Part two of a four-part series
Even if you only grow succulents and cacti, irrigation is an essential part of your operation — but could your operation’s irrigation system be optimized? In the previous article in this series, we looked at three tools that could help you better understand your water reservoir (pond) resources. In this article, we focus on your irrigation system with three additional tools: 1) interception efficiency, 2) coefficient of uniformity, and 3) leaching fraction for salt reduction. You can access these tools and many others for free through the www.cleanwater3.org website under the “Tools” section.
Figure 1. The web interface for the interception efficiency calculator you can use to determine the efficiency of overhead irrigation.
Interception efficiency calculator
Interception efficiency measures how much of the irrigation water applied overhead (mainly impact head and wobbler-type sprinklers) makes it into the container. Drip and spray stakes should irrigate the container surface directly and have 100% interception efficiency. As any grower knows, round containers placed tightly together still have empty area where applied irrigation will land between pots instead of in them. Figure 1 shows the information needed to calculate your interception efficiency using the interception efficiency tool, while Figure 2 provides an explanation of the terms. As the distance between containers increases, your interception efficiency decreases exponentially. For example, an 8-inch container with 8-inch spacing within and 8-inch between rows (8 x 8-inch) has an 79% interception efficiency, while a 9 x 9-inch spacing is 62%, 10 x 10-inch spacing is 50%, and 12 x 12-inch spacing is 35%. A 35% interception efficiency means that if you apply 1,000 gallons of water, only 350 gallons actually makes it into your containers.
As you shift material into larger containers you increase your substrate surface area that can intercept water, but you also typically increase spacing to give larger plants more room to grow. At container sizes of 5 gallons or larger consider switching from overhead irrigation to either spray stake or drip to increase interception efficiency and conserve water. Even though setup and maintenance can be more difficult with micro-irrigation systems, you will save large volumes of water due to higher interception efficiencies.
Some plants have a canopy structure that will more naturally funnel water to the container surface, increasing interception efficiency, while other plant canopies will shed water making it more difficult to get water to the = substrate surface. Canopy structure is not included in these calculations, but regardless of structure or container size, wider spaced plants will have a lower interception efficiency than closer spaced plants.
Coefficient of uniformity
Do some plants within an irrigation zone receive too much water, while others get too little? As we are typically irrigating to sufficiently irrigate the driest plants. Poor irrigation uniformity results in some plants getting too much water, which unnecessarily leaches nutrients and may lead to plants growing at different rates. Testing coefficient of uniformity can help you know exactly how evenly you are applying irrigation in a given irrigation block and decide if you need to make improvements. To measure irrigation uniformity, you need the following items:
- 24 or more of the same size containers (plastic sandwich containers are best but large plastic cups will also work)
- 1 measuring device (graduated cylinder, preferred, or measuring cup)
- An empty irrigation block
- A calm, non-windy day (wind speed < 5 mph)
Set up the empty containers in a grid pattern similar to the one in Figure 3 (pg. 24) making sure to evenly cover the irrigation block you are testing. To keep your containers from moving due to minor wind, you can put a rock in each container to hold it in place but remember to remove the rock before you measure the volume. After the containers are laid out, turn on the irrigation system for a set amount of time (for example 20 minutes). Run the irrigation long enough to collect a volume of water that can be easily measured. It is easiest to run irrigation long enough to half-fill the containers but be careful to not let water overflow any container.
Label each container and note their locations on a map (we recommend numbering the irrigation block and sketching the layout so that you can determine where issues occur). As you collect each container, pour the collected water into the graduated cylinder or other measuring device. Measure and record the water volume in each container (remember to empty the measuring device between containers). If labor is available, have one-person measure and one-person record water volume. The more accurate your measurements, the better your results will be. You can write the volume of each container into the tool directly, on a piece of paper or as a note on your phone etc. Once you have collected and recorded all of the samples you can enter them into the tool, entering one on each line or separating them by commas (Figure 4, pg. 20). If you input both the length of time the irrigation system was run for the test and the surface area of the container, the tool will also calculate your irrigation rate (inches per hour).
