Part 3: Calculating water requirements

Grow your crop steering expertise

Crop steering can optimize crop production and production costs, but to crop steer successfully, you need to do it right. You have to understand how to obtain the right soil water contents and soil electrical conductivities to either stress the crop or avoid stressing the crop in a controlled way. To do this, you’ll need to perform crop steering calculations.

Steer your way to higher quality, productivity, and profit

In part 3 of our greenhouse webinar series, Dr. Gaylon Campbell, internationally recognized soil physics and environmental measurement expert, teaches how to perform crop steering calculations that give you the information you need to stress or de-stress your crop at the right time and in the right way to achieve your goals. In this 30-minute webinar you’ll learn:

• The water balance equation
• How to calculate the irrigation amount
• How to calculate the transpiration variables that affect recharge drainage, and changes in stored water
• How to determine the field capacity of the substrate
• Environmental factors that influence the water balance
• How to determine the leaching fraction
• How to manage substrate electrical conductivity

Next steps

• See part 1 and part 2 and part 4 in this webinar series
• Access the Complete Guide to Soil Moisture
• Explore the TEROS 12 soil moisture, electrical conductivity and temperature sensor
• View past webinar: Soil Electrical Conductivity: Managing Salts for Sustained High Yields
• Request a ZENTRA Cloud live demo

Questions?

Our scientists have decades of experience helping researchers and growers measure the soil-plant-atmosphere continuum.

• Talk to an expert
• Request a quote

Presenter

Dr. Gaylon S. Campbell has been a research scientist and engineer at METER for over 20 years, following nearly 30 years on faculty at Washington State University. Dr. Campbell’s first experience with environmental measurement came in the lab of Sterling Taylor at Utah State University making water potential measurements to understand plant water status.

Dr. Campbell is one of the world’s foremost authorities on physical measurements in the soil-plant-atmosphere continuum. His book written with Dr. John Norman on Environmental Biophysics provides a critical foundation for anyone interested in understanding the physics of the natural world. Dr. Campbell has written three books, over 100 refereed journal articles and book chapters, and has several patents.

Transcript

BRAD NEWBOLD 0:12
Hello everyone, and welcome to part three of our webinar series, Irrigation of Controlled Environment Crops for Increased Quality and Yield. Today’s presentation will be about 30 minutes, followed by about 10 minutes of Q&A with our presenter, Dr. Gaylon Campbell, whom I’ll introduce in just a moment. But before we start, we’ve got a couple of housekeeping items. First, we want this webinar to be interactive. So we encourage you to submit any and all questions in the Questions pane. And we’ll be keeping track of these for the Q&A session toward the end. Second, if you want us to go back or repeat something you missed, don’t worry. We’ll be sending around a recording of the webinar via email within the next three to five business days. All right, with all of that out of the way, let’s get started.

BRAD NEWBOLD 0:54
Today we’ll hear from Dr. Gaylon Campbell, who will discuss how to calculate water requirements for crop steering. Dr. Campbell has been a research scientist and engineer at METER for over 20 years, following nearly 30 years on faculty at Washington State University. His first experience with environmental measurement came into the lab of Sterling Taylor at Utah State University, making water potential measurements to understand plant water status. Dr. Campbell is one of the world’s foremost authorities on physical measurements in the soil plant atmospheric continuum, and his book written with Dr. John Norman on environmental biophysics provides a critical foundation for anyone interested in understanding the physics of the natural world. He’s written three books, over 100 refereed journal articles and book chapters, and has several patents. So without further ado, I’ll hand it over to Gaylon to get us started.

GAYLON CAMPBELL 1:46
Thank you, Brad. Thanks for being with us today to talk about the water requirements of controlled environment crops. As Brad said, this is the third lecture in the series on this topic. And before we start talking about the calculations, I’d like to review some of the things, some of the main points from the first two lectures. The first lecture was on the hydraulic properties of the substrates that we use to grow horticultural crops. We talked about the jobs that we hire a substrate to do. Its purpose is to supply water and nutrients to the plant and oxygen to the plant root. We talked about the two quantities that described the state of water in the substrate, water content and the water potential. And we investigated the relationship between those two variables for typical horticultural substrates. We showed that even the lowest water contents typically used in these coarse textured media, no significant matric water stress could occur. So stressing the plant by reducing the water content is not something that’s in the cards. Finally, we showed how to accurately monitor water content in these media and how to use water content data to maintain the exact right growing conditions for the plant.

