sites

www.senninger.com www.rainbird.com www.toro.com www.orbitonline.com www.rainforrent.com www.valmont.com

1

Maize on Sprinkler System

Sugarcane on Drip

Unit 2 IRRIGATION SCHEDULING I. Irrigation Depth Z a x W 100 MAD d  (2.1)

Where, dx is the maximum net depth of water to apply per irrigation; MAD is

management allowed deficit (usually 40% to 60%); Wa is the water holding capacity, a function of soil texture and structure, equal to FC - WP (field capacity minus wilting point); and Z is the root depth.

• For most agricultural soils, field capacity (FC) is attained about 1 to 3 days after a complete irrigation.

• The dx value is the same as "allowable depletion." Actual depth applied may be less if irrigation frequency is higher than needed during peak use period.

• MAD can also serve as a safety factor because many values (soil data, crop data, weather data, etc.) are not precisely known.

• Assume that crop yield and crop ET begins to decrease below maximum potential levels when actual soil water is below MAD (for more than one day).

• Water holding capacity for agricultural soils is usually between 10% and 20% by volume.

• Wa is sometimes called "TAW (total available water), "WHC" (water holding capacity), "AWHC" (available water holding capacity).

• Note that it may be more appropriate to base net irrigation depth calculations on soil water tension rather than soil water content, also taking into account the crop type - this is a common criteria for scheduling irrigations through the use of tensiometers.

II. Irrigation Interval

• The maximum irrigation frequency is:

d x x U d f  (2.2)

Where, fxis the maximum interval (frequency) in days; and Udis the average daily crop water requirement during the peak-use period.

• Then nominal irrigation frequency, f', is the value of fx rounded down to the nearest whole number of days.

• But, it can be all right to round up if the values are conservative and if fxis near the next highest integer value.

• f' could be fractional if the sprinkler system is automated.

• f' can be further reduced to account for non-irrigation days (e.g. Sundays), whereby f < f'

• The net application depth per irrigation during the peak use period is dn= f'Ud, which will be less than or equal to dx. Thus, dn≤ dx, and when dn= dx, f' becomes fx(the maximum allowable interval during the peak use period).

• Calculating dn in this way, it is assumed that Ud persists for f' days, which may result in an overestimation if f' represents a period spanning many days.

III. Peak Use Period

• Irrigation system design is usually for the most demanding conditions:

Figure 2.1 A typical crop coefficient curve

• The value of ET during the peak use period depends on the crop type and on the weather. Thus, the ET can be different from year to year for the same crop type.

• Some crops may have peak ET at the beginning of the season due to land preparation requirements, but these crops are normally irrigated by surface systems.

• When a system is to irrigate different crops (in the same or different seasons), the crop with the highest peak ET should be used to determine system capacity.

• Consider design probabilities for ET during the peak use period, because peak ET for the same crop and location will vary from year-to-year due to weather variations.

• Consider deficit irrigation, which may be feasible when water is very scarce and or expensive (relative to the crop value). However, in many cases farmers are not interested in practicing deficit irrigation.

IV. Leaching Requirement

• Leaching may be necessary if annual rains are not enough to flush the root zone or if deep percolation from irrigation is small (i.e. good application uniformity and or efficiency).

• If ECw is low, it may not be necessary to consider leaching in the design (system capacity).

• Design equation for leaching:

w e w EC EC 5 EC LR   (2.4)

where LR is the leaching requirement; ECw is the EC of the irrigation water (dS/m or mmho/cm); and ECe is the estimated saturation extract EC of the soil root zone for a given yield reduction value.

• Equation 2.4 is taken from FAO Irrigation and Drainage Paper 29.

• When LR > 0.1, the leaching ratio increases the depth to apply by 1/(1-LR); otherwise, LR does not need to be considered in calculating the gross depth to apply per irrigation, nor in calculating system capacity:

a n E d d : 0.1 LR  (2.5) a n LR)E -(1 d 0.9 d : 0.1 LR  (2.6)

• Eais the application efficiency in fraction.

• When LR < 0.0 (a negative value) the irrigation water is too salty, and the crop would either die or suffer severely.

• Here are some useful conversions: 1 mmho/cm = 1 dS/m = 550 to 800 mg/l (depending on chemical makeup, but typically taken as 640 to 690). And, it can usually be assumed that 1 mg/l « 1 ppm, where ppm is by weight (or mass).

