Crops turn sunlight, water, carbon dioxide, and nutrients into food

The availability of those resources puts limitations on the amount of food a crop can produce. A previous webinar considered the limitations of sunlight. In this 30-minute webinar, world-renowned environmental biophysicist, Dr. Gaylon S. Campbell, discusses how to measure the amount of water a crop will need and how to use that value to predict the amount of biomass it will produce.

Achieve maximum biomass from every drop

Join Dr. Campbell as he discusses the measurements and calculations needed to know how much biomass a given environment can produce. Dr. Campbell will discuss:

• How resource capture models work
• How biomass production and water use are linked
• Examples of effective uses of water resource capture models
• Instrumentation needed to determine water and radiation limitations on yield
• How to use soil and atmospheric measurements to quantify crop water capture
• Water budgets and how they are used to get transpiration and biomass production

Next steps

• Watch part one of the resource capture series
• See the slides
• Discover the ACCUPAR LP-80
• See the ATMOS 41 weather station or the wireless ATMOS 41W

Questions?

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

• Talk to an expert
• Request a quote

Transcript

BRAD NEWBOLD

Hello everyone, and welcome to Water Resource Capture: Turning Water into Biomass. Today’s presentation will be about 30 minutes followed by about 10 minutes of Q&A with our presenter, Dr. Gaylon Campbell, whom I will 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 then 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 will be sending around a recording of the webinar via email within the next three to five business days.

BRAD NEWBOLD

All right, with all that out of the way, let’s get started. Today we’ll hear from Dr. Gaylon Campbell, who will discuss how to manage decreasing water resources while maximizing crop biomass with optimized resource capture. 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 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. He’s written three books, over 100 refereed journal articles, some book chapters, and has several patents. So without further ado, I’ll hand it over to Gaylon to get us started.

GAYLON CAMPBELL

Oh, thank you, Brad. And thanks to all of you for being with us today. I want to continue our discussion that we started in the last webinar that I gave on research capture models. Now to review it a little bit, we defined a resource as a form of energy or matter that organisms need to grow and reproduce, and capture is the process by which organisms remove resources from their environment to maintain metabolism. This is the most important equation there is for living organisms. It’s an example of resource capture, and it shows how carbon dioxide, water, and light become carbohydrate and oxygen. For resource capture, the plants focus on the left side of the equation. For us, as humans, and other animals focus on the right side of the equation, the carbohydrate and oxygen. The capture of nutrients is implied since the plant biomass needs to have those in order to exist. The photosynthesis equation shows water as a resource, and some actually is used in that process, but it’s pretty negligible compared to the main use of water assimilation uses about point six grams, or uses about point six grams of water per gram of dry matter fixed. The plant is mostly made up of water, and they’re roughly 2.4 grams of water per gram of dry matter.

GAYLON CAMPBELL

By far the most important water use though, is shown in the diagram on the left. The process we saw in the equation in the previous slide is diagrammed here, carbon dioxide from the air diffuses through this stomatal pore into the mesophyll cells, where it combines with light and water to form carbohydrate. The humidity inside this stomatal cavity is essentially one or 100% and almost always much lower than that in the atmosphere outside this stomatal pore. So water diffuses out — we call that transpiration — uses orders of magnitude more water than either of the other processes we just talked about. 150 to 300 grams of water per gram of dry matter. We usually talk about the reciprocal of that number, the transpiration efficiency. And that’s typically three to six grams of dry matter per kilogram of water. And we can model photosynthesis and as a stomatal conductance: G sub C in the equation multiplied by the difference in CO2 concentration in the atmosphere and inside the leaf. The second equation is for transpiration. It’s similar. It’s the product of the vapor conductance G sub V. And the difference in water vapor concentration inside the leaf and in the atmosphere. D is called the vapor deficit of the atmosphere. If the leaf is at atmospheric temperature, then D is equal to the difference in vapor concentration inside and outside the leaf. We take the ratio of photosynthesis to transpiration — the ratio the conductance is a constant — and for a given cultivar, the CO2 concentration difference between the inside of the leaf in the atmosphere is also pretty constant. So we’ll call that term the transpiration efficiency P over T or the photosynthesis over the transpiration. We’ll say that that’s a constant K divided by the vapor deficit in the air. The larger the vapor deficit the smaller the transpiration efficiency or the less dry matter we get per kilogram of water.