GAYLON CAMPBELL 3:34
The second lecture, we focused on the measurement of nutrient concentrations in substrates. We showed that we could accurately predict the concentration of nutrients by monitoring the electrical conductivity of the water in the substrate pores. But to determine that concentration, we needed accurate measurements of the bulk electric conductivity and the water content of the substrate. The root environment is dynamic and so continuous monitoring is needed to correctly manage the root environment. Besides telling us the nutrient concentration, electrical conductivity also tells us the osmotic potential at the root zone which is a measure of the water stress that the plant might be experiencing. And this is what we manipulate to give vegetative regenerative cues to the plant for crop steering. Again, careful and timely monitoring is important. And METER’s AROYA system makes that possible. Much can be accomplished by just following water content and pore water electric conductivity values over time and keeping those within set limits.

GAYLON CAMPBELL 5:04
But if we want to schedule irrigation, we need to be more quantitative, we need to know how much water is being applied. And what happens to it. A good context for the discussion of those quantities is the water budget or the water balance that I’ve shown in this slide. The water balance states that the change in water in the plant and the substrate, we’ve called it delta storage here has to equal the irrigation amount I minus the evaporation amount E, and the drainage amount D. And we’ll take a look at each of those terms in turn.

GAYLON CAMPBELL 5:52
Now, we’ll start by the irrigation amount. If we’re using drip irrigation, and the number of milliliters of water we apply per plant per day, is the product of the number of drippers we have per plant, the flow rate per gripper, and the number of minutes per day the irrigation is on. Now this might seem pretty simple, but it’s extremely important to get it right. This is the basis for calculating all the other components in the water balance.

GAYLON CAMPBELL 6:30
So let’s assume that we have two emitters per plant, that each of them is rated at 1200 milliliters per hour or 1.2 liters per hour. We’ll assume that our daily irrigation consists of six shots, each of which lasts five minutes so we irrigate for 30 minutes per day. We also assume a substrate volume of six liters no show events for in a minute are emitting their emitter rate is per hour. And so we need to convert that two per minute. So we divide the 1200 by 60 to get get 20 milliliters per minute. And then we use our formula two emitters times 20 milliliters per minute, times 30 minutes per day, you have 1200 milliliters per day or 1.2 liters per day. And that’s our irrigation rate, that I term in our equation. Now, another calculation is needed to give the volume per shot. Each shot is 1/6 of the 200 milliliters and to calculate a shot volume we use the same equation as for the total except we put in the time per shot, I’ve shown it as a small t rather than the large T here. Now to avoid channeling, we want the shot size to be small compared to the substrate volume. If we divide the 200 milliliters by 6000 milliliters the substrate volume, we see that our shot size is about 3.3%. And that’s appropriate for vegetative steering. We go higher than that when we’re doing generative steering, but we’d like to keep it below 10%. Now if your irrigation is not uniform, your crop production will not be uniform. And you’ll waste water and nutrients and production potential.

GAYLON CAMPBELL 8:44
So you need to assure that the emitters are accurate, and that the water application is uniform. And to do that, you need to check the output from a representative sample of drippers. To do that, put the drip stakes in a cup, turn the irrigation on for a set time say five minutes, measure the water in the cup and calculate the rate. The amount that you get from the different measurements you make should agree within a few percent of each other and should agree with the amount that you assume you’re applying.

GAYLON CAMPBELL 9:29
Let’s now go to the evaporation term, the E term in the equation that includes both the water that evaporates from surfaces and the water that’s transpired by the plants. In controlled environments the exposed evaporating surface is often small compared to the evaporating surface of the plant. So we’ll mainly focus on the water that’s lost by the plant. The factors that affect evaporative loss are the leaf area, the stomatal conductance, the radiation when temperature and vapor deficit. You might be surprised that the light affect the evaporative loss. But what you need to remember is that it requires 600 calories of heat to evaporate one gram of water, and the lights along with the air supply that heat to evaporate the water.