V. Leaching Requirement Example

Suppose ECw= 2.1 mmhos/cm (2.1 dS/m) and ECefor 10% reduction in crop yield is 2.5 dS/m. Then, 20 . 0 2.1 -5(2.5) 2.1 EC 5EC EC LR w E w     (2.7)

Thus, LR > 0.1. And, assuming no loss of water due to application non-uniformity, the gross application depth is related to the net depth as follows:

LR) -(1 d ) ( d d n n    LR d (2.8) n n d 25 . 1 0.20) -(1 d d   (2.9)

Gross Application Depth

0.1 LR for , E d d pa n   (2.10)

Where, Epais the design application efficiency (decimal; Eq. 6.9). And,

0.1 LR for , LR)E -(1 0.9d d pa n   (2.11)

• The gross application depth is the total equivalent depth of water which must be delivered to the field to replace (all or part of) the soil moisture deficit in the root zone of the soil, plus any seepage, evaporation, spray drift, runoff and deep percolation losses.

• The above equations for d presume that the first 10% of the leaching requirement will be satisfied by the Epa (deep percolation losses due to application variability). This presumes that areas which are under-irrigated during an irrigation will also be over-irrigated in the following irrigation, or that sufficient leaching will occur during non-growing season (winter) months.

• When the LR value is small (ECw< ECe), leaching may be accomplished both before and after the peak ET period, and the first equation (for LR < 0.1) can be used for design and sizing of system components. This will reduce the required pipe and pump sizes because the "extra" system capacity during the non-peak ET periods is used to provide water for leaching.

System Capacity

• Application volume can be expressed as either Qt or Ad, where Q is flow rate, t is time, A is irrigated area and d is gross application depth.

• Both terms are in units of volume.

• Thus, the system capacity is defined as (Eq. 5.4):

fT Ad K Qs  (2.12) Where, Qs = system capacity, l/s (gpm)

T = hours of system operation per day (obviously, T< 24; also, t = fT) K = coefficient for conversion of units (see below)

d = gross application depth, mm (in.)

f = time to complete one irrigation (days); equal to f' minus the days off A = net irrigated area supplied by the discharge Qs, ha (acres)

Value of K:

• Metric: for d in mm, A in ha, and Qsin lps: K = 2.78

Notes about system capacity:

• Eq. 2.12 (Eq 5.4 book) is normally used for periodic-move and linear-move sprinkler systems.

• The equation can also be used for center pivots if f is decimal days to complete one revolution and d is the gross application depth per revolution.

• For center pivot and fixed systems, irrigations can be light and frequent (dapplied < d): soil water is maintained somewhat below field capacity at all times (assuming no leaching requirement), and there is very little deep percolation loss.

• Also, there is a margin of safety in the event that the pump fails (or the system is temporarily out of operation for whatever reason) just when MAD is reached (time to irrigate), because the soil water deficit is never allowed to reach MAD.

• However, light and frequent irrigations are associated with higher evaporative losses, and probably higher ET too (due to more optimal soil moisture conditions).

• Frequent irrigations correspond to a higher basal crop coefficient Kcb (due to more favorable soil moisture conditions), and a higher wet soil surface evaporation coefficient, Ks (due to more frequent wetting).

• When a solid-set (fixed) system is used for frost control, all sprinklers must operate simultaneously and the value of Qsis equal to the number of sprinklers multiplied by qa. This tends to give a higher Qsthan that calculated from Eq. 5.4.

Example: Determine the required system capacity for a sprinkler system with following data:

Area, A = 140 acres root depth, Z = 2 ft

T = 22 h/day Ud = 0.21 in 1 day MAD = 50 % Cu = 79% Wa = 1.0 in/ft 1 2 1 0.5 100 Z MAD.W dx  a     

Irrigation Interval, 4.76 (tobeconservative take,f 4) 0.21 1 U d f d x     gpm 720.7 22 4 1 140 453 f.T Ad Q      K s If f = 5 days  Q_s  577 gpm

Unit 3

WATER DISTRIBUTION UNIFORMITY AND APPLICATION EFFICIENCY Water Distribution Pattern from a Stationary Sprinkler

Sprinkler head is the most important component of a sprinkle system because it distributes water over the land. Efficiency and effectiveness of any sprinkle system depend how uniformly water is sprayed from the sprinkler.