GAYLON CAMPBELL

Now, from the last webinar, we had a light capture model, that said that the biomass that could be computed or that said biomass could be computed as the product of radiation use efficiency, fractional interception of the radiation and the cumulative incoming radiation. Now, we have another resource capture model, which says that the biomass is equal to the product to transpiration efficiency and the transpired water, both equations will always be correct, because if radiation is limiting, the plant will adjust the transpiration efficiency. And if water is limiting the plant will adjust the radiation use efficiency. For these models to be useful, we need to see which is the most limiting. We could use the non-limiting value of TE or of RUA and then pick to do the calculation and then pick the smallest as the actual biomass produced for that time increment in our model.

GAYLON CAMPBELL

The idea of using a transpiration efficiency isn’t new. The model we just derived was being used to good effect more than a century and a quarter ago. John A. Widtsoe went from the frontiers of Utah to Harvard to get his degree in chemistry. Then he returned to Utah State Agricultural College — now Utah State University — where he taught chemistry and physics. USAC was Utah’s new land grant college then, and many faculty there had joint appointments in the Agricultural Experiment Station, rather than focusing his research efforts in chemistry, Widtsoe decided that the more urgent need in that arid environment was the knowledge about crop water requirements. So in about 1900, he started some experiments in that area. Here’s a picture of some of that research; crops were grown in pots with various watering treatments. The pots were weighed to determine the amount of water used and harvested to determine the biomass. Widtsoe left USAC to do a PhD in Germany, then was hired back as the director of the experiment station, and went on to be president of the university. He summarized a lot of the research and described its application in two books — one on dry farming and one on irrigation — that he wrote and published when he was pressed into the university.

GAYLON CAMPBELL

Here’s an example of the way he applied the water resource capture model for dry farming. The picture on the left is the way he illustrated the transpiration efficiency. The big bottle on the left is the water required to produce the little bit of grain in the bottle on the right. Let me just read how he used the model. He said one inch of water over one acre of land weighs approximately 226,875 pounds, or over 113 tons. If this quantity of water could be stored in the soil and used only for plant production it would produce at the rate of 45 tons of water for each two and a half bushels of wheat with 10 inches of rainfall, which at present seems to be the lower limit of successful dry farming, there’s a maximum possibility of producing 25 bushels of wheat annually.

GAYLON CAMPBELL

Briggs and Shantz worked in the Midwest about that same time, and their research there was similar to Widtsoe’s — here’s a couple of their conclusions. They said one of the most striking features of water requirement measurements is the marked difference in efficiency exhibited by different plants in the use of water. The millet, sorghum, and corn groups have been found the most efficient while alfalfa and sweet clover are the least efficient in producing dry matter with a given amount of water. The small grain crops have water requirements intermediate between the legumes and the corn. They also made the observation that measurable differences in the water requirements also exists between different varieties of the same crop. This suggests the possibility of developing — through selection — strains that are still more efficient in the use of water. Now, that’s still a hope. But it’s not an area where we’ve actually seen a lot of progress over the last century or more.

GAYLON CAMPBELL

Now if we return to our water resource capture model, that’s shown in the bottom equation, we see the transpiration efficiency is a constant K divided by an environmental factor, the vapor depth of the air. When we talked about radiation use efficiency, the values were crop specific, but conservative over a growing season. Transpiration efficiencies are also conservative over growing seasons, and also crop specific; they depend on the internal CO2 concentration the leaf is able to maintain. Here are a few values of K from a review that Tanner and Sinclair did a number of years ago. The results shown are consistent with the observations of Briggs and Shantz. Vapor deficit is typically measured in kilopascals. The typical daytime value might be two kilopascals. So for the potato that’s shown here, you would have seven pascals divided by two kilopascals, or about three and a half pascals per kilopascal and that would come out three and a half grams of dry matter per kilogram of water. The real power in these models comes when we commit them to computer code and run them for the season in hourly time stamps or smaller, using environmental data that has the same resolution.

