Irrigation sensors for the landscape

I once asked a well-known author a question I thought would stop her dead in her tracks: "What is a writer?" To my surprise, she replied, "A writer is a person who writes." The simplicity of her answer caught me off guard.

In searching for the answer to my own question, "What is a sensor?" I offer a similar answer: A sensor is something that senses.

Sensors are so commonplace that we seldom notice them. In our automobile and homes, sensors surround us. Our lives would be more complicated if it weren't for the sensors that operate all around us.

Sensors serve grounds managers in several ways. Saving water is one of the most important benefits. It's also the one that most often enables you to justify capital outlays to upgrade your system with sensors. However, sensors also allow you to better meet the water needs of your turf and landscape plants. This improves their appearance and health, which in turn results in numerous secondary benefits, such as fewer pest problems.

Sensors measure several factors that affect landscape water needs. Among them are temperature, rain, soil moisture and wind. Sensors also help you manage irrigation by monitoring your system for signs of malfunction.

Sensor systems can be quite simple, such as rain shutoffs, or come in highly sophisticated packages, such as all-in-one weather stations that feed data to a central-control computer and calculates irrigation run times based on mathematical models and evapotranspiration (ET) data. Let's look at sensors available for landscape use, starting with some of the simpler kinds.

Time sensors The most common sensor (even though we tend not to think of it as such) for use in the landscape is the irrigation controller. The irrigation controller is a time sensor. It directs landscape-irrigation valves to open and close at specific times of the day, for specific durations.

A controller is a clock that, like most other sensors, connects to an electrical network serving a specific need. The controller is plugged into an electrical source and the clock is set to the current time, day and month. You program the controller so that at a specific time or day, it sends an electrical pulse to a remote-control valve that tells it to operate for a specific period.

Weather presents challenges that sometimes prevent controllers from meeting the specific needs of your turf and landscape plantings. Adding other sensors helps overcome these challenges. For example, sensors can monitor wind, rain and freezing conditions and can pre-empt a scheduled irrigation cycle. In fact, sensors can monitor all weather conditions and provide a number of secondary functions that make the irrigation controller more dynamic.

Flow meters Flow meters, installed in an irrigation system to monitor flow rates during irrigation cycles, indicate flow velocity and total water quantity used. They allow you to monitor your overall water use as well as identify circuits that are not operating at spec, perhaps due to nozzle problems.

A flow device can also be programmed to shut off the controller (much as a water-pressure sensor will) if a line break occurs. When the flow rate exceeds the intended volume by a significant margin, it's probably because of a broken pipe or sprinkler head. Shutting off the circuit until you can repair the system can save you major headaches.

Rain sensors How many times have you driven down a street on a rainy day and noticed irrigation systems operating? An irrigation controller cannot perceive changes in weather--it reacts solely to the electrical input given by the clock, which prompts it to run a preprogrammed cycle. Rain sensors aid the controller by giving it information that shuts irrigation valves down without changing the actual program of the controller.

It is best to install them where they are unobstructed by buildings and landscape amenities-usually on a building eave or fence post-and close to the controller. Hard-wired to the controller-to-valve common wire, rain sensors signal the valve to shut down when they detect a pre-determined moisture level.

Most rain-shutoff devices resemble a cup, or are a series of hygroscope (moisture-absorbing) ceramic disks separated by distances that correspond to the depth of rainfall required to stop an irrigation cycle. As rain falls, it fills the cup, or the discs absorb it. When the shutoff device senses sufficient moisture, it interrupts the signal to the remote-control valves and overrides the current program. The irrigation controller sends out electrical impulses as it would ordinarily, irrespective of rainfall, but the valve solenoid will not open. This prevents irrigation without altering the program in the controller.

Soil-moisture sensors Soil-moisture sensors provide the same sort of function as a rain shutoff, but you install them below ground at minimum and maximum rooting depths for specific landscape plantings. Soil-moisture sensors operate by matching the soil-moisture level in the upper-level sensor with a sensor set in the lowest rooting zone (see illustrations, page Contractor 52). If the moisture in the lower sensor exceeds the moisture level in the upper unit, the device interrupts the common line to the valve and shuts down the corresponding valves. Using these devices saves water-they eliminate needless irrigations when the soil already contains adequate moisture.