Water application rate and distribution pattern of a sprinkler are a function of i) nozzle size and its angle, ii) nozzle pressure, iii) diameter of throw, iv) wind, and v) sprinkler spacing on the lateral and

spacing of the lateral on the main. Water distribution characteristics of sprinkler heads are typical and change with nozzle- size, shape, angle, and operating pressure. Water application beneath a sprinkler varies with distance from the head. The pattern of this variation, called distribution pattern, is usually consistent for a given pressure, nozzle geometry and wind. Pressure has a significant effect on the distribution pattern. Typical distribution patterns beneath a stationary impact sprinkler with fixed nozzle geometry for different operating pressures are given in Figure 3.1. If pressure is low, larger water drops fall near the sprinkler forming a 'donut-shaped' distribution pattern. Under the normal operating pressure as recommended by the manufacturer a triangular or elliptical shaped distribution pattern is obtained in which the depth of water application is found maximum near the head and decreases toward the outer edge of the pattern. Extremely high pressure produces too many fine water drops that fall finer water drops that fall near the sprinkler distorting the desired distribution pattern (see Fig. 3.2).

Classification of Sprinklers and Applicability

Table 3.1 Classification of sprinkler heads based on operating pressure Agriculture

sprinklers (two nozzle)

Nozzle size mm Operating pressure (bars) Flow rate (m3/h) Diameter coverage (m) Low pressure Medium pressure High pressure 3.04-4.5x2.5-3.5 4.0-6.0x2.5-4.2 12.0-25.0x5.0-8.0 1.5-2.5 2.5-3.5 4.0-9.0 0.3-1.5 1.5-3.0 5.0-45.0 12-21 24.35 60-80

`

Figure 3.1 Water distribution pattern from a stationary rotating sprinkler head (Source: USDA, SCS Handbook, 1960).

Precipitation Profiles

• Typical examples of low, correct, and high sprinkler pressures (see Fig 5.5 below).

Analysis of Water Application under a Stationary Sprinkler

• Agricultural sprinklers typically have flow rates from 4 to 45 lpm (1 to 12 gpm), at nozzle pressures of 135 to 700 kPa (20 to 100 psi).

• "Gun" sprinklers may have flow rates up to 2,000 lpm (500 gpm; 33 lps) or more, at pressures up to 750 kPa (110 psi) or more.

• Sprinklers with higher manufacturer design pressures tend to have larger wetted diameters.

• But, deviations from manufacturer's recommended pressure may have the opposite effect (increase in pressure, decrease in diameter), and uniformity will probably be compromised.

• Sprinklers are usually made of plastic, brass, and or steel.

• Low pressure nozzles save pumping costs, but tend to have large drop sizes and high application rates.

• Medium pressure sprinklers (210 - 410 kPa, or 30 to 60 psi) tend to have the best application uniformity.

• Medium pressure sprinklers also tend to have the lowest minimum application rates.

• High pressure sprinklers have high pumping costs, but when used in periodic-move systems can cover a large area at each set.

• High pressure sprinklers have high application rates.

• Rotating sprinklers have lower application rates because the water is only wetting a "sector" (not a full circle) at any given instance.

• For the same pressure and discharge, rotating sprinklers have larger wetted diameters.

• Impact sprinklers always rotate; the "impact" action on the stream of water is designed to provide acceptable uniformity, given that much of the water would otherwise fall far from the sprinkler (the arm breaks up part of the stream).

The precipitation profile (and uniformity) is a function of many factors:

1. nozzle pressure 2. nozzle shape & size 3. sprinkler head design

4. presence of straightening vanes 5. sprinkler rotation speed

6. trajectory angle 7. riser height

Overlapping sprinkler profiles

Figure 3.3 Water distribution patterns for individual and overlapped sprinklers (Source: USDA, SCS Handbook, 1960).

Set Sprinkler Uniformity & Efficiency Sprinkler Irrigation Efficiency

1. Application uniformity

• It is not enough to have uniform application if the average depth is not enough to refill the root zone to field capacity.

• Similarly, it is not enough to have a correct average application depth if the uniformity is poor.

• Consider the following examples:

Fig. 3.4 Uniform, but average depth applied exceeds the soil water deficit (too much deep percolation)

Fig. 3.5 Average depth is correct, but application is highly non-uniform, with under-irrigation and DP

• We can design a sprinkler system that is capable of providing good application uniformity, but depth of application is a function of the set time (in periodic-move systems) or "on time" (in fixed systems).

• Thus, uniformity is mainly a function of design and subsequent system maintenance, but application depth is a function of management.

Quantitative Measures of Uniformity

Traditional measurements of sprinkler irrigation uniformity only account for the aerial distribution of water.

These measurements of uniformity do not account for redistribution of water within the soil profile, redistribution due to foliar interception of water drops, and surface runoff after the drops hit the ground.

Following are two commonly applied indicators of aerial water distribution: Distribution uniformity, DU (Eq. 6.1):

       depth avg quarter low of depth Avg 100 DU (3.1)

The average of the low quarter is obtained by measuring application from a catch-can test, mathematically overlapping the data (if necessary), ranking the values by magnitude, and taking the average of the values from the low ¼ of all values.