GAYLON CAMPBELL

The models aren’t complex, they can be easily programmed in Visual Basic as macros in the spreadsheet, if you want to. But we can make some general comments based on the equations and ideas that we presented so far. We’ve been talking about the transpiration efficiency, the dry matter produced per unit water loss, have shown that dry matter production is proportional to transpiration because of the way photosynthesis and transpiration are related. There are just two ways of increasing transpiration efficiency. One is by reducing the CO2 concentration inside the leaf. Species like corn and sorghum that have C4 metabolism do this. The other method is to take up the CO2 at night when the vapor deficit is small, and cacti and other plants with CAM metabolism do that. Actually there’s a third way, and that’s to increase the ambient CO2. As a society we’re busy doing that, but the consequences of that on climate change probably aren’t offset by the increase in transpiration efficiency we’re seeing.

GAYLON CAMPBELL

Now here are some more observations we can make. Since transpiration efficiency is constant divided by vapor deficit at the air, humid places have higher transpiration efficiencies than arid places. This has led some to suggest the federal money spent on irrigation projects should be spent to develop irrigation in humid regions. Since we get a lot more bang for the buck there. This hasn’t received a lot of support from those of us in arid regions. Another thing to remember is because vapor deficit is low at low temperatures and high temperatures, the transpiration efficiency changes a lot over the course of a day and even over the course of a season. Biomass production relates only to transpiration, not evapotranspiration. In the quote by Widtsoe that I gave earlier, he talked about the wheat production from 10 inches of rain, if all of that water were transpired. But he knew as well as we do that it isn’t all transpired. The water balance of the crop is shown here. The drainage is typically not a problem in dryland agriculture, and runoff can be kept small by good management practices. That leaves just evaporation. Daniel Hillel, an Israeli soil physicist, used to talk about the plight of the plant. We complain, he said, living under governments where we are taxed 20%, 30%, or maybe even 40%; but consider the plight of the plant. It must live under a regime where it’s taxed 99% and then is forced to keep its money in a bank that’s robbed and embezzled daily. He’s referring of course to the very small fraction of the water the plant takes up that’s used for photosynthesis and production of dry matter. The robbery and embezzlement is the runoff and evaporation. Evaporation occurs from the water intercepted by the canopy from rain or irrigation and evaporation from the soil surface. Water doesn’t benefit the plant, but it subtracts from the water available for transpiration. To get transpiration and dry matter, we need to know the precipitation and then subtract out the change in soil moisture in the evaporation.

GAYLON CAMPBELL

A good weather station is required to provide the measurements one needs to model evapotranspiration. A really convenient one is METER’s new ATMOS 41W that I show here. In addition to solar radiation, it measures temperature, humidity, wind, rain, and a number of other variables. All of these are needed for the water balance calculation. This doesn’t require the ZL6 logger like the ATMOS 41 did. It has its own cell modem to send data directly to ZENTRA cloud, where they’re immediately available to you on your computer. ZENTRA cloud also does the processing of your data to give you a reference evapotranspiration for your crop each day. Potential evapotranspiration is the sum of potential evaporation and potential transpiration. And that’s the reference ET that you can get out of ZENTRA cloud, multiplied by a crop coefficient — typically a value pretty near one. Soil moisture is plentiful in the soil surfaces when evapotranspiration is equal to potential evapotranspiration. The part from transpiration is the fractional interception times the PET, a part from evaporation is the rest — one minus the fractional interception times PET. The actual transpiration is equal to potential transpiration if soil moisture is plentiful; if soil moisture is depleted stomates close, and actual transpiration drops below potential. When the soil surface is wet the actual evaporation equals potential. But, as the soil surface dries evaporation decreases. This is hard to keep track of with a pencil and paper, but it’s easy for a computer model to do. As with a radiation resource capture model, we need to know fractional interception to separate PET into PE and PT. So we need measurements, the NDVI, or measurements with the LP80 ceptometer that we talked about in the last webinar.

GAYLON CAMPBELL

Here’s an example of where we measured soil moisture, electrical conductivity, and temperature at 42 locations in the field at the Washington State University agronomy farm. We’ve made measurements of 30, 60, 90, 120, and 150 centimeters. They’re each foot down to five feet under a crop of winter wheat. And here are some of the data that we collected in the experiment. I’m showing here a picture of the TEROS 12 soil moisture sensor that we sell now. For these measurements, when we did this study, we used a sensor called the 5TE, but the results would be similar or better with the TEROS 12. The purple line is the 30 centimeter, blue is 60 or inches 90, dark green is 120 and light green is 150 centimeters. We harvested around day 200. When the sensors were disconnected from the logger, their wires had to be buried so that we can do tillage on the field. Sensors reconnected around day 320.

