Integrated Irrigation Control: From Clocks to CPUs

December 9, 2011 - 10:31

The first electro-mechanical time clocks and electric watering valves made it possible to water at times when a grower wasn’t handy to turn on a valve. As irrigation time clocks started to incorporate solid-state electronics, it became possible to add more sophisticated programming such as multiple zones, multi-day schedules, and several watering starts per day. Modern computer controls have since integrated the control of large numbers of remote watering valves with other aspects of greenhouse climate control. They have created opportunities for precision irrigation control that were not otherwise possible.

You can think of an irrigation time clock as the most basic form of ‘anticipatory’ control. You anticipate when the crop is going to need watering and you set the time clock accordingly. Chances are your guess will be correct on some days but not on others. This is because plants don’t use water at regular rates. Changes in temperature, humidity and solar radiation have a strong influence on water demand so you need to change the timing of watering quite frequently. If your crop is exposed to rainfall this can also complicate matters.

Computerized controls have extended the function of time clocks by better matching the watering frequency and volume to the actual needs of the crop. This type of advanced decision-making often uses models that measure one or more of the indirect factors that contribute to evapotranspiration as well as direct feedback from sensors.

Sensor Feedback

Instead of trying to guess when water is going to be needed, why not just use a soil moisture sensor to ‘read’ the answer? A properly located and calibrated moisture sensor can provide a better indication of the current water status than a time clock since it measures the end result of all of the factors that influence water use. Not only can it tell you when it is time to water, you can use the sensor to confirm that water has actually reached the target after an irrigation event. Moisture sensors might seem like the ultimate answer for accurate irrigation, but here’s where things can start to break down with a watering strategy that relies solely on feedback control:

1. A moisture sensor can only sample a tiny fraction of the entire irrigated area. Therefore it is only accurate if it truly represents the average conditions in the zone. Consequently, great care must be taken when selecting a sampling point and positioning the sensor.

2. Moisture sensors need to be out in the crop. This makes them a target for all sorts of mishaps. They can be easily damaged, disconnected or dislodged resulting in faulty readings.

3. Unless the feedback sensor is connected to something intelligent (such as a greenhouse control computer) it may fail dangerously turning your greenhouse into a lake or a desert.

4. At least one moisture sensor is usually needed for each irrigated zone. More sensors can aid in redundancy but this can get quite expensive if there are a lot of zones. Also, there is the problem of which sensor to believe when the readings are significantly different. As the old saying goes, “The man with one watch knows what time it is. The man with two is never sure!”

There are a number of other reasons to irrigate besides replacing water lost from evaporation and transpiration. These include:

  • Delivering fertilizers to the crop
  • Flushing salt build-up from the soil
  • Cooling the soil surface to prevent high temperature damage to tender crops (seedlings)
  • Humidifying the air to reduce crop wilting and stress
  • Frost protection for outdoor crops
  • Bringing a crop to a uniform moisture level before packing and shipping

Neither feedback sensor nor time clocks can contend with all of these conditions.

Computerized Irrigation Controls

Integrated irrigation control combines the best features of time clocks and feedback sensors and adds a host of additional capabilities for precision irrigation management. Computers make it possible to more accurately match the needs of your crop while safeguarding against the many things that can potentially go wrong with your irrigation system. They can warn you if a pump has run dry or if a pipe has broken. This is accomplished by using a strategic combination of system monitoring sensors and advanced watering decision logic. You can use any combination of time, sensor feedback, and sophisticated anticipatory models such as light accumulation or evapotranspiration. An added benefit is that all irrigation activities are logged and recorded so you have a complete record of what is being watered and when. Advanced systems can also be accessed from remote locations for reviewing irrigation events and adjusting settings.

With integrated irrigation control you have all of the resources of the control system at your disposal for implementing watering strategies. This can include information from the weather station as well as zone temperatures, humidity, and light levels. It is possible to create highly dynamic watering decisions that are as sensitive to changes in the environment as are the plants. Integrated control provides many additional capabilities and benefits. Here are some of them:

Automated capacity management. This is like ‘air traffic control’ for your irrigation system. With capacity management, any zone can independently request water at any time. You do not need to worry about how many zones can be watered at once. The control system manages all watering execution to ensure optimum system pressures and flow rates are maintained at all times. If too many zones request water at the same time the system efficiently queues them and services each in turn as capacity becomes available.

Intelligent feedback control. Feedback information from moisture sensors, scales, etc. can safely be used as the basis for watering because the control system can help protect your crop and irrigation system from some of their vulnerabilities. This includes automatic failure detection, maximum and minimum application limits, and comprehensive alarm monitoring when problems are detected.

Watering confirmation. Often the best use for a moisture sensor is to confirm that water has actually reached its intended destination. Feedback sensors, and in some cases remote imaging, can be used to confirm that watering applications have actually occurred.

Anticipatory control. Just because plants use water at variable rates doesn’t mean they aren’t predictable. By accumulating and evaluating the factors that influence plant water demand it is possible to create dynamic watering schedules that are very accurate and are much less susceptible to failures than strategies that rely solely on point source feedback sensors.

Nutrient control integration and multiple feed selection. Computerized control makes it possible to select from a variety of feeds available on your system. For example you might want to use a standard feed recipe by default, but switch to plain water or a half-strength feed at times of high demand. Alternatively, you could specify 20-10-20 for four out of every five waterings, and calcium nitrate every fifth watering.

Water temperature control. Automatic controls for equipment used to temper water for cold sensitive crops.

Variable vol-ume application. In some situations you may want to maintain fixed watering frequencies while applying more or less volume depending on the current conditions.

Pulsed waterings. To prevent excessive runoff and achieve more uniform wetting of dry soils you can specify the total volume to be applied as a series of pulses, allowing for sufficient soak-in time in between.

Measurement of runoff volume. This information can be used for management purposes and as the basis for manual or fully automated adjustments to watering volumes, frequencies, and chemical strength.

EC and pH management. Sensors located in either the media or sampling the leachate can be used to modify watering volumes, fertilizer concentrations, and pH.

Recapture, treatment, bulk storage and recycling. With computerized irrigation control you can manage all of the peripheral collection, distribution, and treatment equipment needed for efficient recycling.

About The Author

Alec Mackenzie is director of research and development for Argus Control Systems Ltd. and a director of the Education Committee of the National Greenhouse Manufacturers Association. He can be reached at alec8@shaw.ca. David Flood is a greenhouse horticulture consultant, Flood Technical Services. He can be reached at dflood@twincomm.ca.

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