For example, if there are 60 values, the low quarter would consist of the 15 values with the lowest "catches".

Christiansen Coefficient of Uniformity, CU (Eq. 6.2):

              





Where, z are the individual catch-can values (volumes or depths); n is the number of observations; and m is the average of all catch volumes.

Note that CU can be negative if the distribution is very poor. There are other, equivalent ways to write the equation.

These two measures of uniformity (CU & DU) date back to the time of slide rules (more than 50 years ago; no electronic calculators), and are designed with computational ease in mind.

More complex statistical analyses can be performed, but these values have remained useful in design and evaluation of sprinkler systems. For CU > 70% the data usually conform to a normal distribution, symmetrical about the mean value. Then,

       depth avg half low of depth avg 100 CU (3.3)

Another way to define CU is through the standard deviation of the values,

         2 m 0 . 1 100 CU (3.4)

Where,is the standard deviation of all values, and a normal distribution is assumed (as previously)

• Note that CU = 100% for= 0

• The above equation assumes a normal distribution of the depth values, whereby:



• By the way, the ratio/m is known in statistics as the coefficient of variation.

• Following is the approximate relationship between CU and DU:

CU100 - 0.63(100 - DU) (3.6) Or

DU100 -1.59(100 - CU) (3.7) These equations are used in evaluations of sprinkler systems for both design and operation. Typically, 85 to 90% is the practical upper limit on DU for set systems DU > 65% and CU > 78% is considered to be the minimum acceptable performance level for an economic system design; so, you would not normally design a system for a CU < 78%, unless the objective is simply to "get rid of water or effluent" (which is sometimes the case). For shallow-rooted, high value crops, you may want to use DU > 76% and CU > 85%.

Alternate Sets (Periodic-Move Systems)

• The effective uniformity (over multiple irrigations) increases if "alternate sets" are used for periodic-move systems (1/2 Sl).

• This is usually practiced by placing laterals halfway between the positions from the previous irrigation, alternating each time.

• The relationship is:

CUa10CU (3.8)

DUa10DU (3.9)

• Use of alternate sets is a good management practice for periodic-move systems.

• The use of alternate sets approaches Slof zero, which simulates a continuous-move system.

Uniformity Problems

• From the various causes of non-uniform sprinkler application, some tend to cancel out with time (multiple irrigations) and others tend to concentrate (get worse).

• In other words, the "composite" CU for two or more irrigations may be (but not necessarily) greater than the CU for a single irrigation.

1. Factors that tend to Cancel Out

• Variations in sprinkler rotation speed

• Variations in sprinkler discharge due to wear

• Variations in riser angle (especially with hand-move systems)

• Variations in lateral set time

2. Factors that may both Cancel Out and Concentrate

• Non-uniform aerial distribution of water between sprinklers

3. Factors that tend to Concentrate

• Variations in sprinkler discharge due to elevation and head loss

• Surface ponding and runoff

Water distribution uniformity effectively increases after water infiltrates into the soil because of root-zone redistribution from wetter regions to drier regions. This effect is usually greater in "tight" clay soils than in sandy soils. Thus, the actual application uniformity in the root zone tends to be a little better than the aerial distribution from the sprinklers, at least in the absence of significant runoff.

System Uniformity

• The uniformity is usually less when the entire sprinkler system is considered, because there tends to be greater pressure variation in the system than at any given lateral position.     _  (1 P / ) 2 1 CU CU system _n P_a (3.10)     _  (1 3 P / ) 4 1 DU DU system _n P_a (3.11)

Where, Pn is the minimum sprinkler pressure in the whole field; and Pa is the average sprinkler pressure in the entire system, over the field area.

These equations can be used in design and evaluation. Note that when Pn = Pa (no pressure variation) the system CU equals the CU. If pressure regulators are used at each sprinkler, the system CU is approximately equal to 0.95CU (same for DU). If flexible orifice nozzles are used, calculate system CU as 0.90CU (same for DU)

The Pafor a system can often be estimated as a weighted average of Pn& Px:

3 P 2P P n x a   (3.12)

Where, Pxis the maximum nozzle pressure in the system

Fig. 3.6 Due to parabolic loss vs. flow rate relation, the average is closer to Pn

Computer Software and Standards

• There is a computer program called "Catch3D" that performs uniformity calculations on sprinkler catch-can data and can show the results graphically.

• Jack Keller and John Merriam (1978) published a handbook on the evaluation of irrigation systems, and this includes simple procedures for evaluating the performance of sprinkler systems.