GAYLON CAMPBELL

Now if you look at the soil moisture data, the plants pulled water successively from each layer. If you calculated the derivative or the change of each curve, to get the rate of water update, you take and then you added those together, you could easily get the total transpiration for this environment. There’s very little if any rain during the growing season. So that value that you would get would closely approximate the transpiration for the crop. The transpiration efficiency is a constant for a given species and cultivar divided by the daytime vapor deficit. The vapor deficit is the saturation vapor pressure minus the vapor pressure the air. Since the saturation vapor pressure is strongly temperature dependent, the vapor deficit is near zero at night when the temperatures is low. And it’s large at midday when the temperatures are high. The graph shows a typical daily pattern. The daily average vapor deficit here is 1.2 kilopascals. But the daytime average is 1.6 kilopascals. Since photosynthesis occurs during the day, we want to use the daytime average, not the daily average. The ATMOS 41 and ATMOS 41W that we’ve talked about and our ATMOS 14 that I show here with the ZL6 logger, any of those will give the vapor pressure and vapor deficit numbers. Plants grown outdoors typically have high stomatal conductances during daylight hours, but they close their stomates tightly at night. Most of the time, but not always.

GAYLON CAMPBELL

I was once monitoring stomatal conductance and leaf water potential of a potato crop over a 24 hour cycle. When night came the conductances didn’t drop like I expected them to. I thought my pyrometer was broken. But I tried it on corn in an adjacent field, and the corn conductance was very near zero. Water lost In the dark doesn’t help photosynthesis so it reduces the transpiration efficiency of the crop. This graph is from some recent work by Walden Coleman and others, where they show a strong negative correlation between dark conductance of several cultivars of soybean and their transpiration efficiency. They call it water use efficiency. Even though these conductances are an order of magnitude smaller than we’d expect during the day, this nighttime loss appears to influence the transpiration efficiency. We said in that quote from Briggs and Shantz, that there was a possibility of developing — through selection — strains that are still more efficient in the use of water. This hasn’t been the case with a lot of research but Walden Coleman and his colleagues appear to have found one method of finding such strains. And we might still make progress on that. METER has an instrument for measuring stomatal conductance of leaves. It’s the SC1 stomatal conductance meter, you clamp it on the leaf, and make a measurement, it’s pretty easy.

GAYLON CAMPBELL

Now from this brief introduction to resource capture models, and other sources, we can conclude that the best strategy for maximizing biomass is to maximize the fraction of precipitation and irrigation that goes through the plant during daylight hours. We can minimize the robbery and embezzlement by reducing evaporation and runoff. Those are practices that Widtsoe recommended. We can look for cultivars with low dark conductance and possibly find other useful indicators of non-useful water loss. If it’s an option, we might plan C4 rather than C3 species, we might consider growing crops in more humid climates. Though, being from an arid location and wanting crop production to continue here, that isn’t so palatable to me. I hope it’s clear to you that there are good and simple models that can predict environmental limits on crop production. Finally, we have seen that we can learn a lot just for the form of the model. But the real power comes when we set up the equipment, collect the appropriate environmental data, then feed those data into a computer with the appropriate model to predict the yields that we should be able to get in a given environment. We can then compare those with the yields we measure in the field to see how close we are to the limits the environment sets. Thank you for being with us today.

BRAD NEWBOLD

All right. Thank you, Gaylon. So we’d like to take the next 10 minutes or so to take some questions from the audience. Thank you to those who have sent in questions already, there’s still plenty of time to submit your questions, so please submit them through the questions pane. We’ll try to get to as many as we can before we finish here. If we do not get to your question before we finish, we do have them recorded, and either Dr. Campbell or somebody else from our METER environment team will be able to get back to you via email to answer your question directly. Our first question here is asking “Can the timing of the application of water impact the water use efficiency of the plant?”