GAYLON CAMPBELL 10:32
Here’s the equation that we use to compute transpiration. We just look at the main parts of it. It’s the product of a fractional interception of light by the canopy. So the fraction of light the canopy of the incident light the canopy intercept, vapor conductance for the leaves in the boundary layer, the difference between the saturation vapor pressure at leaf temperature, and the vapor pressure of the air. We divide that by the plant density to convert from milliliters per square meter of milliliters per plant. So now if we knew the canopy temperature, we could easily compute the transpiration rate. But we don’t normally know what the canopy temperature is. It won’t be equal to the air temperature because of the radiant energy that the canopy is absorbing. So we need to compute a leaf temperature.

GAYLON CAMPBELL 11:45
And that takes an even bigger equation. I won’t talk about all of the variables in this equation. But the most important ones are the air temperature, the absorbed radiation, the vapor deficit of the air. We start with the air temperature, then we add a component that’s from the absorbed radiation and we subtract a component due to evaporative cooling. The heat and vapor conductances are a part of the calculation but they’re included in the variable that we show us gamma star in this equation.We don’t have time today to go through the details of the calculation.

GAYLON CAMPBELL 12:34
But the things I want you to take away are these, that there is sound for doing this calculation. It’s been successfully used many times in practice, the equations certainly are big and there are a lot of them. But they’re easy to implement on a computer, you don’t do these calculations by hand. And all of the inputs that are needed to do these calculations are quite easily measured. We need air temperature, the fractional interception that’s the model conductance, radiation, wind, and vapor deficit.

GAYLON CAMPBELL 13:14
And here are some of the ways that we can get those needed. In the upper left with the canopy in the AROYA Nose has a solar panel in it. And that’s used for charging its internal batteries, but also is set up so that it’s useful as a light sensor. Since they’re known as modules below and above the canopy we have measurements of light below and above the canopy and the ratio of those gives us the fractional interception. The picture on the right shows the apogee pyranometer or the apogee quantum sensor hooked up to an AROYA nose that’s mounted above the canopy and gives us the light interception information that we need for doing the calculations. The lower left picture shows the METER SC 10 leaf barometer clip, clamped onto a leaf to measure stmotal conductance. That’s my annual measurement. But if we make those measurements on leaves during the light period, during the dark period, those measurements can be applied over long for periods of time, and we can use those to do the calculation. In the bottom center is the AROYA climate station that can give us the air temperature and the vapor deficit and accurate measurement of that. And finally, on the lower right, is a sonic anemometer, the ATMOS 22 from METER that could be used for monitoring wind, although wind probably stays fairly constant in a typical uncontrolled environment situation, and that could be measured manually and input that way. And moving on we want to talk about measuring drainage and change in storage. To do that, I’d like to show results from an experiment that we did last summer in a controlled environment chamber that you see here in the picture.

GAYLON CAMPBELL 16:07
You see plants growing in three different substrate, cocoa, rock wall and potting soil. The middle one on rock wall is the one that we’ve used to illustrate the components of the water budget today. These plants are on load cells, so that we’re able to continuously monitor the weight of each set of plants and their substrate. We also have AROYA water content, electrical conductivity, and temperature sensors the TEROS 12 in the substrate. Other sensors monitored light temperature, humidity, and wind for the calculation of E that I just described. So you will see the load cells allowed us to directly measure irrigation amount, transpiration rates, and the change in moisture of the substrate.

GAYLON CAMPBELL 17:03
Now this graph shows just a single day, day 36 during our experiment. The blue line is the weight from the load cell, and the value is shown on the left axis. The yellow line shows the six irrigation shots that were given for the day. At each shot, the mass of the plants and the substrate increases, each shot was a little over 500 milliliters. So with a shot the way jumps 500 grams, at least with the early ones, the ones early on in the irrigation. The lights came on at 36.33 tenths of the day into the day. And you can see that the way it starts to drop faster as the transpiration increases, when the lights come on. There isn’t a lot of stomotal control in this crop. So the main effect of changing the light is to change the radiation intercepted by the canopy. The lights went off at 36.8 or eight tenths in the day, and the slope decreased then. Now if we take the slope of the mass versus time line, we can compute transpiration rate.

