Planning a Sprinkler or Micro Irrigation
 

In This Advisory

- Steps in Planning a Sprinkler Irrigation
- Estimating Set Times for a Sprinkle Irrigation
- Estimating Set Times for a Micro-irrigation System
- Estimating Set Times for a Drip-Tape ("row-crop drip") System
- Estimating Soil Moisture Depletion from the Feel of a Soil Sample
- Common Suggestions for Improving Sprinkler Irrigations
- Alternate-Set Sprinkle Irrigation
- Making and Using a Soil Probe
   
Purpose - The purpose of this advisory is to...
 
- Explain how one might plan a sprinkler or micro irrigation
- Also provide a short discussion concerning micro systems
- List common suggestions for improving sprinkler irrigations
   
 

INTRODUCTION

Irrigation has often been called "both an art and a science".   Sprinkler irrigation should be one of the easier system types to manage since there is so much engineering and control involved.  That is, a well-designed and maintained sprinkler system should produce little surface runoff and good distribution uniformity.  Since it is a piped system you should have control over the total application also.  So what can go wrong?  Wind, bad maintenance, no flexibility in your water supply so that you have to run the system longer than you want, poor design, an application rate too high for the soil, and no knowledge of soil moisture depletions at irrigation- just to name a few.

Science has developed rational concepts and equations for helping to design and manage irrigation systems.  However, even with all the engineering that goes into the modern sprinkler system it will always be important to have an irrigator that knows both the art and the science of irrigation.


STEPS IN PLANNING A SPRINKLER IRRIGATION

 


Step 1.  

Estimate the SOIL MOISTURE DEPLETION in the EFFECTIVE ROOT ZONE.  Using an auger and the Merriam "Feel" chart (Table 1 below) is fast, flexible, and inexpensive.  Using a neutron probe is expensive, less flexible (constrained to the sampling site), but more accurate.  A water budget irrigation scheduling system (refer to the tutorial for a discussion)will provide guidance as it will provide an estimate of total crop evapotranspiration (water use) since the last irrigation.

the EFFECTIVE ROOT ZONE may or may not be the total depth of crop roots (deeper or shallower). It is the depth of soil in which you want to control moisture contents.

Soil will hold only so much water, the FIELD CAPACITY. The difference between the current soil moisture and the soil's field capacity is called the SOIL MOISTURE DEPLETION, SMD. Soaking in more water than the SMD will result in some deep percolation below the effective root zone. At some irrigations you may want to do this to maintain a salt balance. In a drought year you want to reduce deep percolation as much as possible.

 

Step 2 .  

Know the application rate of your sprinkler system.  The application rate (APP RATE) is how fast water is being applied through the system.  APP RATES are expressed in terms of a depth per unit time, commonly "inches/hour".  If I say that the application rate of the system is .25 inches per hour, that means that the system is applying an average depth of .25 inches over the set for each hour it is run (i.e. it is raining .25 inches each hour).

Each combination of nozzle size, sprinkler spacing, and system pressure produces a distinct application rate. For example, a 7/64" nozzle running at 50 pounds per square inch (PSI) pressure on a 30 x 40 spacing has an application rate of about .2 inches/hour.  Table 1 shows the APP RATES for different combinations of nozzle and sprinkler spacing running at 50 pounds per square inches pressure. 

 

TABLE 1- Approximate Application Rates in Inches/Hour for Various Sprinkler Spacings and Nozzle Sizes (running at 50 psi)

 
  Nozzle Sizes
Sprinkler Spacings 3/32 7/64 1/8 9/64 5/32 11/64
30x30 .19 .27 .34 .44 .54 .65
30x40 .14 .20 .26 .33 .41 .49
30x35 .13 .18 .23 .29 .26 .43
30x40 .12 .17 .22 .28 .35 .42
30x45 .11 .15 .20 .25 .31 .37
30x40 .11 .15 .19 .25 .31 .37
30x45 .10 .13 .17 .22 .27 .32
 

If you wish to determine your system's APP RATE exactly, use the following equation...

APPLICATION RATE = GPM  x 96.3 / AREA

where:

    APPLICATION RATE is the application rate of the system in inches/hour
    GPM is the flow through the sprinkler nozzle in gallons per minute

AREA is the area covered by each sprinkler in square feet =  distance between sprinklers on the lateral * distance between laterals

Example, if you had 30 foot sprinkler lateral with a lateral move of 45 feet, and the flow through each nozzle was measured at 2.8 GPM, then...