• The ASAE S436 (Sep 92) is a detailed standard for determining the application uniformity under center pivot (not a set sprinkler system, but a continuous move system).

• ASAE S398.1 provides a description of various types of information that can be collected during an evaluation of a set sprinkler system.

General Sprinkle Application Efficiency

The following material leads up to the development of a general sprinkle application efficiency term (Eq. 6.9) as follows:

Design Efficiency:

Epa = DEpaReOe (3.13)

Where, DEpais the distribution efficiency (%); Reis the fraction of applied water that reaches the soil surface; and Oeis the fraction of water that does not leak from the system pipes.

• The design efficiency, Epa, is used to determine gross application depth (for design purposes), given the net application depth.

• In most designs, it is not possible to do a catch-can test and data analysis you have to install the system in the field first; thus, use the "design efficiency".

• The subscript "pa" represents the "percent area" of the field that is adequately irrigated (to dn, or greater). For example, E80 and DE80 are the application and distribution efficiencies when 80% of the field is adequately irrigated.

Simulate different lateral spacings by "overlapping" catch-can data in the direction of lateral movement (overlapping along the lateral is automatically included in the catch-can data, unless it's just one sprinkler).

Figure 3.7 An example to calculate data to measure irrigation uniformity for sprinkler irrigation system using single lateral

Field Evaluation of Sprinklers

• Catch-can tests are typically conducted to evaluate the uniformities of installed sprinkler systems and manufacturer's products.

• Catch-can data is often overlapped for various sprinkler and lateral spacings (Se & Sl) to evaluate uniformities for design and management purposes.

Se

Sl

• Typical catch-can spacings are 2 or 3 m on a square grid, or 1 to 2 m spacings along one or more "radial legs", with the sprinkler in the center.

• Set up catch-cans with half spacing from sprinklers (in both axes) to facilitate overlap calculations.

• See Merriam & Keller (1978); also see ASAE S398.1 and ASAE S436

Choosing a Suitable Sprinkler Heads

• The system designer doesn't "design" a sprinkler, but "selects" a sprinkler.

• There are hundreds of sprinkler designs and variations from several manufacturers, and new sprinklers appear in the market quite often.

• The system designer must choose between different nozzle sizes and nozzle designs for a given sprinkler head design.

The objective is to combine sprinkler selection with Se and Sl to provide acceptable application uniformity, acceptable operating costs, and acceptable hardware & installation costs.

• Manufacturers provide recommended spacings and pressures.

• There are special sprinklers designed for use in frost control.

General Spacing Recommendations

• Sprinkler spacing is usually rectangular or triangular.

• Triangular spacing is more common under fixed-system sprinklers.

• Sprinkler spacings based on average (moderate) wind speeds:

1. Rectangular spacing is 40% (Se) by 67% (Sl) of the effective diameter

2. Square spacing is 50% of the effective diameter 3. Equilateral triangle spacing is 62% of the effective

diameter [lateral spacing is 0.62 cos(60°/2) = 0.54, or 54% of the effective diameter,

• See Fig. 3.8 about profiles and spacings.

Rectangular: 40% x 67%

Square: 50% x 50%

Triangular: 62% x 54%

Windy Conditions

• When winds are consistently recurring at some specific hour, the system can be shut down during this period.

• For center pivots, rotation should not be a multiple of 24 hours, even if there is no appreciable wind (evaporation during day, much less at night).

• If winds consistently occur, special straightening vanes can be used upstream of the sprinkler nozzles to reduce turbulence; wind is responsible for breaking up the stream, so under calm conditions the uniformity could decrease.

• For periodic-move systems, laterals should be moved in same direction as prevailing winds to achieve greater uniformity (because Se< Sl).

• Laterals should also move in the direction of wind to mitigate problems of salt accumulating on plant leaves.

• Wind can be a major factor on the application uniformity on soils with low infiltration rates (i.e. low application rates and small drop sizes).

• In windy areas with periodic-move sprinkler systems, the use of offset laterals (Sl) may significantly increase application uniformity.

• Alternating the time of day of lateral operation in each place in the field may also improve uniformity under windy conditions.

• Occasionally, wind can help to increase uniformity, as the randomness of wind turbulence and gusts helps to smooth out the precipitation profile.