GAYLON CAMPBELL 18:46
This graph shows the same data as the last one. But I’ve added now the water content data measured with the AROYA TEROS 12 sensor, and that’s the orange line. And you read that on the right axis. And the changes in water content are very similar to the changes in light. That shouldn’t be surprising, but it should give us confidence in the TEROS 12 for managing irrigation. The TEROS 12 is fairly inexpensive, it’s pretty easy to install, and it’s easy to monitor. A load cell is much more expensive. It’d be pretty difficult to use a load cell in a commercial controlled environment situation. And in addition, the TEROS 12 gives us information on the nutrients and the osmotic stress that the load cell wouldn’t be able to do. Now this graph again has the weight and the water content traces but there are other than the irrigation. It shows the drainage.

GAYLON CAMPBELL 20:03
We installed a tipping spoon monitor on the outflow of the rockwool block to see when and how much water came out. And it’s perhaps obvious if we put 500 plus milliliters of water into the substrate and its mass and water content don’t increase, the water has to be going somewhere besides the substrate. Here you see that the water content continues to increase with subsequent shots tell about 50%. Subsequent shots increase it a little, but most of the added water drains out. The highest water content the substrate is able to maintain is around 54%. Now in soil we call the upper limit of plant available water field capacity. When the water content of the soil profile exceeds field capacity, the excess water mostly drains out. While there is no field involved in controlled environment crops, the idea of a field capacity still applies. When irrigation occurs at water contents below field capacity, the water is retained for plant uptake. Water applied at or near field capacity will mostly drain out. We find the field capacity the way we showed here by adding enough water so that the water content plateaus and drainage starts. Field capacity can change by managing the substrate water but if we do a good job of irrigating it’ll be fairly constant during the growth of the crop. So here are some of the important points to remember relative to our water balance model and the measurements that I’ve shown you.

GAYLON CAMPBELL 22:11
First, field capacity is the water content to which substrates drain after recharge with an excess of water. If you look at the orange line in the graph here, the final shot causes the water content to reach about 55% but the substrate quickly drains to about 54%. We therefore set the field capacity in this situation to 54%. Shots given after the substrate is at field capacity are mostly lost to drainage shots given before drainage starts replace yesterday’s transpiration. Since we know how much water is added by an irrigation, this gives us a good way to estimate the crop water use. Here are some additional points to remember that change in storage term in the water budget equation is directly measured by the load cell.

GAYLON CAMPBELL 23:25
But we can also estimate it by knowing the change in water content. The TEROS 12 tells us and the water capacity of the substrate. Since we know the irrigation amount and the corresponding changes in water content we can estimate that substrate capacity. We talked already about the slope of the load cell line being equal to the transpiration rate during times when we’re not irrigating, but again if we know the substrate capacity, and we know the slope of the water content line, we can also estimate the transpiration rate from that.

GAYLON CAMPBELL 24:14
Well if we know the irrigation amount, the substrate water content in the field capacity, we can classify shots as recharge or as drainage. By adding up the drainage shots for a day, we get the d value for the water balance equation. We divide the daily drainage by the daily irrigation, we get what’s called the leaching fraction. You can see that equation on the top here. And we’re irrigating with nutrient solution that affects the electrical conductivity. The leaching fraction is the main tool that we have for controlling the electrical conductivity and the root zone of the crop. Now, to simplify this idea of how we would use electrical conductivity, we can start by assuming a steady state and no planned uptake of salts. In that case, we can also calculate leaching fraction from the ratio of electrical conductivity that I’ve shown in that top equation. And the example here then, the nutrient solution is three decisiemens per meter. And if 50% of the irrigation is drainage, then the drainage water would be at six decisiemens per meter. Now of course, there is uptake of nutrients, hopefully a lot of it. But this gives a feeling for the amount of drainage that would be required to bring down the rootzone EC electrical conductivity. With that, let’s summarize the things that we’ve gone over today.