In a mixed landscape planting, ornamentals with various water needs or different rooting depths limit the usefulness of soil-moisture sensors. However, the sensors function beautifully in landscape plantings with one plant and soil type and a generally uniform microenvironment.

Perhaps their best use is with golf courses, parks and commercial properties. The soil-moisture sensor on a property with homogenous turf (in terms of grass type, soil and exposure) can control water applications closely enough to supply adequate water but prevent it from leaching below the rooting depth of the turf.

Imagine the benefits of such devices. By setting the controller as frequently as you need for seasonal conditions, the sensors allow for variances in water percolation due weather factors that could alter water needs. They allow you to attend to other aspects of your job without having to worry about over-watering.

Another good use for soil-moisture sensors is managing water on an irrigated slope. On a steep hillside, divide the irrigation system into upper, mid- and lower-level irrigation circuits. Program the controller to begin the irrigation cycle at the top of the slope, and then cycle to a few other circuits in other areas of the landscape. This will give any excess irrigation water applied to the top of the slope time to move down to the mid-range of the slope. The mid-level circuit of the slope is then activated; after which, again, the controller switches to other areas of the landscape to give water time to flow down the slope.

Finally, the controller signals the lower-level irrigation valve to operate-unless a soil-moisture sensor installed below grade in lower-level zones indicates that sufficient water already exists. In this way, you reduce puddling and saturated soil at the bottom of the slope.

The wind variable Wind presents a two-fold challenge in landscapes. First, it dehydrates plants and forces them to draw more water from the soil to prevent their leaves from drying out. Second, wind prevents irrigation water from hitting its targets, sometimes causing as much as 75 to 80 percent of the water to miss the intended turf or ornamentals. (That's one reason, in addition to the fact that nighttime irrigation is less disruptive to site use, why irrigation managers prefer to program irrigation cycles before sunrise-winds usually are less intense at night than during the day.)

Anemometers are instruments that measure wind velocity. You can install an anemometer on site and, as with other sensors, program it to create a break in the irrigation common wire. If the wind is gusting, the anemometer reads the gust cycles and shuts down irrigation while allowing operation when winds are calm. Be sure to mount anemometers in an open area where the wind will not be directly affected by structures.

Temperature In addition to the physical problems freezes can cause with irrigation piping and equipment, frost also can harm landscape plant stock. Frozen water on shrubs and trees can break branches and threaten the plants.

Installed in an open location (like a rain sensor), a freeze sensor can interrupt the current running to the valves to prevent operation during low temperatures.

Telemetry, computer controllers and weather stations At large facilities where sensors may be far from a central-control computer, the cost of hard wiring to link the two can be prohibitive. Telemetry solves this problem because it is based on wireless technology. The sensor's data is sent by radio signal to a central-control computer. Once the computer receives the information, it can use it to alter irrigation cycles. Computer-controlled irrigation systems are relatively sophisticated and don't need to physically create a break in the electrical circuit the way simple sensors do. The computer simply decides whether to run a circuit or not, and, perhaps, for how long.

Your system can be further refined if you supplement your data with regional weather information, such as evapotranspiration (ET) values. ET values are published by public agencies, or you can measure them on your site.

On-site weather stations are a sophisticated way to bring sensor technology to bear on your irrigation management. Weather stations are relatively expensive, but are practical for large sites such as golf courses where the water savings can be large enough to more than offset the cost of such systems. In addition, they are valuable for monitoring conditions that may lead to disease outbreaks.

Weather stations can measure all of the parameters discussed here (and more) and submit the data to a computer controller. Software that uses mathematical models that incorporate the various inputs (from sensors), as well as the needs of the plants, determines water requirements for the site. It then computes actual run times based on the calculated needs.

Even if your site cannot justify a sophisticated, computer-controlled irrigation system, look for opportunities to integrate sensors into your grounds care operations. Simple units can help your irrigation applications more closely match the needs of your plants. They also can help solve specific problems in landscape areas where irrigation is difficult to manage.

Perhaps best of all, something as simple as a rain shutoff can prevent your site from being the one that people notice being irrigated on a rainy day.

Steve McGuirk is a licensed landscape architect specializing in maintenance issues with Madrone Landscape Group (Soquel, Calif.).

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