Soil moisture sensors are coming of age

Lorne Haveruk
June 08, 2011
By Lorne Haveruk
How do you determine what is the best method for controlling irrigation? SMART or MOISTURE? Eliminate the human factor and you will be controlled by science. Science utilizing devices that measure weather, soil tension and/or soil conductivity levels to allow or suspend an automatic irrigation cycle from occurring.
After all, shouldn’t the plants that are utilizing the water be the ones to determine when water is added to the root zone?

How much water is enough

If, and this is a big IF…you are going to calculate the actual water requirements based on system design, quality of installation, and level of maintenance…on a weekly basis, you will need to know the following.lorneweb

Volume of water required per unit time (week, month, year), per unit area to sustain turf.

• 27,154 US gallons of water are required to cover 1 acre with 1 inch of water.

• 1 acre = 43,560 square feet.

• The  irrigated vegetation area = 100,834 square feet or 2.31acres.

• 2.31 acres x 27,154 US gallons = 62,725 US gallons per week is required to irrigate the turf areas to 2.5 centimetres per week.

Note:  The volume of irrigation water required to be applied will vary considerably depending upon the amount of water (moisture) currently present in the root zone soil area (known as available water [AW]). This will vary depending upon the time of year that irrigation is to commence, as evapotranspiration (ET) changes daily throughout the year, and if the irrigation begins prior to or directly after a substantial summer rain event.
Not many folks will bother to work out the actual water use calculations for each irrigated site they manage. Why? Even though the cost of delivered water to a site has gone up 200 per cent or more and is going up every year for who knows how long, the cost is still lower than the two-year payback…more like 10 years…so it is just ignored.

Available Water (AW)

The above noted information is excellent and sets the base point for how much water is enough and when the glass is full and when the glass is empty.

These points that I am referring to are known as field capacity (top of glass minus saturation so about 80 per cent full soil root zone (SRZ) and permanent wilting point (PWP) about 20 per cent of water left in the root zone, but soil tension is too high for plants to withdraw water from the soil.

This is a great segue into soil moisture sensors as they work on tension or conductivity of electrical current through moist soil.  Depending upon the manufacturer of the soil moisture sensing equipment will determine how the equipment will operate.

Soil moisture sensors

“One of the problems with ‘automatic’ irrigation systems is the tendency of the user to ‘set it and forget it.’ This can result in a huge amount of water waste due to the system operating when water is not needed by the turf or plants. The concept of using soil moisture sensors to control an automatic irrigation system was pioneered by Irrometer Company in the 1960s. The instruments, when wired into the control/valve wiring of these automatic systems, act like ‘thermostats.’

“Constantly measuring the soil moisture status, and allowing the irrigation system timer to open the valves as programmed only after the soil moisture has been allowed to deplete to the level where irrigation is actually needed. These instruments are adjustable for the desired moisture level, so you can ‘fine-tune’ the settings for individual needs and results. They are primarily used by the larger commercial, industrial, and recreational area irrigators, not so much by homeowners.

“Their effective use usually requires a higher level of irrigation management which is necessary to adjust, fine-tune, maintain, and manage the instruments themselves as well as the overall system efficiency. Many irrigation management consultant firms are now getting involved in this key area of landscape management.” 1

Green roof & green wall soil moisture sensor pilot project

Currently, a pilot project is underway at the University of Toronto to try to establish species of plants, soil media, and irrigation method plus quantity of delivered water required for the survival of the green roof or green wall plants. Why Toronto? The City of Toronto mandated that commercial and government buildings must have a percentage of the roof as a living green roof helping to control stormwater runoff. Why greenroof? To absorb rainfall as it occurs and slowly release excessive rain to help control creek, stream, and infrastructure piping systems that overflow water into the Great Lakes…our source of drinking water for many that live near the lake. 

Plus, help reduce the heat island effect that building roofs have on the city during the warm summer months. This can be as much as a five-to-seven-degree Celsius gain which increase energy use for air conditioning. Reduce the heat and we help the environment by not requiring excess energy production currently produced by use of water (hydro), coal (adds to greenhouse effect) and nuclear (Pickering power generation).