GAYLON CAMPBELL 26:16
We introduced the idea of a water budget, the inputs and losses from the soil plant system, and how they have to sum to zero. This is a simple but powerful idea. We then went through term by term to quantify the terms in the water budget equation. We talked about how to check the quantity of irrigation amounts and how important it is to know how much water each blind is getting. We talked about determining transpiration rate for measured plant and environmental variables, about what field capacity is for horticultural substrates and how to determine it. That background we talked about ways of determining drainage in short and how to use that in connection with the known irrigation amount to determine leaching fraction, which we in turn use to control the substrate salt concentration of the osmotic potential. Oh, again, thank you for being with us today. We hope you find this useful.

BRAD NEWBOLD 27:35
All right. Thank you, Gaylon. And we’d like to use the next 10 minutes or so depending on how things go to take some questions from the audience. Thank you again to everybody who’s already sent in a question. And there’s still plenty of time to submit your questions there in the Questions pane. And we’ll try to get to as many as we can before we finish today, just as a heads up as well. If we do not get to your question, we still have them recorded and Gaylon or somebody else from our team will be able to get back to you via email to answer your question directly. So feel free to submit any and all questions that you have. There are several questions yes, we will be sharing the PowerPoint we will be sharing slides and recording of the webinar to everybody who registered today. So no worries there. All right. Let’s see here. One question really quickly are AROYA and TEROS 12 water content readings always expressed as volume per volume?

GAYLON CAMPBELL 28:37
Yes. Okay. To measure volumetric water content. Okay.

BRAD NEWBOLD 28:44
All right. Let’s see, if transpiration is high, and the substrate water content reaches the minimum acceptable content during the day, how would you suggest restarting the irrigation cycle?

GAYLON CAMPBELL 29:04
Yeah, I think you certainly can give irrigation shots anytime you need to. And, and I would do that to keep the substrate water content from going too long. That’s one of the things that your system ought to be monitoring. And the examples that we’ve shown generally give the irrigation right at the start of the day. But I would try to irrigate in such a way that the water content didn’t dropp below 30 35% Yeah, the substrate keeps doing its job properly.

BRAD NEWBOLD 29:56
Alright. What adjustments need to be done if other substrates are used for example cocoa or other mixtures of substrates?

GAYLON CAMPBELL 30:05
I think all of the things that we’ve talked about here apply equally well for any article substrate. Typically they have large pores hold the water pretty weakly and all of them are designed that way to make sure that the roots get plenty of oxygen.

BRAD NEWBOLD 30:32
Is irrigating by several shots to reach my target moisture content the optimal way to irrigate rather than giving one full shot?

GAYLON CAMPBELL 30:40
It is the optimal way and we covered this in more detail in the first lecture, you could go back and look at that a little bit, but with these coarse textured substrates, the water will tend to channel if you maintain them at too high water content and so, you give small shots of water to allow the water to redistribute within the substrate between shots and that way you get to use most of the substrate volume for the purpose of the substrate.

BRAD NEWBOLD 31:26
Alright. This next question I’m curious about what you said regarding intentional water stress in plants. If it’s not possible to impose low soil water potentials by withholding water, you hinted at using osmolytes to lower the osmotic potential of the soil. Is this something you do in practice using certain electrolytes?

GAYLON CAMPBELL 31:49
This is something that you do in practice and we covered that in quite a bit more detail in the second lecture. But since these coarse textured materials don’t allow us to apply a matric stress for crop steering the way we might do in natural soils we do it osmotically. And we can accomplish the same things and in fact that is what people talk about when they talk about crop steering, that’s the meaning of that term. It’s to apply an intentional stress to get the crop to generate a plea respond.

BRAD NEWBOLD 32:49
All right. Are there any tensiometers that can be useful in substrate production to avoid plant stress?

GAYLON CAMPBELL 33:03
A tensiometer could be used but because the matric potential isn’t of interest in horticultural substrates, I wouldn’t use one. water content tells you if you have a moisture release curve and a water content measurement you know everything you need to know about the state of water in the substrate and the thing that you need to know is the osmotic potential or the electrical conductivity, which is a separate thing and a tensiometer can’t tell you that.