Wind effects on the diameter of throw:

0-3 mph wind: reduce manufacturer's listed diameter of throw by 10% for an effective value (i.e. the diameter where the application of water is significant) over 3 mph wind: reduce manufacturer's listed diameter of throw by an additional 2.5% for every 1 mph above 3 mph (5.6% for every 1 m/s over 1.34 m/s)

In equation form:

For 0-3 mph (0-1.34 m/s):

diam = 0.9diammanuf (3.14) For > 3 mph (> 1.34 m/s):

diam = diammanuf[0.9 - 0.025 (windmph- 3)] (3.15) or,

Example: a manufacturer gives an 80-ft diameter of throw for a certain sprinkler and operating pressure. For a 5 mph wind, what is the effective diameter?

80 ft - (0.10)(0.80) = 72 ft (3.17) 72 ft - (5 mph - 3 mph)(0.025)(80 ft) = 68 ft (3.18) or, diam = 80(0.9-0.025(5-3))=68 ft (3.19) Pressure-Discharge Relationship Equation 5.1: q = KdP

where, q is the sprinkler flow rate; Kdis an empirical coefficient; and P is the nozzle pressure The above equation is for a simple round orifice nozzle. It can be derived from Bernoulli’s equation like this:

2 2 2 2 2 P gA q g V    (3.20) q P K g P d    2 2gA (3.21) Where, the elevations are the same (z1 = z2) and the conversion through the nozzle is assumed to be all pressure to all velocity.

• P can be replaced by H (head), but the value of Kdwill be different

• Eq. 5.1 is accurate within a certain range of pressures

• See Table 5.2 for P, q, and Kdrelationships

• Kdcan be separated into an orifice coefficient, Ko, and nozzle bore area, A:

• P A K q _o (3.22) Where by,  / 2  o K (3.23)

A q K

I  (3.24)

Where,

I = application rate (mm/h, in/h) A = wetted area of sprinkler (m2, ft2)

K = unit constant (K equal to 60 for I mm/h, q l/min and A m2) or equal to 96.3 for I in/h, q gpm, and A ft2

A single stationary sprinkler applies water on a circular area. Water depth observe maximum near the head and decreases outward as shown in Figure 3.1. Thus to obtain a reasonable uniform water application, the wetted circular areas of adjacent sprinklers are overlapped as shown in Figures 3.2 and 3.3 above. The resulting accumulated depth is almost uniform.

The average application rate along a lateral can be calculated as:

LS

Q

K

I



(3.25) Where, Ql is the discharge of the lateral, L is the length of lateral, and S is the spacing between adjacent lateral, I and K are as previously defined.

Allowable Application Rate

Sprinkle systems are normally designed for no surface runoff. Thus, application rate of a sprinkle system is designed to apply less water than the infiltration rate of the soil. Complication arises as the soil infiltration rate decreases with time of application as illustrated by a curve in Figure 3.9.

Figure 3.9 Relationship between infiltration rate of a soil and constant application rates. Inf il tr at ion R at e (L T -1 ) Curve a b c Time (T)

Curve in the above figure shows for an unlimited amount of water available at the soil surface that the infiltration rate is initially higher than the application rate (line b). Thus, no runoff occurs in the beginning of irrigation. As the infiltration rate decreases with time, runoff may occur if irrigation is continued at rate b. No runoff will occur if the application rate is everywhere less than curve corresponding to line c.

EVALUATION OF SPRINKLE SYSTEMS

Irrigation efficiency and uniformity of water application are often used to describe effectiveness of any sprinkle system. Keller and Bliesner (1988, page 86) have identified the following factors which affect water application efficiency of sprinkle systems.

 Variation of individual sprinkler discharge throughout the lateral lines. This variation can be held to a minimum by proper pipe network design or by employing pressure or flow-control devices at each sprinkler or sprinkler nozzle.

 Variation in water distribution within the sprinkler-spacing area. This variation is caused primarily by wind. It can be partly overcome for set sprinkler systems by close spacing of the sprinklers. In addition to the variation caused by wind, there is a variability in the distribution pattern of individual sprinklers. The extent of this variability depends on sprinkler design, operating pressure, and sprinkler rotation.

 Loss of water by direct evaporation from the spray. Losses increase as temperature and wind velocities increase, and as drop size and application rate decrease.

 Evaporation from the soil surface before the water is used by the plants. This loss will be proportionately smaller as greater depths of water are applied.

Uniformity

The performance of any irrigation system is judged by the evenness or uniformity of its water application. Only an ideal system will apply water with 100 percent uniformity. For sprinkler systems catch-cans normally placed in a grid pattern are used to estimate the uniformity with volumes of water collected in the cans measured in graduated cylinders. Christiansen's coefficient of uniformity, after used to measure the uniformity of application in sprinkle irrigation, is calculated (Christiansen, 1942) from:

] x -[1.0 100 CU MN   (3.26) Where

The higher the coefficient of uniformity, better the water application of the system will be. Usually, a system is considered satisfactory if it has a coefficient of uniformity higher than 70 percent.

