Good Measure: Landscape Moisture

Monitoring the landscape’s moisture is a key element to any successful irrigation system.

When it rains, it pours. But when it doesn't rain, an irrigation system is a contractor's answer to regulating landscape moisture. A key to having an efficient irrigation system is being able to monitor the landscape moisture levels in order to keep plant material in perfect health.

"It is important to monitor landscape moisture to determine if you need to make adjustments in your irrigation system," explained Takeshi Yamamoto, irrigation division manager at Del Conte's Landscaping, Fremont, Calif.

Ken Peshka, service manager at Cascade Lawn Sprinkling, Ada, Mich., explained the biggest importance to monitoring soil moisture is that grass is a cool-season plant and it will go dormant during dry spells.

"Grass needs moisture or it will shut itself down," he noted.

Both contractors and manufacturers defined the importance of frequent moisture monitoring along with how to make an adjustment, when needed, in the irrigation system.

METHOD FOR MEASURING. Contractors have many ways of keeping track of the moisture in a landscape. Some prefer the soil probe method while others opt for high-tech solutions.

"We use conventional soil probes," remarked Yamamoto. "This method is mostly through visual observation. The probe pulls a ½-inch diameter of soil out of the ground so the contractor can see how far the moisture has gone through the soil. It's a more subjective method. It's one of those things where you just know what you are looking for based on past experience."

David Wickham, president of David Wickham & Associates, Lake Mary, Fla., believes that using an irrometer is the best way to measure moisture.

"Using an irrometer is one of the better ways to measure and it also is the best known," Wickham commented. "It's beneficial if you have a large irrigation area like a commercial park or even a golf course. In this case, it's good to have one irrometer for each irrigation controller."

According to the Weather Systems Company, Santa Clara, Calif., an irrometer operates on the tensiometer principle, which is different from systems that measure percent moisture. The irrometer consists of a sealed, water-filled tube equipped with a special vacuum gauge. A porous tip is installed in the ground at desired root-zone depths. In dry soil, water is drawn out of the instrument, reducing the water volume in the instrument and creating a partial vacuum, which is registered on the gauge. The drier the soil, the higher the reading.

Today's Technology

    A common way of monitoring the moisture in any landscape is simply observing the plant material with the naked eye. However, this method requires years of experience and knowledge in the green industry. According to Environmental Sensors, Escondido, Calif., there is now a more scientific way to check the landscape's moisture level. Based on a technology known as Time Domain Transmissometry, Gro-Point is a portable sensor that offers an immediate response to changes in soil moisture. TDT technology measures the time that an electromagnetic wave travels along a given length of transmission line in the soil. This wave signal travels at different rates in wet soil and dry soil, then it is measured at the other end of the line from the transmitter. This wave will measure differences in the soil moisture up to 3 inches from the rods along all sides. The sensor can simply be installed in the landscape's root zone, and the percentage of soil moisture will appear on the display. A similar technology, Time Domain Reflectometry, is the underlying technology that provides moisture profiling capabilities in a broad range of soils. TDR technology also measures the time a fast pulse edge takes to travel along a section of transmission line, however, TDR is based on a reflected wave and the time measure in a two-way or round-trip travel time. – Angela Dyer

Using a different method, Peshka puts his customers in charge of watching their own landscapes. He explained that when the grass is too dry, it gets a purple haze and the customers know to watch for this.

"I have customers in charge of their own irrigation systems," Peshka noted. "I set up the timings on each zone and as the landscape becomes drier, the length of the application needs to increase."

He added that some of the probes on the market can cause an added expense to the customers, which can be a big factor in how the moisture is monitored.

Another option on the market now is a moisture sensor designed to measure the water content of the soil to find the trends of where the moisture is in the soil, according to Jan Windscheffel, sales and marketing manager for agricultural products, Environmental Sensors, Escondido, Calif.

"A sensor is a way to make sure that the adequate amount of water is being supplied and that the plants are not being over-watered," said Windscheffel.

"This will work like radar in the soil," Windscheffel mentioned. "It measures the moisture in the radius of the sensor to about 2 or 3 inches. It can be integrated into an irrigation system where it will help make the decision whether or not the landscape needs irrigation."