BRAD NEWBOLD 33:48
Okay. If we want to reduce overall water applied per crop, what changes in monitoring or calculations are needed to account for substrate surface evaporation?

GAYLON CAMPBELL 34:03
We didn’t spend much time on the evaporation from the substrate surface here. The same equations apply. We need to have to know the evaporative areas and resistances or conductances for those but your points well taken if we’re trying to reduce water use then we need to quantify the evaporative area and do the things we need to do to reduce the evaporation from those areas.

BRAD NEWBOLD 34:42
Okay, I think we’ve got time for a couple more questions here. So what okay, this is about leaching fraction. Would this leaching fraction equation of irrigation EC to drainage EC, apply directly to a substrate that contains and really uses nutrients? Or would there be more complicating factors in this case?

GAYLON CAMPBELL 35:05
There are more complicating factors, that the assumptions behind those calculations are a steady state and no uptake of nutrients. And neither of those really applies very well in the situation that’s a dynamic situation. And so the point of going through that calculation is, I mean, it’s still true that the only tools that you have, if you’re maintaining a constant concentration of nutrients in the irrigation water, the only tool that you have for controlling the substrate, electrical conductivity, is leaching. And so, the point of doing the calculation is to get a feeling for what you would need to do if those assumptions apply. Now, the uptake of nutrients makes it so that you you have to leech a lot less. But I think you need to understand that system. And the leaching fraction is a simplification that allows you to understand that so that you can use it to properly control the substrate electrical conductivity or portal electrical conductivity.

BRAD NEWBOLD 36:44
In the experiment you did, what was the interval between shots? And was this dependent on some climactic factor? I think this is just in general in determining that interval, what are the the variables that go into that?

GAYLON CAMPBELL 36:57
you know, you just need to wait between shots long enough so that the water can redistribute. And I think we waited about 15 minutes between shots.

BRAD NEWBOLD 37:13
Okay. Let’s see. This question is saying, we know that our soil has a field capacity at negative 33 kPa. So what is the pressure point that we can take in determining field capacity of that substrate?

GAYLON CAMPBELL 37:37
In soil, we take minus 33 kilopascals typically as a kind of a rough estimate of field capacity. In these horticulture substrates, it’s more like five kilopascals or something like that. It’s an entirely different system, because the outflow from the substrate the water won’t flow out of the substrate until you have a positive pressure. And so the substrate is saturated at the outflow point. And so it’s an entirely different system than you have in the soil or you have a deep soil profile with water potential gradient all the way down.

BRAD NEWBOLD 38:33
Alright, I think this is going to be our final question here. For a lot of folks, they’re new to this kind of system. So there are some concerns about how much should they worry about taking exact measurements as opposed to maybe estimating. How much will that affect their overall plan?

GAYLON CAMPBELL 38:59
Well, that’s a good question. The good thing about that water balance is that the stuff still has to add up. And so let’s say that you did some estimating of the E term or whichever term. You will know that the next day when you irrigate, because it’ll show up as more drainage or less drainage. If you’re not irrigating enough, why you will see that the you never get back to field capacity. And the average water content’s going down and so it’s kind of self correcting in that way that you always will know, as you go along and see the result of the decision that you made. You will know what you need to do to adjust whatever factor you have to estimate.

BRAD NEWBOLD 39:58
All right. Thank you again, Gaylon. That’s going to wrap it up for us. Thank you to everyone who submitted questions. We had a ton of questions come in, and we did not definitely did not get to all of them. So just wanted to let you know that again, if we did not get to your question here during the live webinar, we do have them recorded, and we will be able to get back to you via email to answer your question directly. Please consider also answering the short survey that will appear after the webinar is finished, just to let us know what types of webinars you’d like to see in the future. And again, for more information on what you’ve seen today, please visit us at metergroup.com. Finally, look for the recording of today’s presentation in your email. And stay tuned for future meter webinars. Thanks again, stay safe and have a great day.