APPLICATION RATE     = GPM x 96.3 / AREA
                                 = 2.8 x 96.3 / (30 x 45)
                                 = .20 inches/hour

 

Step 3.  

Know the distribution uniformity of your system.  Remember that the first consideration for effective, efficient irrigations is distribution uniformity (see the Advisory on Irrigation Performance Measurements for an explanation.)   With sprinkle systems there are three main factors in determining distribution uniformity (DU), pressure uniformity, device uniformity, and overlap uniformity.

Pressure Uniformity - Check the pressure throughout the irrigation system. It should not vary more than 10-15% at the device outlets throughout the field.  Also make sure the system is operating at the correct base pressure.   That is, if the design pressure is 65 PSI at the pump outlet, make sure that it is 65 PSI at the pump outlet.

Device Uniformity - Device uniformity means that each sprinkler is flowing the same amount of water.  Assuming that the pressure uniformity is okay, check for worn or plugged nozzles and systems with more than one size nozzle or sprinkler head.

Overlap Uniformity - Overlap uniformity is important with field sprinkler systems and depends on the correct choice of sprinkler spacing, pressure, and head/nozzle size.  Wind is a major factor in affecting overlap uniformities.   Using alternate sets is always a good idea with field sprinkler systems.  Also make sure that the risers are high enough so that the crop doesn't interfere with the stream. 

There should not be excessive surface runoff with sprinkle/trickle/spitter systems. If there is, either the set times are too long or the system was not designed properly. The soil/water chemistry may also have changed, check for required amendments.

It is a fairly straightforward process to evaluate   sprinkle/trickle or furrow/border irrigation systems for DU.  Check to see if there is a Mobile Irrigation Laboratory in your area.

 
 

Step 4.  

Match the set times to the soil moisture deficit.  Sprinkler systems provide good control over the total application, assuming you have the flexibility to water on and off as desired.  And, assuming you are dealing with wind and are obtaining good DU, achieving good irrigation efficiency is a matter of determining correct set times.   Remember, good DU is necessary for high efficiency but it is not sufficient,   as the figure below show.  

In the figures, the heavy dashed line is the soil moisture depletion (or deficit) at irrigation. This is how much water was needed.   The dashed line is how much actually was applied.  Notice that in both figures the applied line is relatively flat, indicating a good DU for both irrigations. However, the set time on the right was twice as long as needed (notice how far below the SMD line is the applied line).

 

Figure 1 - Good DU with about the right set time

Figure 2 - Good DU but excessive set time resulting in excessive deep percolation
   
 

Determining Optimum Set Times - Determine the optimum set time and then try to "best-fit" it into your field operations.  The equation for determining set times is:

RUNTIME = SMD / (APPLICATION RATE * DU * (1 - OTHER) )

where

    RUNTIME - the desired set time in hours
    SMD - the soil moisture depletion in inches
    APP RATE - the application rate of the system in inches/hour
    DU - distribution uniformity as a decimal
    OTHER - other losses as a decimal (mainly evaporation and leakage- possibly on the         order of 10%, .1 as a decimal)

Example -

Assume that you determine a soil moisture depletion of 2.5 inches just before the irrigation. You've had the system evaluated and it is operating at about an 65% distribution uniformity (.65 as a decimal) in your windy conditions.   The application rate is .25 inches/hour.  You think that 10% of the applied water immediately evaporates or is lost to leakage in pipe joints.  An optimum runtime is calculated as:

    RUNTIME     = SMD / (APPLICATION RATE * DU * (1 - OTHER) )

                           = 2.5 / (.25 x .65 x (1 - .1))

                           = 17.1 hours

This may not fit too well into normal field operations. However it is a starting point and will allow you to do some calculations to determine the benefits/costs of hiring night irrigators (so as to change sets at night) versus lengthening the irrigation cycle and running a 22 or 23 hour set.  22-24 hours would probably match normal field operations and may be a fit with water district operations if the primary supply is from a district. 