Professor Robert M. Wright, associate dean, research, Daniels Faculty of Architecture, Landscape, and Design;  and  assistant professor Liat Margolis, John H. Daniels Faculty of Architecture, Landscape & Design, University of Toronto, provide the desired outcomes for the pilot project as noted herein: “We are aiming to measure the quantity of water relative to different substrate types and planting compositions in order to assess water conservation, water retention, water runoff, evaporative cooling, and plant growth. These are the important factors for us in terms of irrigation:

“1. compare timed irrigation vs. soil moisture sensor irrigation vs. no irrigation

“2. compare irrigation quantities per bed

“How is the irrigation monitored in terms of software and controllers? Could we collect and compare the sensor data from each bed and compare to water quantities? We will have soil moisture sensors in the soil as well as a weather station to acquire climate data in real time. That means that we are aiming to compare climate to irrigation relative to each bed’s specific composition.”

Manufacturers have a soil moisture sensing system they have either created or have purchased from other vendors. The issue is what system will work with the soil you are testing, as we are discovering that the soil that many green roof applications are receiving do not allow water to travel due to particle size being closer to the size of a salt crystal rather than a combination of soil similar to a loam mixture (sand, silt and clay.) Will the water travel throughout the soil or just pass through to drainage to the green roof drain system, as some of the manufactured and modified “light” soils allow? In this case many of the soil moisture sensing devices will not work. Why, you might be asking. Let’s ask an expert that makes the products.

Irrometer and soil moisture

Tom Penning of Irrometer Co., Inc. is providing information on the Irrometer Tensiometer and the Watermark soil moisture sensor. These devices have been around and in use since the 1920s even though many others have their own iteration of what they deem works as per Mother Nature. The proof is always in the plant, the farmers say. Each system performs a little different from the other, so the task is to choose the best system to deliver the outcome you require. 

Sounds easy, but as the pilot project for the College Street Green Roof and Green Wall test slowly advanced, it became apparent that it would involve many experts. A volunteer team slowly materialized that consisted of Richard Buist of Bioroof Systems, a soils expert; Tom Penning, Irrometer Co, a soil moisture sensor manufacturing expert;  Dr. Robert Wright, professor of landscape architecture; Liat Margolis, assistant professor of landscape architecture; Richard Laffin, Campbell Scientific ET (evapotranspiration) weather station company and data logger; Kara Gibbons Toro Canada, irrigation product for system creation; Brent Sleep, professor of civil engineering; and Maxim Batourine of Daniels Faculty IT; and of course, myself.

What did Tom need to know to better understand what was to be achieved by this 32-plot, three-greenwall, and one living wall in downtown Toronto (a very busy traffic area of the city) greenroof experiment? He asked, “Can you also give me some idea of the design, such as type and depth of the soil or media being used?”

Tom continued to say “using Watermarks [soil moisture sensing system]; the medium will have to be soil. If an artificial medium, such as a greenhouse non-soil growing media, is going to be used or experimented with, we will need to switch to a (Irrometer) tensiometer-controlled system using automatic switching versions of our Irrometer Model LT. Our Watermark devices act as an inhibitor only. The controller initiates each event and then the sensor decides whether to allow it or not. Essentially, you program the controller for maximum consumptive use, broken up into as many start times as possible. Then the sensor either allows or interrupts each start time, thereby limiting the irrigation delivered in segments of the run time interval, i.e. if dry at the start time then that whole run time will be delivered. This can be controlled for each valve, groups of valves or the entire controller. Will this suffice for your project?

 If you want actual soil moisture activated on and off, then we will have to use switching tensiometers and you will need a programmable logic controller to turn on with the dry switch of one instrument and turn off with the wet switch of another one. Typically, when used in conjunction with an irrigation controller, these simply interrupt the common, like the Watermark devices, although they will turn off mid-cycle. Often they are used without controllers and simply switch the circuit between a 24 VAC transformer and the solenoid, but this does not allow much differential in moisture level, but is good at keeping things close to a fixed value... like at field capacity for drip irrigated vegetables. He continued, “If you are going to be using Campbell (Scientific) gear to log everything, you can just connect our sensors, either Watermarks and/or Irrometers to the Campbell (Scientific).