Once you enter the volume from each container, irrigation duration, and the diameter or length x width of your collection container press the “Recompute” button and your results will be calculated. Ideally, overhead irrigation uniformity should be at least 85%, while drip irrigation uniformity should be over 90%. Measuring and correcting your distribution uniformity is a critical first step if you want to introduce automation, increase irrigation efficiency, or save water. If you want to apply 1 inch of water, at 90% efficiency you need to apply 1.11 inches to achieve 1 inch everywhere or for 80% efficiency, 1.25 inches needs to be applied. If you are irrigating 50 acres at 80% efficiency, you are applying an extra acre-foot of water every irrigation cycle.
Improving your irrigation uniformity can save significant amounts of water as well as disinfection costs and equipment maintenance costs.
If uniformity is below the recommended level, it is important to determine why. Some places to start include:
- Checking nozzle orifice sizes (are they old and worn?) and replacing as needed.
- Determining if all of the sprinklers in each irrigation block have the same flow rate.
- Determining if all risers are plumb and the same height.
- Ensuring system pressure matches the pressure recommended for the heads.
Wind can also reduce distribution uniformity. When possible, minimizing irrigation when conditions are windy will help to maintain irrigation distribution uniformity.
Leaching fraction
A leaching fraction, in general, tells you how much of the water that was applied to your container drains out of the bottom. If you add a liter (1,000 ml) of water to the top of a container, and 150 ml comes out the bottom, 15% of the water left (leached out of) the container. Ideally, when irrigating with high-quality, low-salt content water, leaching fractions should be low (below 10%) since water coming out of the bottom of the container also carries dissolved nutrients (which means money).
Application of most commercial fertilizers results in a build-up of salts in the growing media over time. This salt build up occurs more quickly when irrigation water has a higher salt content, like recycled irrigation runoff water. Source irrigation water with higher electrical conductivity (EC) values are more common in the southwestern and western U.S., where water is taken out, used, and then put back into the same river or lake repeatedly, increasing the salt content. Common water treatment processes do not remove salts before water is discharged back to the water body. Although high EC source water may be more common in the western U.S., any grower recycling irrigation runoff water will experience higher salt content in recycled water versus source water. As water EC increases, plants have to work harder to absorb the water from soil while leaving the salt behind. Some species of plants tolerate the presence of salt in water better than others, salt tolerance is species and cultivar specific. As plants absorb water, some salts are left behind in the substrate making it increasingly more difficult for the plants to remove the water with each additional irrigation cycle. The easiest solution to managing salt buildup within substrates is to regularly slightly over-irrigate plants, so that salts dissolve into the water and leach out of the bottom of the container.
You can use the leaching fraction calculator (Figure 5, pg. 25) to help determine how long to run your irrigation system to prevent substrate salt concentrations from becoming too high and damaging your plants. First, you need to know the EC of your irrigation water, which can easily be determined with a handheld EC meter. In the leaching fraction tool: enter the EC of your irrigation water, and then select the plant you are interested in from the dropdown list. They are listed alphabetically by common name. You can also search for a plant or select a generic plant sensitivity. Then you enter EITHER your irrigation amount (inches per day) and application rate (inches per hour of your irrigation system), OR the length of time your irrigation system is run for that species. The “Compute Irrigation Time” button will calculate how long you need to irrigate to accommodate the proper leaching fraction (normal irrigation time + additional time for leaching accumulated salts). Generally, your irrigation time will increase as the EC levels in your irrigation water increase or the plant’s salt tolerance decreases.
These three tools give you some of the information you need to help you determine which components of your irrigation system need attention. When you integrate information from these tools with your practical knowledge, you can help your operation save water, energy, money, and grow plants more efficiently and uniformly. Give them a try and get a little more efficient today.