On the other hand, using "alternate set" sprinkler irrigation (see below) can improve DU's up to 10%. Let's assume that you can improve the DU of the system to 75% (.75 as a decimal).  Also, you decide some needed maintenance will reduce leakage somewhat so that other losses are not only 8%.  Now:

    RUNTIME     = SMD / (APP RATE * DU * (1 - OTHER) )

                           = 2.5 / (.25 x .75 x (1 - .08))

                           = 14.5 hours

You now might decide to run 12 hour sets and shorten up the irrigation cycle slightly.  However,  this will probably increase labor costs.

One final option may be to lower the system pressure somewhat, hoping to maintain DU at the same time, and lower the application rate.  Assume you can do so and the new application rate is .20 but the DU is reduced to 70%, even with alternate sets.  Now:

    RUNTIME     = SMD / (APP RATE * DU * (1 - OTHER) )

                           = 2.5 / (.20 x .65 x (1 - .08))

                           = 19.4 hours

Now,  just a slight lengthening of the cycle will probably give me a good fit for 22-23 hour set times.

  IMPORTANT!!
Obviously, the big question when changing the length of irrigation cycle is the impact on the crop.

 
 

Step 5.

Monitor the results of the irrigation.  Use a soil probe 1-2 days after the set to see how far water penetrated.  Check at various places within a set of sprinklers.  That is, check water penetration right at a sprinkler riser, about 1/3 of a spacing away, in the center of a pattern from overlapping sprinklers, etc.   This will give you an idea of the overlap uniformity.  Check water penetration in different parts of the field to check differences due to pressure uniformity. 

   

ESTIMATING OPTIMUM SET TIMES FOR MICRO-IRRIGATION SYSTEMS

 

Micro-irrigation systems are much like sprinkler systems in that the design and maintenance of the system is the main determinant of the DU and they both give good control over the total application.  In micro-irrigation systems we speak of an Emission Uniformity (EU) rather than a DU for a couple of reasons.   Note that EUs can be determined just as DUs and Mobile Irrigation Laboratories or irrigation specialists can provide this service.

  IMPORTANT!! 

Preventive maintenance is all important with micro-irrigation systems.  Make sure that the filtration system has been designed correctly and kept in proper working order.  Make sure that you have analyzed the water supply for any amendment needs and compatibility with any injected chemicals or fertilizers.  Make sure that you are preventing algae growths or other contaminants from plugging up the system.
 

A big difference between sprinkler systems and micro-irrigation is how we determine set times.  Since we are covering the total field with field sprinkler systems we can compare the application rate to the soil moisture depletion to determine how long to run the system.  With trickle, micro-sprinkler, or spitter systems we are not covering all the field.  And, these systems are usually operated frequently so as to maintain optimal soil moisture conditions (one of the big advantages of drip irrigation).  Many times we compare the gallons per hour per plant of the system design to the daily water use of the crop (evapotranspiration, ETc) to determine required set times.

The equation to convert daily ET to hours of system operation with micro-irrigation systems is...

RUNTIME = ETc * AREA / (GPH * EU * 1.605)

where:

    RUNTIME = daily hours of system operation
    ETc = daily crop water use in inches/day
    GPH = total flow to each plant in gallons per hour
    EU = system emission uniformity as a decimal 0 - 1.0

For example, there is a grape vineyard with two 1-gallon per hour emitters per vine. The vines are spaced 8 by 12. The estimated system emission uniformity is 80% (.8 as a decimal) and the current daily crop water use is estimated at .25 inches/day. Then...

    RUNTIME = ETc * AREA / (GPH * AE * 1.605)
    RUNTIME = .25 * (8 * 12) / (2 * .8 * 1.605)
    RUNTIME = 9 hours of operation per day

Graphical irrigation scheduling systems are used often with high frequency systems.  Refer to that Tutorial for ideas on managing a micro-irrigation system.

   
ESTIMATING OPTIMUM SET TIMES FOR DRIP-TAPE SYSTEMS
 

The equation for determining set times for drip-tape systems, also termed "row-crop drip" systems is different since the flow rating system for the product is different.  Drip tape is rated in terms of gallons-per-minute per 100 feet of tape.  For example, a common "high flow" tape will have a rating of .33 gpm/100 feet.

The equation to convert daily ET to hours of system operation with micro-irrigation systems is...

