by Tianna DuPont, WSU Extension; Troy Peters, Biological Systems Engineering; Lee Kalcsits, WSU Horticulture. Project partner Cascadia Conservation District. June 18, 2021
Irrigation sensors are important tools for fine tuning your irrigation. Kicking the dirt or doing a “the grass looks green irrigation check” is not always enough. Old pear orchards have deep roots and what is happening in the top two inches can be quite different from what is happening below two feet. Irrigation sensors help you make more informed irrigation decisions based on soil moisture conditions in the root zone.
Types of sensors
Soil Water Tension
Soil water tension is how hard a dry soil is pulling (sucking) on soil water and is measured using vacuum or pressure units such as pounds per square inch (psi), kiloPascals, or centibars (cbar).
Tensiometer: Tensiometers are an example of this type of sensor. Tensiometers are sealed plastic tubes that have a porous ceramic tip on the end (Figure 1 and 2). The tube is buried with good soil contact and then filled with water. As the soil dries, it pulls the water from that tip creating a vacuum/tension that we can measure with a tension gauge. When is the soil is full, tension will be at or near zero. Trees generally experience water stress when the probe reads 50-70 centibars.
Figure 2. The porous ceramic tip of a tensiometer (left) and one that has been broken to show the inside (right). Photo credit Troy Peters.
Granular Matrix Sensors: Since water helps conduct electricity, these sensors use the resistance in a known material surrounded by a uniformly packed media to estimate the soil water tension (Figure 3). These use similar units as tensiometers and are usually less expensive than tensiometers.
Soil Water Content
Soil water content is the actual amount of water in the soil. It is most often measured as a percentage of water by volume. cm/cm, m/m, cm3/cm3, m3/m3, in/in, or ft/ft are all equivalent measures of soil water content. These can be multiplied by 100 to show the soil water content as a %, or multiplied by 12 to get inches of water per foot of root zone soil depth (in/ft). in/ft is a convenient measurement since it can be multiplied by the root zone depth (in ft) to get the total inches of water held in the root zone.
Capacitance: Capacitance sensors measure the ability of soil to hold an electrical charge for a short period of time (Figure 4). Most capacitance probes have a positive and a negative plate with a space in between (the dielectric). The probe measures the change in the ability to conduct electricity due to an increase or decrease in the ions from soil water when it dries or becomes moist. Advantages to these probes include the ability to have multiple sensors at different depths and the ability to connect to a telemetry unit which transmits the information to your computer. A disadvantage is that bulk density and soil texture will affect readings and so proper calibration is necessary.
There are two strategies to consider when placing an irrigation sensor in the field. One strategy is to pick the most representative part of the field. With this strategy, the manager would choose a location with the average soil type/texture in the field. This would be a useful strategy for orchards trying to implement water limitations. The other strategy often used is to choose a location with the lowest water holding capacity in the field (sandiest section). In this scenario, the operator would be managing for the part of the field that will dry out the quickest and consequently the rest of the field should also receive water before it dried out. This strategy aligns with goals of maximizing fruit size and or tree growth. It is not recommended to place sensors in an area that is always water-logged. Avoid placing sensors in outside rows of trees which generally use more irrigation than trees on the interior of the orchard.
Deciding When to Water
When you install a new irrigation sensor you will need to calibrate it by setting the full and refill points.
Step 1: Determine when the soil is full (field capacity).
The first step to sensor calibration is to determine what is ‘full’ or field capacity for a given soil and soil probe. Field capacity is the maximum amount of water the soil can hold after it has drained but not begun to dry. An easy way to determine the full point is to use it to take a measurement when the soil profile is full. The soil is often full in the Pacific Northwest in the early spring as soon as the soil thaws as a result of winter snow melt and spring rains. Taking a measurement at this time will indicate the full point. If installed in the spring, completing several irrigation cycles can help designating the full point for your soil. You can also use the dynamics of the soil moisture signal itself (Figure 6) to determine where the field capacity point is as the free water will drain through the profile, but at field capacity the rate of decrease in soil water will change since the soil can hold the water at that point and the decrease is then caused by the crop water use.
Setting full point example. Using the soil moisture output from your sensor you can see that there is a peak when irrigation occurs and then the soil moisture level line drops very quickly before starting to step down slowly. The quick drop is the soil draining the free water. The line steps down slowly as the soil is able to hold the water and the soil then starts to dry. Generally, the full point is set at the transition between when the line drops very quickly and when it starts to step down.
Step 2: Determine when to irrigate (re-fill point).
The point where trees begin to experience water stress (re-fill point) can be estimated based on the previously measured field capacity (above), the soil’s available water capacity (AWC) and the recommended maximum allowable depletion (MAD). For tree fruit the MAD is generally 50% of the AWC (Figure 7).
Example determination of re-fill point. In this example field the soil moisture probe is 36 in long.
- Determine the available water holding capacity: The grower looked up the available water capacity for his soil on the web soil survey < https://websoilsurvey.sc.egov.usda.gov/> The soil at the sites was a Birch Loam with available water capacity of 0.19 cm/cm. Of course, cm/cm is equivalent to in/in. So,
- Determine field capacity: Field capacity is determined based on the soil moisture measured when the soil has been fully irrigated and had time for the excess water to drain through but not dried. At this site when the soil was full there was approximately 12 in of total water in the profile in the top 36 in.
