Home Lawn Irrigation Audit and review

 

By checking several very important aspects of your sprinkler system you can determine if it is performing at peak efficiency and uniformity. A properly functioning system is vital to the health of your landscape and greatly benefits your watering expense.

     Benefits:

         Improved soil-moisture uniformity

         Lower water bills

         Easier irrigation management

         Reduced runoff and deeper soil penetration

         Healthier turf grass.

 

DISTRIBUTION UNIFORMITY of IRRIGATION WATER

 “UNIFORMITY” vs. “EFFICIENCY”

SYSTEM EVALUATION AND INSPECTION

Location to test

Inspect your system

Matched-Precipitation Rate Sprinkler Heads

Head-to-Head Coverage

Check your watering schedule

PERFORMING FIELD TESTS

Test the system

Gather

Marking Flags

Where to place collectors

Turn on the zone to be tested

Analyze the test results

          Calculations

                   WHAT DU & SC MEANS

Precipitation rate vs. Infiltration rate

Understanding your soils Infiltration or Porosity

How OFTEN should you water your SHRUBS and TREES ?

PROCEDURE REVIEW

 

 

 

DISTRIBUTION UNIFORMITY of IRRIGATION WATER

When discussing watering uniformity the term for measuring this is called Distribution Uniformity or "DU”. So what is it, and why is it important? Simply put, DU is how evenly a sprinkler delivers water over your lawn.

 

As you can see in the photo to the right, poor distribution of water can create a lawn with varying shades of green.

Due to uneven watering your lawn can look like this, unless you over-water the green spots to make the yellow ones healthy.

 

It is important to know that uneven or non-uniform systems create areas that are either too wet or too dry. This non-uniformity causes havoc with your irrigation scheduling. If you apply enough water to adequately dampen the dry spots, you'll place too much water on the wet spots. If you manage the wet spots, the dry spots most likely turn to "dead" spots. Both of these options create plant stress and make efficient water use impossible.   (top)

 

The terms “uniformity” and “efficiency are often interchanged. In reality however, these individually define different parameters. Efficiency is the actually the ratio between how much water the plant uses compared to how much water the irrigation system applies. One published study (B1) found that tall fescue required .33 inch of water per day during July in Las Vegas. It can become quite complex to be aware of the water required (or used) by all your plants. For this reason the focus of this audit will be uniformity rather than efficiency.  (top)

 

Percent efficiency = 100 x (water required by the plant /. water applied)

 

Uniformity, however, relates to how evenly water is applied over the lawn. This takes into account the equipment selection (sprinkler types, nozzle size, pressure, pipe size) and the design, installation and maintenance of the irrigation system. To get optimum performance from your irrigation system, you must properly design, maintain and manage it.

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SYSTEM EVALUATION AND INSPECTION

Now that we've established the importance of uniformity, you are ready to follow the process to determine your systems’ uniformity rating.

 

1) Location to test. Are there problem areas in your landscape? Do you have irregular coloration to your lawn? Where?

     You will want to test an area where there looks to be a problem, and compare that to an area that looks good.

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2) Inspect your system. Perform an inspection of your system while it is operating.

Check and correct these items before you analyze yours systems uniformity:

        

     Broken heads

            Poorly adjusted heads (wet sidewalks)

           Heads that are crooked or sunken

           Plugged nozzles

           Mismatched heads & nozzles

           Pressure too high or too low

           Spray deflection by plants or hardscapes

           Lawn compaction or heavy thatch

           Electric valves malfunctioning (incomplete shut off?)

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Broken heads                  Misdirected heads            Excessive pressure

 

Typically, you can repair the above items at a minimal cost of time and money. You then can observe and correct most of these problems when the irrigation system is operating.

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Matched-Precipitation Rate Sprinkler Heads [check and record this]
All sprinkler heads on a given zone should be the same type (e.g. rotors, pop-up spray, etc.) and have matched precipitation rate (in inches per hour). Heads with differing precipitation rates have widely varying operating times, which can lead to the over-watering of one area in order to sufficiently water another. If you have this problem it will show up in your audit. 
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Head-to-Head Coverage [check and record this]
Pop-up sprinkler systems are designed to operate with head-to-head coverage, where the spray from one sprinkler head reaches to the next, resulting in the needed overlap. Precipitation rates are based on the assumption of overlap. Places where the sprinkler's spray pattern does not overlap are likely not getting sufficient water and may develop brown dry spots. These spots indicate the system has low "uniformity" of coverage. To compensate, one has to run the sprinklers for longer times to get adequate water to the dry spots, while the rest of the lawn is getting over-watered. 
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3) Check your watering schedule. How have you programmed your controller? Are you using some sort of a smart controller; a rain switch or soil-moisture sensor? How is the system scheduled ­ guesswork, weather stations, historical ET or the popular "pour on the water until the dry spots are gone" method? Do you modify the schedule for seasonal weather changes?   (top)

 

PERFORMING FIELD TESTS

Once you have fixed all the obvious problems, you are ready to analyze your irrigation system. Reschedule the test if wind conditions exceed 8 mph.