In the above equation, it is assumed that each catch-can represents an equal sample area. This is the case in sprinkle systems other than center pivots. For the center pivot system if cans are set out radially from the pivot point, this would only be true if the can spacing is varied inversely with the distance from the pivot. Heermann and Hein (1968) suggested a method of calculating the coefficient of uniformity for the center-pivot system for radially located cans. They determined the sample area represented by each can and then by considering the volume of water applied to this area rather than the depth applied at a point, a formula very similar to above equation was used to calculate the coefficient of uniformity. If the distance between the pivot and first can is one-half of the distance between the succeeding cans, then the relationship proposed by Heermann and Hein reduces to the following (Ring and Heermann 1978):

s s s R D D D R     100(1 ) CU_w (3.27) Where,

CUw = weighted Christiansen coefficient of uniformity, Ds = depth at any point

D = weighted mean depth, given as D = (RsDs)/Rs

Rs = weighting factor equal to the distance from the pivot, given as Rs = 0.5S + (i - 1) S where:

i = subscript referring to the ith can from the pivot S = spacing between cans

Merriam and Keller (1978) recommended another indicator called Distribution Uniformity (DU) to measure uniformity. The Distribution uniformity indicates the uniformity throughout the field and is given by:

Distribution Uniformity = Average low-quarter depth of water applied x 100 (3.28) Average depth of water applied

The average low-quarter depth is the average of the lowest one-quarter of the data set (assuming each value represents an equal area) and it does not refer to the lowest one fourth of the field.

The CU and DU are approximately related as (Keller and Bliesner, 1988, p. 87):

CU = 100 - 0.63 (100 - DU) (3.29) or

Irrigation scheduling and water application practices can be combined to determine the water application uniformity and efficiency of an irrigation system as shown in the equations below.

DU = f(P,ΔP,S,dn,WDP,WS) (3.31) Ea= f(P,ΔP,S,dn,WDP,WS,I.is,ti,SWD) (3.32) Where, P = pressure available at the sprinkler; ΔP = variation of the pressure along the lateral; S = spacings of the sprinklers along and between the laterals; dn = nozzle diameter; WDP = water distribution pattern of the sprinkler; WS = wind speed and direction; I= intake characteristics of the soil; is= application rate of the sprinkler; ti= duration of the irrigation even; and SWD = soil water deficit before the irrigation event.

Application Efficiency

Effectiveness of a given sprinkle system can be determined from how much of the applied water is stored in the crop root zone. Many definitions of application efficiency are found in the literature therefore the readers are cautioned to use this term carefully. Sometime less water is applied than the required soil moisture deficit (SMD) in order to efficiently utilize rainfall. This practice is called under or deficit irrigation. The application efficiency is usually defined as:

Ea = Average depth added to the root zone storage x 100 (3.33) Average depth applied to the field

Data Collection: Data for evaluation of a sprinkle system may be collected while actual system is being normally operated. Cans of uniform shape, size and diameter are placed between the sprinklers and laterals in a uniform grid except center-pivot system. First row of cans should be placed at one-half of the grid spacing so that each data point represents equal area. The system should be operated for at least one to two hours to collect sufficient water in the containers. The test should be conducted during calm day in order to lessen the drift loss due to wind. Evaporation during the test period should also be measured. For this purpose, use cans of the same size like the one used for collecting catch cans.

If actual system is not in operation, data for DU and CU may be collected by operating a few sprinklers on a lateral. Place the cans between any two sprinklers and note the data as discussed above. The uniformity coefficients should be calculated by overlapping the data collected on the other side of the lateral. If symmetrical conditions are assumed on both side of the lateral, data collected even on one side of the lateral may be used to calculate the uniformity coefficients by overlapping as illustrated in the following example.

1.0 1.7 3.5 5.0 X 5.1 3.6 2.0 1.0

0.9 2.0 3.7 6.1 5.2 3.9 1.5 0.8

1.0 2.1 3.6 5.5 5.5 4.0 2.0 1.0

0.8 1.8 3.8 5.2 5.6 3.8 1.5 1.0

Figure 3.10 Data collection to measure Distribution Uniformity and Christiansen Uniformity Coefficient. Values shown are water depth collected in cans in cm (Grid 3m x 3m).