RUNTIME = ETc * SPACE * 1.039 / (GPM 100 * IE)

Where:

     RUNTIME = daily hours of system operation
     ETc = daily crop water use in inches/day
     GPM 100 = tape flow rating, gpm per 100 feet of tape
     IE = irrigation efficiency as a decimal 0 - 1.0
     SPACE = tape spacing in the field (which is usually the furrow or bed spacing) in feet

For example, there is a melon field using tape rated at .33 gpm/100 feet. The melon beds are spaced 5 feet with one tape per bed. The estimated irrigation system efficiency is 80% (.8 as a decimal) and the current daily crop water use is estimated at .25 inches/day. Then...

RUNTIME = ETc * SPACE * 1.039 / (GPM 100 * IE)
RUNTIME = .25 in/day * 5 feet * 1.039 / (.33 gpm/100 feet * .8)
RUNTIME = 4.9 hours/day

 

ALTERNATE-SET LATERAL PLACEMENT

 

"Alternate set" lateral placement is illustrated by Figure 3.  The lateral positions marked by "1" are the positions for the first irrigation.  The lateral positions for the next irrigation are marked by "2".  You can see that the same spacing is used for each irrigation.   It's just that the laterals are offset by a half-spacing, placed in alternate locations for each irrigation.

 

Figure 3 - Schematic of a sprinkler system indicating lateral placement for "Alternate Set" sprinkler irrigation

This alternate placement of laterals can improve the distribution uniformity by up to 10% with no increase in labor costs.  It does so by moving the field areas getting the lowest application rates around.  Thus, area A may get a low application rate one irrigation, but a higher application rate the next.  The assumption is that the theoretical under-watering on the one irrigation will not be too harmful for crop development.  Alternate sets are a good idea anytime, but especially important in areas with consistently windy conditions.

 
COMMON SUGGESTIONS FOR IMPROVING SPRINKLE AND MICRO-IRRIGATION SYSTEM PERFORMANCE
 

The following are common management practices that are recommended for improving the performance of sprinkle and micro-irrigation systems.

Those practices that can be used with any irrigation system type include:

 
- Measure all water applications accurately
- Monitor pumping plant efficiency (see the Energy Advisory )
- Evaluate the irrigation system using SCS or Cooperative Extension procedures
- Know required leaching ratios to maintain salt balances
- Use irrigation scheduling as an aid in deciding when and how much to irrigate (see the Water Budget or Graphical Irrigation Scheduling Tutorials)
- Practice total planning of individual irrigations (see the Steps above)
- Use two irrigation systems in special situations (sprinklers for pre-irrigations then furrows; portable gated pipe to reduce furrow lengths for pre-irrigations; sprinklers to germinate crops irrigated by micro-irrigation; over-tree sprinkler for cooling with undertree irrigation)
- Consider changing the irrigation system type if it is not clearly adaptable to the physical situation
- Use aerial photography to identify patterns that indicate problems with irrigation/drainage
 


Implementation practices for sprinkle irrigation systems include:

 
- Have an irrigation engineer/specialist check hand-line and side-roll sprinkle field layouts to ensure correct combinations of spacing, operating pressure, sprinkler head, and nozzle size/type
- Have an irrigation engineer/specialist check field layouts for flow uniformity - use flow control nozzles, pressure regulators as necessary
- Maintain sprinkle systems in good operating condition
- Use the " alternate set " technique with hand-line, side-roll, or "big gun" field sprinklers to improve overlap uniformity
- Operate in low-wind situations if possible
- Modify hand-line and side-roll sprinkle systems to smaller spacings and lower pressures if wind is a problem
- Ensure that center pivot sprinkler/nozzle packages match the infiltration rate of the soil
- Minimize surface runoff from sprinkle-irrigated fields
- Use reservoir tillage (dammer/diker) techniques to reduce field runoff
- Install runoff-reuse systems
 
Implementation practices for micro-irrigation systems include:
 
- Consult experienced agronomists/engineers to ensure that the appropriate volume of soil is being wetted by the system design
- Have an irrigation engineer/specialist check the design for emission uniformity (pressure uniformity, correct pressure for the device) - use pressure regulators and pressure compensating emitters as necessary
- Have the irrigation water analyzed to enable design of an adequate system of water treatment and filtration
- Have a chemical analysis of irrigation water/fertilizer/other additives to ensure compatibility and prevent clogging of the system
- Practice good maintenance procedures to ensure that the system performs as designed
   
