WSU Biological Systems Engineering’s Troy Peters describes common types of irrigation sensors and how to use them. Part of ‘Virtual Fielday’ May 2020. Virtual Field Day hosted by Tianna DuPont, WSU Extension, Troy Peters, WSU Biological Systems Engineering, Lee Kalcsits, WSU Horticulture. Project funders and supporters include the Fresh Pear Committee, WSU Extension, Bonneville Environmental Foundation, Cascadia Conservation District, S&W Irrigation, Wilbur Ellis.
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| (Music plays in background) Thank you very much Troy for joining us in our virtual field day. You’re going to tell us a little bit about irrigation sensors. What’s the major thing that irrigation sensors can tell us? | Tianna and Troy stand facing each other in a fruit orchard. Text on screen reads “Tianna DuPont, WSU Extension Tree Fruit” and “Troy Peters, WSU Irrigation Specialist” |
| Irrigation sensors tell us one of two things, one is how much water is in the soil, or what the soil water tension is. With that measurement we can decide when to irrigate and how much water to apply. | |
| So, you told me before there’s kind of two major classes of sensors, right? So, what are those two types and what’s different about them? | |
| There are two different categories that we kind of group sensors in. There’s soil water tension, which measures how hard the soil is pulling on the water; it’s actually a vacuum that we’re measuring. And then also soil water content, what the volume of water is in the soil. | Cut to a closer shot of Troy answering the question. |
| And so, there’s two different types. Let me talk about soil water tension sensors first. So, the main way that most people might be used to looking at soil water tension sensors is a tensiometer. A tensiometer is basically just a plastic tube that has a porous ceramic tip on the end of it. This one is actually broken, and you can see that it goes up into this tube. So we fill this with water, then we seal it off tight, then we bury this tip in contact with the soil. And then as the soil dries out it pulls the water out of there and it pulls a vacuum inside of there. And there’s a tension that we can measure directly with a tension gauge. It’s a vacuum gauge basically, and so we usually use centibars. A bar is also a measurement of pressure; it’s like 15 psi per bar. So, 1/100th of a bar, you can measure that tension in there. And then we use that to irrigate. So, when it’s full that soil water tension is going to be at or near zero. So, field capacity when it’s full is going to be close to zero. When it’s begun to see water stress is going to be 50 or 60 or 70 or something like that. We use that to do soil water tension. | Troy reaches down to pick up a tensiometer off of the ground. He gestures to the clear tubing, the broken ceramic tip, and the tension gauge. |
| The other class of sensors measures soil water content. And there are many more of these types of sensors and there are different technologies that are used. One is called capacitance; it basically measures the soil’s ability to hold an electric charge for a short period of time. So, this is buried in the soil and it uses and electric signal to measure what that soil’s dielectric constant is across that soil which we can then correlate to soil water content. | Troy reaches down to pick up a different sensor off of the ground, this one is a long metal tube rather than clear, and has an electrical wire coming out of the top of the sensor. |
| But this is good because we can also bury it in the soil and we can plug it into a datalogger and it can report to us and we can have the telemetry connect to it so that we’re looking at it on the internet. | He gestures to the disconnected electrical wire. |
| This is called a TDR, Time Domain Reflectometry, it’s a different technology. It’s measuring the response of a signal that goes down these two different wires. This is a little bit better in that it measures a larger soil volume. But again, we would dig a hole and install this very carefully, according to the manufacturer’s recommendations, and then plug this into a datalogger and then use that to measure the soil water content. Soil water content is measured in the volume of water in the soil. So it’s usually cubic centimeter by cubic centimeter or inches of water per foot of soil water depth. | Troy reaches down to pick up a different sensor off of the ground, this one has two short metal prongs and is connected to an electrical wire. |
| So, we’ve got a great big field, obviously we’re going to have a limited number of these sensors how’s a grower going to decide where to put them to be the most representative? | Camera changes to wide shot of Troy and Tianna standing in the orchard. |
