Tracking Every Last Drop: Irrigation Audits

The rate at which the water is applied varies with the type of sprinkler being used, the spacing and the pressure. These and other factors positively or negatively affect the system’s efficiency and water application uniformity. The best irrigation efficiency is accomplished when the largest percentage of the water applied by the irrigation system is used by the landscape material being watered. By having high irrigation efficiencies the amount of water is reduced and costs are less. The efficient application of water is based on proper scheduling and good uniformity. To properly schedule the irrigation system operation and to understand the system’s uniformity, an irrigation audit should be performed.

WHAT IS AN AUDIT? An irrigation system audit consists of a series of steps performed in a logical order to arrive at a base irrigation schedule to reduce water use. The steps include a system tune up, data collection, calculation of appropriate station run times based on collected data, and developing schedules based on weather that reflects actual site conditions.

When performing a system tune up, a record drawing of the irrigation system installation is helpful. But most systems do not have a record drawing, hence a visit to the audit site usually is in order. Once on site, the auditor should locate all of the irrigation system components, including the point of connection, water meter, backflow preventer and shut off valve. The controller needs to be located and operated so that the sprinkler can be observed while operating. The auditor should record the current schedule and available features in the controller, such as water budgeting, repeat cycles, number of programs and any sensors that affect the schedule.

Record the station number, zone location, plant material being watered and types of operating sprinklers for each station. The auditor should also note if the station stands alone or if its coverage interfaces with another station. While operating each station, note valve malfunctions, pressure, whether sprinklers are level, interference with the spray or throw, misaligned arcs, clogged nozzles, broken sprinklers or seals, and low head drainage.

A simple spreadsheet for the site inspection (see sidebar below) can be used for recording data. Many times, a system tune up can be performed at the same time you prepare the system audit. If the system problems are more severe, the audit should be postponed, as auditing a poorly operating irrigation system is a waste of time. Items such as old or worn out equipment, valves that don’t close, unmatched nozzles, different types of sprinklers on the same zone, poor spacing, incorrect pressure, limited controller functions and poor zoning should be repaired/fixed before an audit takes place.

PERFORMING THE AUDIT. Before the field data is collected, there are many different steps that must be taken. 1) review the record drawings, 2) observe the system operation, 3) perform the system tune-up or verify that it has been completed, and 4) select what zones on the site are to be tested as not all zones will be audited.

Also, please note that this field work is best accomplished early or late in the day when wind and evaporation losses are at their lowest. When the audit is performed, conditions should be consistent with the irrigation system’s normal operation. Pressures and amount of wind should be similar.

The audit will test multiple and/or single stations. As implied, a single-station audit tests the uniformity of a single station. All of the water from that station covers a specific area. A multiple station area has water applied to it from more than one controller station or zone. The stations should be marked in groups, which is easily done by using colored flags. Assigning different colors to zones helps quickly identify multiple station groups, but flags may have to be removed before testing to avoid interfering with the sprinklers’ distribution pattern. The audit should be performed on representative stations. If there are five zones of spray sprinklers on turf at a 15-foot spacing, only one needs to be audited and that data can be used for the other four.

Next, check the pressure for each station being audited using a pressure gauge. This is best done within the zone being tested.

Make sure that the system pressure is approximately the same as when the system is operating to see if the pressure is uniform throughout the zone. The pressure should be taken on each sprinkler zone and at the beginning, middle and end of each zone to verify that the pressure doesn’t significantly differ within the zone. Then compare the pressure to the sprinkler’s required operating pressure from the manufacturer’s literature, the record drawing or the original design drawings. The easiest way to measure the pressure is to use an adapter that screws onto the sprinkler or a pitot tube can be used to get the pressure at the nozzle.

The sprinkler spacings need to be measured in order to calculate the precipitation rate of each station. Sprinkler spacing should be head-to-head and row-to-row. This is easily accomplished with a 100-foot measuring tape. The type of sprinklers and nozzles should be recorded. The zone should be checked for match precipitated nozzles.

Using a soil probe, a soil sample should be taken to measure the root zone depth and estimate the soil texture and structure so that the soil can be classified. Once the preliminary data has been collected, the distribution data can be collected using catch devices.

Catch devices are small containers that are evenly spaced throughout the area being irrigated in a set pattern. Catch devices (cups) can be many different types such as cat food cans, tuna fish cans or expensive containers specifically made for auditing. The most important thing to remember is that the cup should have a consistent top area (throat) and have some weight to it so that it will stay in place. Once the cups are in place, each station to be audited is operated for a specific time period and the amount of water is measured. The results are used to calculate a net precipitation rate and distribution uniformity for the area. The irrigation system is then scheduled based on actual field data as opposed to the manufacturer’s catalogue theoretical data.

Proper cup placement is critical to proper auditing. Correct placement of the cups will save time, minimize the number of cups needed, and provide good data. In most cases, cups should be placed near each sprinkler and halfway between one sprinkler and the next. In multiple groups, all stations that contribute water to an individual cup need to be tested before that cup’s data is recorded as they all effect the total amount of water the area will receive. When placing cups it is important that the cups are lined up and evenly spaced. When looking down a row of cups, all the cups should be in a straight line. Be careful not to put the cups too close to the sprinkler, as they can get knocked over when the sprinkler comes on or the stream will be deflected by the cup. This may require that the test be started over again or that the test be completed with one less cup. For irregular shaped areas with irregular sprinkler spacings, cups are better placed in a straight grid pattern. The larger the sprinkler spacing the larger the grid spacing, but a 20-foot by 20-foot or 15-foot by 15-foot grid seems to work best.