Calculations are given in Fig. 3.11 for 9 m x 15 m spacings using the above data. 5.1 3.6 2.0 1.0 0.0 0.0 1.0 1.7 3.5 5.0 5.1 4.6 3.7 4.5 5.0 (0.34)* (0.16) (1.06) (0.26) (0.24) 5.2 3.9 1.5 0.8 0.0 0.0 0.9 2.0 3.7 6.1 4.2 4.8 3.5 4.5 6.1 (0.44) (0.04) (1.26) (0.26) (1.34) 5.5 4.0 2.1 1.0 0.0 0.0 1.0 2.0 3.6 5.5 5.5 5.0 4.1 4.6 5.5 (0.74) (0.24) (0.66) (0.16) (0.74) 5.6 3.8 1.5 1.0 0.0 0.0 0.8 1.8 3.8 5.2 5.6 4.6 3.3 4.8 5.2 (0.84) (0.16) (1.46) (0.04) (0.44) *Figures in parentheses are deviations from the mean.

Figure 3.11 Calculation of Distribution Uniformity and Christiansen Coefficient of Uniformity by Overlapping water depth collected in cm.

X 1.5m 3 m 3 m Sprinkler head Lateral Imaginary lateral

There are 20 data points. Mean, M is equal to 4.76 cm. Mean of the lowest one quarter, MLQis MLQ = 3.3 + 3.5 + 3.7 + 4.1 + 4.5 = 19.1/5 = 3.82 cm

DU = 3.82/4.76 x 100 = 80.25% Sum of deviations = 10.72

CU = (1 – 10.72/20 x 4.76) x 100 = 88.74%

Similarly data given in Figure 3.11 may be used to calculate the uniformity coefficients for other lateral spacings. Table 3.1 summarizes the results for DU and CU for different spacings. The results in the table shows that the uniformity coefficients decrease as lateral spacing is increased as expected.

Table 3.2 Measured DU and CU values for different lateral spacings.

Spacings (m)

9 x 12 9 x 15 9 x 18 DU, Percent 89.50 80.25 67.25 CU, Percent 93.20 88.74 75.85

C A T C H 3 D Sprinkler Overlap Program

Dr. R.G. Allen, Dept. Ag ansd Irrigation Engr. Utah State University, Logan, Ut 84322-4105 ph (801) 750-2798

The CATCH3D program is an interactive IBM-PC program written in Microsoft Basic. A compiled version (CATCH3D.EXE) should be on this disk. CATCH3D is designed to simulate the water application uniformities of rectangular sprinkler patterns (Se x Sl) by overlapping catch can measurements from either a single sprinkler head test or single lateral line test.

Sprinkler (Se) and lateral (Sl) spacings evaluated must be integer multiples of the catch can grid. The catch can grid must be square. In other words, if the catch can grid is 2m by 2m, the following sprinkler spacings could be evaluated by CATCH3D: 4m x 4m, 4m x 8m, 8m x 4m, 16m x 12m, etc.

Catch can data can be stored by CATCH3D in a data file for future use. Therefore, the data needs to be typed in only once. Sprinkler and lateral spacings to be evaluated can also be stored in this same file, along with descriptive information concerning the catch can test: number of rows and columns of catch-cans, location of the sprinkler, measurement units, catch-can size, duration of test, nozzle discharge, wind speed and direction, and grid spacing.

Program calculations include estimates of the uniformity coefficient (CU), distribution uniformity, application efficiencies of the low half and low quarter (AELH, AELQ), and catch can efficiencies. Example data sets on this disk include CATCH3D.DAT, CONE.DAT and DONUT.DAT. These files can be read into CATCH3D to provide example calculations and demonstration of program operation for an actual test, and for synthetic conical and donut shaped patterns.

The resulting overlapped application patterns can be printed on a line printer, or 3-dimensional graphs of the patterns can be plotted on the screen (color graphics card with 640x200 resolution required), or plotted on a Hewlett-Packard 7475A table top plotter. Depicted plots are of a rectangular overlap section with sprinklers located at each corner of the pattern. To print a copy of the screen generated 3-d plot onto an Epson-type printer, you must have previously had this disk (with IBM DOS3.1, GRAPHICS.COM, and AUTOEXEC.BAT) in drive A upon turning on the computer. Then, after the plot is completed on the computer screen, press the SHIFT and PRT SC keys simultaneously.

To evaluate a catch-can test from a single lateral with multiple sprinklers, set the sprinkler spacing (Se) to the actual spacing of the tested lateral, and vary the lateral spacing (Sl) to evaluate effects of various lateral spacings.

The CATCH3D program is copyrighted, 1986, by the author and by Utah State University. Neither the author nor Utah State University assumes any liability resulting from use of this program.