| Yeah, good question. So, there’s two philosophies and it kind of depends on what you’re trying to do. One is to pick a soil that is representative of the entire field and then manage the entire field for that average soil, for like an average soil condition. The other one is to pick your soil with the lowest water holding capacity, the soil that’s sandiest or rockiest, the location wherever you see the water stress first. Because if I have small water holding capacity because it’s sandy or rocky or something it’s going to run out of water soon, and if I want maximum yields across my whole orchard then I might want to put it there so that I apply water so that is never stressed, but also I don’t apply too much water so that I’m leaching water in that soil. And then if I manage for that, so with a really small water holding capacity, then the rest of the field is going to be fine. One thing we don’t want to do is put it in an area that’s always waterlogged because there’s poor drainage there. Also, not right at the edge. You want to come in a little bit from the edge of the orchard because the trees on the edge of the orchard actually use a little bit more water, they dry out a little sooner than the trees on the inside so it’s just not as representative of the whole field. | |
| That makes a lot of sense. So, we’ve got sensors in a field and it’s telling us some numbers how do we decide what those numbers mean to us? When do we actually trigger irrigation? | |
| So, we get these soil water sensors and then they give us a signal; they give us this line that comes back. So, with soil water tension it goes low when you irrigate and then it gets higher as it gets water stressed. And the soil water content sensor, more water in the soil increases that number and then it gets drier as it comes down. So, what we want to do with those numbers, in order to make them meaningful to us, we need to know when the soil is full and when are the trees going to begin to experience water stress. | Cut to a closer shot of Troy answering the question. He gestures up and down with his hands to show what the output graphs look like for each type of sensor. |
| So, here’s my little demonstration. I have my soil and then I’m irrigating and I’m applying the water to the soil. So obviously soil has the ability, like a sponge, to hold water and store it for use later by the trees. But at some point, adding additional water even though it makes us feel good because we’re irrigating, and it goes into the soil we see it go in, but the soil can only hold so much water. So, the maximum amount of water that soil can hold long term against that pull of gravity is called field capacity. That is a very important number that we need to know for irrigation water management. Why? Because that’s the top, we don’t want to put more water in the soil than it can hold because what happens is we’ve wasted our time, we’ve wasted our money, we probably also rinsed a lot of those soluble nutrients out of the soil. We lost that water and we lost those good things that our trees need. So, field capacity and then the other point that we need is know when it’s going to begin experiencing water stress. | Troy reaches down to grab a container of water which represents the irrigation and a large sponge which represents the soil. He pours the water onto the sponge which soaks up the water. As more water is added, the bottom of the sponge begins to drip water out. |
| So, when my soil is at field capacity, just like when I squeeze a sponge, lots of water comes out very easily. When it’s at or near field capacity, the trees don’t have to work very hard at all to get that water away from the soil. However, at some point the water is so depleted that it takes a long time for the water to get to that soil root hair and the root’s got to pull really hard to get that water away from the soil. Remember soil water tension. It’s pulling on that water; the tree has to pull that water away from the soil. So, it begins to be a point where the trees start to shut down and starts to begin experiencing water stress. | Troy begins to squeeze the sponge between his hands, and a large amount of water flows out of it. |
| Any other indication of water stress may be the leaves start curling or something like that. Then we draw that line on that graph and we say “ok, we’re not going there in future”. Another good rough estimate for a lot of soils is you take your field capacity line, you take about 75 or 80 percent of that number and that’s going to be a first guess as to where to draw that line. It doesn’t really hold that well for sandy soils, it’s more like 50 or 60 percent of that number where you’re going to begin to see water stress. | Camera changes to wide shot of Troy and Tianna standing in the orchard. |
| But still, we want those two numbers, Field capacity, first water stress, and then we want to stay between those two lines. Anywhere between those two lines the trees are going to be fine. And that’s how we use that to do water management. | He gestures with his hands to indicated that field capacity is the top line on the graph, water stress is the bottom line on the graph, and the ideal condition for the orchard is to stay between those two lines. |
| Perfect. Well I think it’s super useful for folks to have sensors in their blocks and we’re going to talk a little bit more about how that can impact folks in different blocks. Thanks so much for explaining what they are and how we can use them. | |
| Music plays | Credit slide containing information on collaborators, funding, authors, and videography. |