GAYLON CAMPBELL

Certainly it can. Frequent applications of water allow a lot more evaporation of the water without it being used, and oftentimes water is supplied through sprinklers, and so the leaves are wet from that, and that water evaporates without going through the plant, and keeps the soil surface wet for longer periods of time. And so if we store more of the water in the soil and increase that period between irrigations that would have an effect. Applying the water too infrequently will also affect the transpiration efficiency because water becomes limiting, then that also has an effect.

BRAD NEWBOLD

There’s another question that kind of follows on this, can you talk about how soil type might affect water use efficiency?

GAYLON CAMPBELL

Again, one of the most important things there is the evaporative losses compared to the transpiration; that higher clay soils hold more water and therefore, keep the soil surface wet for a longer period of time after an irrigation. Fine textured soils will push that balance in the direction of evaporation. Some of that can be mitigated by tillage practices. And again, that’s something that Widtsoe talks about him in his irrigation book.

BRAD NEWBOLD

Another question here is saying, Can all of these measurements be interpreted or used solely for the germination process, or until an early growth stage, in order to predict biomass of seedlings or germinating seeds depending on the humidity of the environment?”

GAYLON CAMPBELL

I think these models apply at all stages; the seedling stage and throughout the growth of the crop. During that early stage, way more of the water is lost by evaporation than by transpiration just because the soil surface is bare. That could be adjusted some I suppose by adjusting planting densities, things like that that would determine how quickly the soil surface was covered with leaves. I can’t think of any other factors that would apply here.

BRAD NEWBOLD

All right. There’s another question asking what the relationship is between low internal CO2 concentration and CO2 sequestration?

GAYLON CAMPBELL

That sounds like that applies. CO2 sequestration relates to trying to help out with climate change, I guess, in that the species that can produce more biomass certainly sequesters more CO2, and so C4 species will be better at that than C3. That’s the only connection I can see there.

BRAD NEWBOLD

All right. Another person wanted you to go into more detail about how we can use stomatal conductance measurements to better understand water use efficiency of a plant.

GAYLON CAMPBELL

Well, the paper — the reference that I shared with you here is the only one that I’ve found so far that shows that directly. It was a beautiful study. I’m hoping more people will notice that and do more of that kind of research. So we know something about soybeans but there are lots of other species that could be looked at to see whether those same relationships exist.

BRAD NEWBOLD

Great, so there’s a lot more research to be done then right?

GAYLON CAMPBELL

I think so.

BRAD NEWBOLD

All right. Let’s see. Another one is saying “Are there ways to encourage plants to shut their stomates more to reduce water loss and create more biomass?

GAYLON CAMPBELL

Well stomates are under hormonal control, so I suppose we could use hormones. I had one PhD student who did a study where he applied chitosan to plants to reduce transpiration, and that actually worked, that did reduce transpiration. So, we know that it’s possible to do that. Some of those methods have been used, for example, on vegetation in medians, and freeways, where they irrigated to try to keep the vegetation growing there — I think this study was in California a number of years ago — where it’s pretty dangerous to go out and irrigate there because of the traffic, and so they used antitranspirants on those to try to get the plants to use less water.

BRAD NEWBOLD

Alright, we got one more minute, maybe we’ll do one or two more questions. We’ll see how this goes here. This question here. If stomatal conductance is reduced later in the day, would that indicate the plant is in some kind of stress? Or is that just a typical daily rhythm of conductance?

GAYLON CAMPBELL

It could be either. The plant closes its stomates when it has produced as much assimilate as it can store for that day. So it’s not wasting water. And so when it reduces stomatal conductance late in the day, that could well be a part of the daily pattern and relate to the rate of assimilation not to the to water stress. But if water is limiting, then it also would reduce stomatal conductance to limit transpiration just to preserve itself. So it could be either one, the only way to know would be to measure the soil moisture to see if whether the water is available to the plant.

BRAD NEWBOLD

All right. Well, I think our time is up. So that’s going to wrap it up for today. Thank you all for joining us. We hope that you enjoyed this discussion. Thank you again, for all your great questions. There were several that we did not get to. Again, we do have those recorded and somebody from our meter environment team will be able to get back to you to answer your questions directly. Also, please consider 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 for more information on what you’ve seen today, please visit us at metergroup.com. Finally, look for a recording of today’s presentation in your email, and stay tuned for future METER webinars. Thanks again, stay safe, and have a great day.