Eyes on Earth Episode 73 – Global Water Use

SAVANNAH COOLEY:

I think the big implications of this study point to a lot of potential damage that could happen if climate change is left unmitigated. At the same time, there's opportunity for curbing global emissions in this next decade that could potentially have big implications for many generations to come.

JOHN HULT:

Hello, everyone, and welcome to another episode of Eyes on Earth. We're a podcast that focuses on our ever-changing planet. And the people here at EROS sent across the globe who use remote sensing to monitor and study the health of Earth. I'm your host for this episode, John Hult. Today, we're diving into a measure of ecosystem health that's been rather difficult to get a handle on at the global scale: water use efficiency. Some plants are simply better at making use of their water supply than others. More efficient plants can capture more carbon with less water, which has implications for carbon sequestration and ultimately for climate change modeling. In other words, the more we understand about water use efficiency, the more reliable our climate change models can be. And the only way to measure efficiency at the global scale is from space. Our guest today did that. Savannah Cooley is an applied science systems engineer at NASA's Jet Propulsion Laboratory. She's just released a study on water use efficiency built from data collected by the ECOsystem Spaceborne Thermal Radiometer Experiment on Space Station, or ECOSTRESS. Data from that sensor lives at the USGS EROS Center in NASA's Land Processes Distributed Active Archive Center, or LP DAAC. Savannah scanned the Earth's plant life using ECOSTRESS and came up with some surprising results. You might assume a plant species in a hotter, colder, drier or wetter part of the world would adapt to use more or less water based on those conditions. But Savannah's research found that more than half of the plant types she studied use similar amounts of water, regardless of where they grow. She also found, perhaps less surprisingly, that plants with longer lifespans, like trees and shrubs, tend to be more efficient than plants that don't live as long, like grasses. Let's talk about that. Savannah, welcome to Eyes on Earth. 

COOLEY:

Thank you, John. It's great to be here.

HULT:

We're super excited to talk about this. And let's get started with some basics. What are we talking about when we talk about water use efficiency? Why is it important and how is this been measured in the past?

COOLEY

I'll start with the why to your question. Droughts are increasing in both severity and length as a result of humans continuing to emit carbon dioxide and other greenhouse gases, which leads to climate change. We expect that plants that use water efficiently will be better adapted to these changes relative to plants that use water inefficiently. Moving from the individual plant scale to the ecosystem scale, we would expect that ecosystems with a wide range of plants with different water use efficiencies may also be more resilient to these changes, because the plants in those ecosystems are suited to a variety of climate conditions. To get to the other component of your question around kind of what water use efficiency is, you can think of this measure as a measure of the connection between the carbon and water exchange. Specifically, we define water use efficiency as the amount of carbon a plant or ecosystem can take in for photosynthesis per amount water used.

HULT:

So on a very basic level, we're talking about how good a plant is at sucking up the carbon, carbon, which of course, we are pumping more and more of into the atmosphere, year by year.

COOLEY:

Yes, exactly. And other component of your question around how this has been measured in the past ... The answer is yes, there's quite a few ways that water use efficiency can be estimated. So a few of those techniques include what's called leaf gas exchange, stable isotope discrimination, Eddy covariance, as well as remote sensing, which is what our study used.

HULT:

And when you're talking about something like Eddy covariance you're talking about on the ground measuring if Evapotranspiration.

COOLEY:

Yes, evapotranspiration is one component-the water use component, right? And then also, estimating gross primary production, which is the carbon component.

HULT:

Gotcha. So we got a little background here. Let's talk about your study. Tell us about the study. Where did you look, and what types of plants did you look at? And what kind of a time frame are we talking about? Did you just look at one year or several years.

COOLEY:

We looked at the entire available record of ECOSTRESS data. We obtained data starting in 2018, in the summer of 2018 through 2020. With these two years of data, we looked at water use efficiency in 11 different study regions all over the world. Each study region consisted of an area of about 500-kilometer radius, spanning all major climate types globally. That includes tropical, temperate, and boreal ecosystems. And as I think previous Eyes on Earth episodes have mentioned, ECOSTRESS is on board the International Space Station. Being on that platform allows the instrument to capture data every three to five days for any given location on Earth. So we used all of the data available for these 11 study regions.

HULT:

What did you expect to find when you looked at this information, and what did you actually find?

COOLEY:

We expected to see a more confined or narrow range of water use efficiency values within study regions compared to the different plant types that we looked at, which grew in many of the study regions and therefore kind of spanned many different climates. But in contrast to expectations, we found that over half of the plant types that we studied have a consistent water use efficiency, regardless of where they grow in the world. So ranging from hot and wet to dry and cold regions. So these plant types that we found to be constrained include grassland, wetland, savannah, deciduous broadleaf forest, and deciduous needle leaf forests. The results indicate that it is the type of plant rather than the climate in which they are growing that mainly dictates water use efficiency. Our findings also indicate that plant types with longer lifespans such as shrubs and trees have higher water efficiency than plants like grasses that have shorter lifespans.

HULT:

You could have a particular species of tree in a very cold climate or a completely different sort of climate and the water use efficiency would be similar.