 
 

ESTIMATING SOIL MOISTURE DEPLETION FROM THE LOOK AND FEEL OF A SOIL SAMPLE

 

The chart below is a compendium of years of experience by Dr. Merriam and his colleagues in the Soil Conservation Service, Bureau of Reclamation, University Extension, and private industry.   It is used to gauge soil moisture depletions by observing the look and feel of soil samples.  Many consultants would prefer to work with a sample from a coring tool rather than a screw auger or bucket auger.

 

Table 2 - Factors for estimating soil moisture deficit from the look and feel of a soil sample
Soil Texture Classification

Moisture
Deficiency
in/ft
Coarse
(loamy sand)
Sandy
(sandy loam)
Medium
(loam)
Fine
(clay loam)



.0


.2


.4


.6


.8


1.0


1.2


1.4


1.6


1.8


2.0

(field capacity)

--Leaves wet outline
    on hand when
    squeezed.

    Appears moist,
    makes a weak ball.

__ Appears slightly
   moist, sticks
   together slightly.

   Dry, loose, flows
   through fingers.
   (wilting point)

--






__







---
(field capacity)

--Appears very dark,
   leaves wet outline
   on hand, makes a
   short ribbon.

   Quite dark color,
   makes a hard ball.
___

   Fairly dark color,
   makes a good ball.

   Slightly dark color,
   makes a weak ball.

--Lightly colored by
   moisture, will not
   ball.

   Very slight color
   due to moisture.
   (wilting point)
__







---
(field capacity)

--Appears very dark,
   leaves wet outline
   on hand, will ribbon
   out about one inch.

   Dark color, forma a
   plastic ball, slicks
__ when rubbed.

   Quite dark, forms a
   hard ball.


   Fairly dark, forms
   a good ball.
--

   Slightly dark, forms
   a weak ball.


   Lightly colored,
__small clods crumble
   fairly easily.

   Slight color due to
   moisture, small
   clods are hard.
   (wilting point)

---
(field capacity)

--Appears very dark,
   leaves slight moisture
   on hand when
   squeezed, will ribbon
   out about two inches.

   Dark color, will slick
__and ribbons easily.


   Quite dark, will make
   a thick ribbon, may
   slick when rubbed.


--Fairly dark, makes a
   good ball.

   Will ball, small clods
   will flatten out rather
   than crumble.

__Slightly dark, clods
   crumble.


  Some darkness due to
  unavailable moisture,
  hard & cracked clods
  (wilting point)
---

 

 
  THE SOIL PROBE
 

A soil probe is simply a 4 -6 foot long piece of 3/8 steel with a rounded tip at one end and a handle at the other.  One configuration is shown in Figure 4 below.

 
 
Figure 4 - One configuration of a soil probe
   
 

The soil probe is one of the handiest tools in your pick-up.   It has many uses:

 
- It can be used to check when it is time to move and irrigation set.  Just press the tip into the soil at an appropriate spot in the field.  If the tip goes in easily to about 1/2 to 2/3 the depth of the effective root zone it is about time to move the set.  The area that the tip goes in is probably near saturation.  Excess water in this area will drain down to take the rest of the effective root zone to field capacity.
- It can be used to check whether an irrigation was sufficient.   Wait a day or two after the irrigation to press the tip in.  Now it should go easily to the depth of the effective root zone- too shallow and the irrigation was insufficient, too deep and you over-irrigated.
- It can be used to check the uniformity of an irrigation.   One or two days after an irrigation, go to the most-watered part of the field and press the tip in and record how far you can easily push it in.  Then, go to the least-watered area and do the same thing.  The difference in the depths is an indication of the uniformity of the irrigation.  For sprinkler systems, check at a sprinkler and in the middle of a pattern of four over-lapping sprinklers.
- You can also use the soil probe to check the lateral spread of water.  This may be especially handy when irrigating every-other furrow and there is a question of whether the beds are getting wetted through sufficiently.
- It can be used to check the wetted soil volume under a drip irrigation system.  Press the probe in laterally from the emitter to see how far out and deep water spreads.
   
 
 
 
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