 

4) Test the system.

 

4a) Gather - You will need the following tools to evaluate your system's performance and uniformity:

·         Watering collectors

·         Moisture meter

·         Measuring tape

·         Marking flags

·      Forms (see below)

 

·         Landscape Layout form  (Print this out – to be filled in)

 

·         Soil Moisture Charts » (1h). (4h). (22h).  (Print these out – to be filled in)

 

 

·         DU calculation form  (Print this out – to be filled in)

 

 

·         Clock Scheduling form  (Print this out – to be filled in)

 

·         Pencil and note pad

·         Stopwatch or watch with second hand

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4b) Marking Flags -After you select the test site or zone, operate the system for a few minutes so that you can place marking flags near each head in the zone. If you use more than one valve, select a different color flag to mark each zone. Take a moment to make sure everything is working and operating correctly.

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4c) Where to place your collectors. Note: collectors should be low- profile, especially when testing low-angle sprinklers. Use cans of the same type and set level. Lay out your catch cans based on your zone's style of head layout ­ either triangular or rectangular. Keep the collectors at least 1 yard from the head to avoid spray deflection. Place your collectors near a head and in between heads. Try not to vary your pattern of layout within the zone. Use at least 10 collectors per tested zone. Sketch a map of the test zone showing the location of the heads and catch cans.

 

Before testing, note whether your zone is stand-alone or overlapping. A stand-alone test area receives its water from the operation of only one valve. Additional valves or zones don't contribute to the coverage. This is typical of residential spray-head systems. An overlapping zone is typical of large commercial areas such as a sports field or park where several valves or zones contribute and overlap during operation.   

 

Overlapping zones require multiple-valve operation. You'll need to set your cans, run a valve for a set period and operate the overlapping zone for the equivalent or appropriate time. In some instances, you may need to operate more than two valves before you can evaluate the test area. Operate the valves as you normally schedule them. That is, if you run the valves sequentially, you should operate them sequentially during the test. Otherwise, it may affect the operating pressure.   (top)

 

4d) Turn on the zone to be tested for 3 to 10 minutes for sprays and 10 to 30 minutes for rotors. You want an average of about 1 inch of water collected before shutting off the valve. Keep track of the runtime in minutes. Record the data from the collectors. Hold each collector level and read the water's depth. Be accurate; to 1/10”! Record amounts and locations on the sketch.

During your field work, you also should collect information on head and row spacing, nozzle pressure, soil type and root-zone depth. By taking a few samples and examining the rooting depth, you can determine whether you need soil amendments or deeper irrigations.

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5) Analyze the test results. After you complete your test, you can analyze the results to determine whether your system operates efficiently. The test shows you how evenly your irrigation system distributes water over the area in question. It also points out weak spots you can modify and repair. You can determine your precipitation rate. With the soil and root-zone information, you can develop more accurate watering schedules as well.

 

5a) Calculations In order to complete the analysis of your test results there are several calculations that must be completed. Formulas  (for AC, ALC, AD, DU, CU, SC) are provided in order that you may perform these calculations, or you can take your test results into your local Star Nursery, and they can complete the calculations for you.

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5b) DU. Distribution uniformity (DU) emphasizes under- watered areas and is the calculation that irrigation auditors typically use in landscape audits. Its formula is:

Percent DU = 100 x (Average catch in the low quartile /. Average catch overall)

 

Rank the collector readings from low to high. Take the average of the lowest 25 percent of the catch-can readings. (For example, if you use 16 catch cans, take the average of the four lowest readings.) Divide the average lower quartile by the average of all the catch-can readings. Multiply the resulting decimal by 100 and express as a percentage.

 

WHAT DU MEANS

As a basic guideline, you should have a DU (Distribution Uniformity) of 70 percent or higher for rotor systems and a DU of 50 percent or higher for spray-head systems. The higher the DU the better. If your DU falls below these values, you may need to upgrade your system to achieve greater uniformity.

A disadvantage to determining DU is that it treats under-watering as the critical element but does not tell you how big or severe the dry spot is. You also can use the CU Coefficient of Uniformity and SC (Scheduling Coefficient) formulas to determine your system's uniformity. Set the collectors in the area you want to evaluate and operate the sprinkler system for a set time. Measure and record the amount of water in each collector. Then use that information in the following equations:

Percent CU = 100 x (1 - average deviation /. Average catch)

One of the limitations of the CU calculation is that it treats under under-watering and over-watering the same.