COOLEY:

Yes. And to be clear, we weren't looking at the species level, we were looking at what are called plant functional types. So for example, a deciduous broadleaf forest, right, which numerous species are within that type ... but like you said, we can find a range of climate conditions where deciduous broadleaf forests grow in the world, and we found that that particular plant type is relatively constrained.

HULT:

The follow up there would be what are the implications of these findings for climate modeling? And what are the next steps for the research?

COOLEY:

These findings suggest that climate might not be as important as plant physiology. By plant physiology, I mean the physical characteristics and limitations that are associated with these different plant type categories for constraining water use efficiency, under current and future global change. This finding has direct implications for how climate models represent ecosystem water use efficiency now and in the future. For instance, as temperature continues to rise and drought frequency and severity continue to occur, these climate models now have a better way of representing how ecosystems do or do not adapt to these changes. Another implication and important direction for future research is around climate adaptation and biodiversity preservation. We know that the pressures of global change on ecosystems currently and in the future may surpass the ability of certain plant species and functional groups to adapt. So for instance, traits that enable plants to conserve water during photosynthesis will be advantageous as the intensity and frequency of droughts increase in many parts of the world. The plant groups and study regions that we looked at that have low water use efficiency and a narrower range of water use efficiency values may therefore be especially vulnerable. And then in contrast, ecosystems with high variability and no convergence-so in other words, no constraint to a small range of water use efficiency values-might be more resilient to change, as they already have certain plant types that are suited to a variety of climate conditions. And our study essentially helps to identify these differences and ecosystem vulnerability to global change and lays the foundation for climate adaptation modeling and planning at large spatial scales.

HULT:

Interesting. So you're offering a data point in the future survival of these ecosystems, or certain plant types in these ecosystems, you're offering a metric that helps us to understand who's going to make it and who's not, to put it bluntly and darkly.

COOLEY:

There's certainly a lot more research to be done to control for numerous other confounding factors that might be also influencing water use efficiency at ecosystem scales under climate change. We didn't actually look at climate change impacts directly. But yes, our study kind of lays the foundation for that future work to happen.

HULT:

Let's rewind a little bit and talk a little bit more about the sensor, because we like to talk remote sensing on this show. Why use ECOSTRESS? Why this sensor? What can it do that other sensors can't? And how did it help you to come to these conclusions?

COOLEY:

ECOSTRESS provides the highest spatial resolution estimates of evapotranspiration that are currently available at global scales. We found that this fine resolution allows us to observe local scale variability in water use efficiency that we otherwise would not be able to detect. For example, we observed a 25% difference in water use efficiency across a 50-kilometer tract of intact forest in the southwestern Amazon. One possibility as to why this 25% difference arose could be due to changes in soil conditions and topography. This local scale variability where GPP, or the carbon component, is relatively unchanging, might reflect the importance of evapotranspiration in regulating ecosystem water use efficiency. So the evapotranspiration component being what ECOSTRESS provides at the local scale, at the small, fine scale. ECOSTRESS also absorbs finer scale differences between high water use efficiency of intact forest and the low water use efficiency and adjacent cattle pasture.

HULT:

If we're zooming out here, just a little bit, ECOSTRESS allowed you to see differences in small areas that you wouldn't have been able to see otherwise. And you were able to look at those measurements worldwide. And as you say, you could tell the difference between an intact forest and an area that had been cleared for grazing. And that's the advantage, that's what you were able to do because of the sensor.

COOLEY:

Exactly. And in the case of the intact forest versus an area that's been cleared for grazing, this is particularly important to have fine spatial scale information. Because a lot of times, the landscape has small spatial scale, kind of patchwork configurations of forest and agriculture, where lower resolution satellites simply can't detect and can't identify these spatially nuanced patterns of water use efficiency.

HULT:

Pretty cool that you get to do that from your desk, isn't it? Just get to kind of travel around the world and look at forests and pastures and see how well the plants are doing at taking up carbon.

COOLEY:

It truly is an amazing tool set. And it really opens up a lot of possibilities for making access to doing this kind of research possible for anyone who has a computer and internet access. And so it's really exciting to see all the different ways that these data can be used and shared among the scientific and broader environmental communities.

HULT:

Savannah, any closing thoughts on ECOSTRESS, remote sensing in general, or water use efficiency that you'd like to leave our listeners with?

COOLEY:

What we as human civilization choose to do in the next decade in terms of curbing global emissions has massive implications for the wellbeing of many generations to come, both for the human species and many other species. That's really a big component of why I do the work that I do. And thinking about how different ecosystems will or some will not be able to adapt to these rapidly changing climate conditions just for me highlights the importance of communicating these implications to policymakers and environmental decision makers such that meaningful action on climate change can happen.

HULT:

We've been talking with Savannah Cooley of NASA's Jet Propulsion Laboratory about water use efficiency at the global scale. This has been a fascinating conversation. Savannah, thank you so much for joining us!

COOLEY:

Thank you for having me on the show.

HULT:

And thank you to the listeners for joining us as well. You can find all our shows on our website at usgs.gov/eros. That's U-S-G-S-dot-g-o-v, forward slash e-r-o-s. You can also follow us on Facebook and Twitter to find the latest episodes, or you can find our shows on Google podcasts. You can subscribe there. Just hit a button, you'll get the new show every time.

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