Richard Laffin, sales and technical support/ventes et support technique, Campbell Scientific (Canada) Corp., noted that “the individual sensors should be fine to connect to the logger, but do not know if the control system will play well with the Campbell logger since the way they make measurements of the sensor are different. It probably won’t be a problem, but I think we’d need to hear that from the Watermark folks. Tom clarified the controlling issues and how the two systems could come together and operate as one by saying to “use Campbell’s logger, such as a CR1000, for both monitoring and controlling. I believe their system can initiate outputs based on sensor inputs, thus acting as a controller. So by itself (plus our sensors) it could provide the soil moisture activated irrigation as well as the monitoring.”

Light soils for green roofs

Due to weight of soil on roof structures, engineered “light” soils are currently the norm when building a green roof landscape area. Weight needs to be managed not only for dry but also for saturated soil as the water-holding capabilities of soil changes best described by the soil triangle and what comprises a natural mineral-based soil. Look at the soil triangle at the bottom where sand is located. Read this from right to left from zero to 100 per cent. On one end we have very little sand, 10 per cent so we would have to a lot of silt, 90 per cent and a silty soil. Sand allows water to infiltrate but does not hold water, thus the reason to amend sandy soils with silt, clay, organics or inorganic such as foam particles that can absorb and hold water. Clay on the other side does not allow water to infiltrate as the particles are so small and millions make up a small portion of soil.

Infiltration rate is around 6.35 millimetres per hour which, for irrigation, is manageable if care is taken to application rates of water. Typical subsurface landscape dripline like the Toro DL2000 specified for this pilot project have an inline emitter flow rate of 0.063 litres per minute. Long station run times exceeding 20 minutes will end up causing runoff as precipitation rate exceeds infiltration rate. Loam…40 per cent sand, 40 per cent silt, 20 per cent clay…this is our friend. Light, fluffy, high infiltration and holding capacity plus gives up its water easily. Plants love loam and loam also provides nutrients so a win-win for the plants and the irrigator.

Who else makes soil moisture sensing systems? Toro, Rain Bird, Acclima, Baseline are all leaders in this field with new technology that has its specific applications for their devices. These devices can be used specifically for one station watering control to say…control the watering of vegetables, which is where soil moisture sensing was born. Or, perhaps a dedicated rose garden that is to receive watering from sub-surface irrigation dripline buried in the middle of the plant’s root zone about 100 centimetres down. Be sure to add an air/vacuum relief valve so the soil does not get drawn into the emitters after watering stops. An automatic flush valve is a good idea as well. This device will expel the air so all emitters receive water at close to the same time and will flush the system ever time the water comes on. But, do not have these devices close to your soil moisture sensor or the system will shut down as it will think that all areas have enough water. You will discover this when the plants die and you don’t know why.

Where to use the soil moisture sensors

So where to use soil moisture sensors is the issue…complete landscape or specific locations? OK, green roof and green wall planters are a good location for single soil moisture sensors that specifically control the watering into these areas. Sunny side of life in a home or commercial landscape where you have annuals, perennials, native plants, drought-tolerant plants, sunny and shady areas and sand and clay areas? No! They were not meant to be the be all and end all of irrigation control similar to ET that can only be as good as the weather, the data and the controlling system that controls when the watering is to occur.  Yes, you would need about six or more soil moisture sensors connected to irrigation 24VAC electric solenoid valves that the irrigation consultant/designer took the time to ensure each of these above noted hydrozones (different exposures, soils, plants) were actually designed to be watered as separate stations. Even if this was the design intent, did the installer follow the plan or leave it in the truck and he did it my way (his way)? Soil moisture sensing technology can be the supreme method for matching plant water needs. Is the industry ready?

Credit

Lorne Haveruk, CID, CWCM-L, CIC, CGIA, CLIA, WCP, Principal, DH Water Management, a water resource management consultancy firm focused on innovative rain, ground, storm, grey and  city water supplied indoor and outdoor watering solutions. Visit  http://www.dhwatermgmt.com or email  This e-mail address is being protected from spambots. You need JavaScript enabled to view it for info. This material is for information purposes and is not intended to provide legal advice.

References

1 Irrometer Co, Inc., Bioroof, University of Toronto, Campbell Scientific (Canada)

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