He added that on a commercial building's irrigation system, for example, the sensor will check the moisture before the system starts to make sure the soil can hold that amount of moisture.

FREQUENCY FACTOR. No matter what method a contractor uses, following through on a monitoring schedule is important to the health of the plant material.

When the contractor is monitoring on a regular basis, it is often necessary to use a more exact method than just looking at plant material to determine if the soil will require moisture.

"Contractors have to measure the moisture in the soil," Wickham suggested. "It takes a lot of experience to look at the actual plant material and determine the moisture content."

Yamamoto explained that to some degree a contractor can check the moisture level by looking at the plant, but it isn't a tell-all method.

"Contractors could easily be severely overwatering and the plants would still look good for quite awhile," Yamamoto said. "This means that they may not even realize the damage being caused, such as root damage. This method is acceptable for the short term, but contractors have to measure soil for long-term monitoring. Contractors often over-water and it isn't obvious right away. This is unlike underwatering, which is apparent by looking at plants if there is too little moisture."

"Our account managers monitor on a regular basis - about twice per month," Yamamoto remarked. "Then we get feedback so we can adjust the controller."

According to Windscheffel, if a sensor is connected to an irrigation controller, then it can be set to check the moisture level twice each day, if necessary.

Monitoring should be an ongoing process with irrigation, according to Peshka.

"During rain spells, customers may need to shut the system down," Peshka added. "We have to tell customers what their soil type is because this will be a factor in how much water the landscape needs."

Percentage Perception

    According to Environmental Sensors, Escondido, Calif., moisture readings must be interpreted for site-specific conditions. The following percentages are the actual figures the moisture sensor reads. When checking levels, the field capacity percent is the maximum amount of water a soil can hold. The permanent wilting point is the level where the plant will die from a lack of moisture. Available moisture content is the field capacity percent minus the permanent wilting point.

    Sand 10 4 6
    Loamy Sand 16 7 9
    Sandy Loam 21 9 12
    Loam 27 12 15
    Silt Loam 30 15 15
    Silty Clay Loam 36 20 16
    Clay Loam 32 18 14
    Sandy Clay Loam 29 18 11
    Sandy Clay 28 15 13
    Silty Clay 40 20 20
    Clay 40 22 18

    Source: Blaine Hanson, Ph.D., Extension Irrigation and Drainage Specialist at the University of California at Davis

AN ADJUSTMENT PHASE. When contractors are out on the properties taking measurements and checking plant material, it is important to know the range that soil moisture levels can fluctuate before a change needs to be made.

"If you rate the levels of moisture as 'bone dry' being a one and 'super saturated' being a 10," Peshka noted, "the moisture level should be right in the middle, around five, but it could drop to even a three or four without being too damaging."

Windscheffel recommended placing a sensor in an area that has the highest moisture demand and tying the application of water to that area's need. This will keep any plants from being underwatered.

"The range of soil moisture depends on the soil type," Windscheffel mentioned. "A general range is about ¾ to 2 inches of water per foot of soil. However, in the top 6 inches of soil, some types can hold more than an inch of water each day and some as little as ¼ inch.

"With some sensors, contractors measure the volumetric water and soil," Windscheffel added. "On a typical loam soil, you need to maintain 25 to 30 percent of volumetric soil moisture. Clay soils require a higher percent and sandy soils need less to maintain optimum growth." (see sidebar, page 98)

When a contractor determines that the moisture level is not in the proper range for a certain soil type, it's time for a change. Fixing a soil problem is often as easy as changing the controller program.

"Fixing a problem can be this easy, but it really depends on the specific situation," Wickham stated. "Sometimes I can simply adjust the irrometer."

Yamamoto agreed that dealing with moisture problems often a simple matter of controller adjustment.

"However, sometimes within one zone where you have a controller, there may be different watering needs in that area," he remarked. "This becomes a distribution problem because some spots may need more water while some may need less, but those areas might be in the same zone. When this happens, contractors may have to make adjustments by adding or removing sprinklers to get the proper distribution."

The author is Assistant Editor of Lawn & Landscape magazine.

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June 1999
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