The amount of time to run each cup test depends on the sprinkler being operated. Sprinklers should be operated until a minimum of 25 ml of water is collected in the average cup. This translates into about five rotations for rotary sprinklers (12 to 15 minutes) and about seven minutes for sprayheads. Record the amount of water collected in each cup and the location needs to be recorded as well as the amount of time the sprinklers were operated. The cups should be recorded to the nearest millimeter, and the same person should read all the cups so that the resulting data is consistent and accurate.

Problems always occur when testing, but if the problem is a result of the test sequence, either throw out the data from that cup or repeat the test after correcting the problem. If the problem is with sprinkler operation, then the data should remain as it is consistent with the normal operation of the system.

After comleting the cup tests, make sure that all the necessary field data has been collected. This includes cup test data, sprinkler spacings, wind speed and direction, sprinkler pressures, sprinkler type and nozzle, soil information, test run time, plant material, stand alone vs. multiple stations and controller information. Making a small sketch of the areas tested with the sprinklers and cup locations noted also helps. Lastly, document any problems that occurred during the testing.

CALCULATIONS. Now that all the field data has been collected, calculate a base schedule for each irrigation system station. This can be done manually or with the use of a simple computer program. Base schedules are for a watering time and more than one base schedule will be needed. Depending on the irrigation system location, a monthly base schedule may be required.

In order to determine the base schedule, not only is the amount of water being applied needed, but also the amount of water required by the plants. The plants’ water requirement will be based on a number of weather factors, including solar radiation (sunlight), wind, temperature, rainfall and relative humidity. All of these factors can be used to calculate the evapotranspiration rate (ET) - a combination of evaporation from the soil and transpiration from the plant - of the plants to be irrigated. ET is difficult to calculate without a weather station, so find a local source, which might include universities, Internet Web sites, extension service historical data or evaporation pans.

An auditor needs to be familiar with several terms to calculate base schedules, such as distribution uniformity, efficiency, precipitation rates, scheduling coefficient and coefficient of uniformity.

The distribution uniformity (DU) measures how uniformly water is applied to the irrigated area. It is tied to the sprinkler spacing and how the individual sprinklers distribute water. Conditions such as wind, pressure and worn nozzles can affect sprinkler distribution. In auditing, usually the lower quarter distribution uniformity is used as opposed to the overall distribution uniformity, which can skew the number toward drier areas. Distribution uniformity is determined in the field with the cup test results and is the main reason for performing the cup tests. It is the average water applied in the 25 percent lowest amount of cups compared to the average of all cups.

Irrigation efficiency is a measure of the amount of water applied vs. the amount of available water. It is affected more by subjective factors rather than mechanical factors like uniformity. Time of day, over-watering, low head drainage, and drift all affect efficiency. Although efficiency and uniformity are related, they are two very different measurements. A system can have a high uniformity but a low efficiency due to poor operation, such as over-watering, where the excess water is lost to deep percolation or runoff occurs. Irrigation efficiency is very difficult to measure and is usually assumed.

Gross precipitation is calculated in inches per hour using the general formula or by looking it up in the manufacturer’s catalogue.

By conducting an audit, a more accurate precipitation rate can be calculated using the cup tests results. This form of the "net" precipitation rate is calculated as:

Scheduling coefficient measures uniformity to compare the lowest precipitation rate for a defined area to the average precipitation rate over the entire area. It is calculated with the formula at the top of the next page.

This measurement looks at the driest 1, 2, 5 or 10 percent of the area being watered and is very skewed to the dry side. It compares the absolute lowest precipitation rate to the average. This figure is difficult to calculate without a computer, but it effectively distinguishes between wet and dry spots.

Once the distribution or other uniformity measurement has been established, the total run time per station can be calculated using the irrigation water requirement, which is presented in the box on the bottom of the next page.

AN EXAMPLE. Let’s use a small irrigation system audited as an example. During the audit, 20 cups (16.5-square-inch throat area) were used and the cup data is outlined in the chart below along with the calculations. The test lasted 15 minutes. The average spacing of the sprinklers was 35 feet by 35 feet, each using 4.0 gpm at 40 psi pressure. The plant water requirement is 0.35 inches.

Based on the audit, that station representing the 20 cups would require an 86-minute run time to apply 0.35 inches of water. That could be on a daily, weekly, monthly or annual basis depending on the plant requirement. In this case, it is most likely a daily requirement and the controller would be set up to irrigate in cycles to percent runoff. This is why noting the controller features and determining whether the existing controller can implement a schedule that allows for multiple start times per station is so critical.

The same procedure would be performed for each station for which data was collected. This data would then be applied to similar stations. Ultimately, the auditor provides a number of base schedules for the irrigation system with the required operating time for each station. The base schedules are for specific periods of time and amounts of water to be applied. In addition to run time, they include days per week and number of cycles required per day for each station.

Auditing an irrigation system is a fun and educational experience. It is a quick and visual way of learning how sprinklers apply water and the problems associated with irrigation system operation. The audit needs to be performed in steps in a logical manner to obtain accurate results. The cup tests are the main part of the audit and provide the data necessary to calculate the distribution uniformity and net precipitation rates. With these figures, base schedules can be determined and implemented to reduce water use throughout the audited irrigation system.