- Determine the wilting point: The wilting point is the point where none of the water left in the soil is available to plants.
- Determine the refill point:
Note that to convert from a measurement given as a % soil moisture to inches of water you simply multiply by the soil depth that the measurement represents. For example, if there are three soil moisture sensors at 6, 18, and 30 inches deep and the % soil water measurements are 25%, 30%, and 35% respectively, then each measurement is multiplied by the 12 inches of soil that it represents (25% water x 12 inches = 3.0 inches of water in the top foot of soil, 30% water x 12 inches = 3.6 inches of water in the middle foot of soil, and 35% of water x 12 inches = 4.2 inches of water in the bottom foot of soil. These would then be added to get the total amount of water in the soil profile: 3.0 + 3.6 + 4.2 = 10.8 inches of water in the 36-inch root zone.
If you do not know what the soil’s available water capacity (AWC) is, then a rough estimate of the refill point is about 75-80% of the full point (field capacity). In the above example it would yield a starting refill point at: 75% of 12 inches of water at field capacity is 9.0 inches of total water at the refill point. Field capacity and refill points can be revised and refined based on experience and observations in the field over time.
Example Sensor Outputs
Many modern sensors are connected to telemetry which transmits the data from the sensor in the field (Figure 8) to where it can be accessed on a computer or app (Figures 9, 10, and 11). After calibration, most software gives users a graph which shows the soil moisture level compared to the full and re-fill points set by the user both on average and at multiple sensor depths.
How soil moisture sensors can help
In pears we often irrigate in standard twelve or twenty-four sets once per week. But year-to-year and over the course of the season our irrigation needs vary greatly.
In a case study conducted in 2019 we followed soil moisture and fruit quality in two adjoining blocks where the grower irrigated using weekly sets compared to when he looked at the output from his irrigation sensors. Using outputs from the irrigation sensors the grower was able to maintain soil moisture between the full and refill points in the area of the block designated as experimental. In comparison, the grower standard of irrigating weekly using 12 or 24 hr sets kept soil overly moist with moisture above the full point (field capacity) (Figure 12). In the section of the field where irrigation sensors were used to determine watering frequency (Figure 13) the amount of cork was reduced resulting in higher packouts (79% vs 70%) and 3,660 lbs fewer culls due to cork.
|Figure 12. Standard irrigation in weekly sets resulted in over saturated soil above the full point.||Figure 13. Irrigation following the irrigation sensors kept soil moisture between full and refill points.|
Trouble shooting sensors
When soil moisture sensors are giving no reading or a reading that does not make sense there are multiple common problems to check:
- Is the battery dead?
- Is the station still on the post?
- Is the solar panel charging the battery extremely dirty (e.g. covered in kaolin clay) and consequently no longer providing power?
- Is the cable cut? Occasionally cables too close to trees to be pruned etc will end up with cut cables.
- Are the sprinklers close to the irrigation probe working properly? If sprinklers are clogged, not rotating properly or blocked by weeds there may be a dry spot close to the sensor which does not represent the rest of the field (Figure 14).
- Is the probe installed in an area which receives full overlap coverage from the sprinklers in the field? If a probe is installed behind a tree or in another area which does not receive full coverage the reading will not be representative.
- Did you just cultivate? Cultivation too close to the probe can disturb the soil leaving air spaces which change the reading.
- Have you waited for the system to update? Depending on your settings the system may update only hourly or less often. Check the sensor output date/time. If you are putting on water now and no change is showing in the ap it may just need time to update.
- Has foliage (fruit/ leaves) grown up blocking the signal since the probe was installed?
Practical Use of Soil Moisture Sensors and Their Data for Irrigation Scheduling Troy Peters, Kefyalew Desta, Leigh Nelson, 2013. Washington State University. FS083E https://s3-us-west-2.amazonaws.com/treefruit.wsu.edu/wp-content/uploads/2017/12/FS083E.pdf
Using Irrigation Sensors (Video with Troy Peters) http://treefruit.wsu.edu/videos/using-irrigation-sensors-troy-peters/ Part of a virtual field day in 2020.
Irrigation Sensors with Jac LeRoux (Video) http://treefruit.wsu.edu/videos/irrigation-sensors-with-jac-leroux-improving-irrigation-efficiency-in-pears-virtual-field-day/ Part of a virtual field day in 2020.
Cost Share Availability (Video with Cascadia Conservation District) http://treefruit.wsu.edu/videos/cost-share-availability-irrigation-efficiencies-virtual-field-day/ Part of a virtual field day in 2020.
This article is part of a project funded by the Fresh Pear Committee and the Bonneville Environmental Foundation with in kind support from Wilbur Ellis, Sentek, and Irrigation Technology Control. Thank you to project cooperators Cascadia Conservation District, Larry and Renee Caudle, Erica Bland, Brandon Long, Aaron Hargrove, Phil Guthrie, and Bob Gix.
WSU Extension Specialist, Tree Fruit
Office: (509) 293-8758
Mobile: (509) 713-5346
Biological Systems Engineering
Washington State University
Endowed Chair Tree Fruit Physiology
Washington State University
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