SC = average catch overall /. Average catch in the critical dry area.

SC allows you to define how big the critical dry area is and to determine the irrigation run time required to alleviate the dry area. Because grounds managers normally irrigate to the dry spot, DU and SC are more useful for establishing irrigation schedules. Irrigation auditors have used the DU in audits for many years. However, due to limitations with the DU approach, the SC concept is gaining a large following, particularly in the turf and golf industry.

 

Note: Any single catch that is less than average catch x 0.70, or is more than average catch x 1.40 should be inspected to see why, and if any correction can be made to improve uniformity for this area.

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6) Precipitation rate vs. Infiltration rate. The irrigation system’s Precipitation Rate should not exceed the Soil's Infiltration Rate!

Based upon soil characteristics, water will soak into the ground at differing rates. Sandy soils have high (fast) infiltration rates, while clay soils have low (slow) infiltration rates. If the precipitation rate of the sprinkler heads exceeds the soil's infiltration rate, then runoff and erosion occur (especially on slopes). On flat ground, this will also lead to puddling. If your sprinklers' precipitation rate exceeds the infiltration rate, lower precipitation rate heads can be installed, or you can shorten your watering times and use multiple start times (e.g. 3 start times at 5 minutes each at 1-hour intervals instead of 15 minutes all at once) to allow the water to soak into the soil.

 

The soil surface acts as a filter that lets water pass through at a rate known as the infiltration rate. Runoff may be produced when precipitation rate of water to the soil surface is faster than it can be absorbed. For example, if the precipitation rate is 2 inches per hour, but the infiltration rate is only 1 inch per hour; surface runoff is produced, even if the soil is not entirely saturated.

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6a) Understanding your soils Infiltration or Porosity. If you know how long moisture stays in your soil this will enable you to schedule your watering cycle to be efficient. This will also help to prevent diseases (fungus) from getting started. Recommended watering frequencies for the Las Vegas Valley are seasonally adjusted as follows:

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6b) HOW OFTEN SHOULD YOU WATER YOUR SHRUBS and TREES ?

 

These recommendations are only typical – Conditions may differ for individual landscapes.

Please consult with a qualified professional to make certain proper watering is selected for your landscape.

Typical watering frequency

Winter

Spring – Fall

Summer

Fescue Lawn

Once per week

Three times per week

Daily (as needed)

Bedding plants and vegetables.

Once per week

Twice per week

Three times per week

Trees

Twice per month

Once per week

Twice per week

Shrubs

Once per week

Twice per week

Three times per week

Desert Plants

Once per month

Twice per month

Once per week

 

Use a moisture meter and check the moisture content in the soil following irrigation. Check at 1 hour, 2 hour, sunset and following morning. Then again just before the next irrigation or watering. Measure the moisture at a depth of 1.5”, 3.0” and 6” (if possible). Compare your readings with the moisture retention chart included with this audit kit. If the soil at 1.5” still reads a moisture content greater than 7 when your next irrigation is about to begin, you are watering too often. The plant has not yet had a chance to breathe, and further watering could damage or kill it. Record this information in the “Soil Moisture” charts that you printed from this document.

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PROCEDURE REVIEW

 

As a review, remember the steps necessary to evaluate your sprinkler irrigation system. First, select and prioritize your sites. Second, inspect and repair the irrigation system as needed. Finally, conduct the test and analyze the data.

 

At this point, you can make decisions about how, when and if to make changes to your system. Now you also can calculate more accurately the watering schedules for your site.

 

If, for example, your system has a high uniformity, then you will spend most of your efforts in perfecting the system's scheduling. If you have a low uniformity, then you must make decisions regarding how much time and money to spend on upgrading the irrigation system. Usually it is an accumulation of minor things that contribute to low uniformity. However, in some cases, only a major redesign of the system will make an appreciable difference.

 

The single most important aspect of conducting an evaluation of your irrigation system is to look at the system and make sure it is operating properly. By closely evaluating your irrigation system, you will gain a much clearer picture of its performance.

 

If you do not periodically inspect and audit your irrigation system, you may not be aware of repairs that your system might need. Be sure to inspect and maintain your sprinkler- irrigation system at the designed efficiency and uniformity to achieve optimum performance.

 

When you have finished these inspections and tests, feel free to bring the results into your local Star Nursery and ask our irrigation specialists to help you understand what steps you should take to make needed improvements.

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© courtesy Frank J. Rauscher – Star Nursery