Eyes on Earth Episode 67 - ECOSTRESS and Water Use

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 on the people here at EROS and across the globe who use remote sensing to monitor and study the health of earth. I'm your host for this episode, John Hult. If you want to know how much rain fell yesterday, you can catch it and measure it. Water vapor? That's not so easy. Which is a problem. If you want to know how quickly that rate is returning to the atmosphere. If you're a farmer trying to decide how much to water and when, that's useful information. It can also signal drought or plant stress. Water vapor is the single largest part of the water budget, but without space-based observations, it would be all but impossible to measure at wide scale. Today's guest is an expert in the measurement of evapotranspiration, or ET, which is the combined rate of evaporation from the Earth's surface and transpiration from plants. Dr. Kerry Cawse-Nicholson is a member of the Carbon Cycle and Ecosystems group at NASA Jet Propulsion Laboratory. She's the deputy science lead for ECOSTRESS as well, which is a sensor that was a fixed to the international space station in 2018. ECOSTRESS data are available through NASA's Land Processes Distributed Active Archive Center, or LP DAAC, which is located here at EROS. For one of her many projects at JPL, Kerry uses ECOSTRESS data to estimate ET across the contiguous United States. And she was the lead author of a recent study designed to test the accuracy of those estimates. Kerry, welcome to Eyes on Earth.

KERRY CAWSE-NICHOLSON:

Thank you so much for having me, John. I'm so pleased to talk to you about this.

HULT:

Yeah, well, it's exciting stuff. We've done a few shows about ECOSTRESS already, but you are the deputy science lead for this project, so I want to ask you to begin with, what is ECOSTRESS? What is it designed to do, and why did NASA send it up in a space in the first place?

CAWSE-NICHOLSON:

So ECOSTRESS is what we call a thermal radiometer. Essentially it measures temperature from space. So in some cases, we measure really large temperature changes like urban heat waves, wildfires, and volcanic eruptions. And in other cases, we measure very, very small changes in temperature. When plants photosynthesize, they open up tiny pores on their leaves called stomata that allow them to take up carbon, but in doing so small amounts of water escape, what's called transpiration. If plants are heat to or water stressed, they'll close their stomata to keep that water inside. Now this inhibits photosynthesis, so it's delicate balance, but when the stomata close, the plant starts to heat up ever so slightly, like a human who can't sweat. Those are the tiny temperature changes we're looking for using ECOSTRESS.

HULT:

That's fascinating. So this is really a tool designed for this job. 

CAWSE-NICHOLSON:

Yes, absolutely. 

HULT:

Let's step back a second and talk about why it's important to measure ET from space in the first place. What is this information good for?

CAWSE-NICHOLSON:

A farmer is able to look at their field and use their expert judgment on the state of their crops in their field that they're looking at. But remote sensing allows us to understand ET at a regional scale, looking for patterns to understand how different crops in different regions might respond differently to drought. But at the spatial resolution of ECOSTRESS, it means that we can also see the detail down to individual fields. We also monitor unmanaged landscapes, which don't have those human eyes on them at all, and ET enables us to detect plant stress some weeks before plants visibly start to wilt. So this is an extremely useful measurement, both in terms of agriculture, and in terms of natural landscapes, and monitoring the way that plants respond to drought. 

HULT:

If you wanted to measure evapotranspiration without a spaceborne sensor, how do you typically do that?

CAWSE-NICHOLSON:

We can make measurements on the ground. Often it's done by using towers and fields that measure eddy covariance. And so they establish what's called the water balance, the amount of water that might come down onto the Earth, and the amount of water that may rise up back into the atmosphere. And they use that to estimate exactly how much water is lost by the plants, and we compare our ECOSTRESS measurements to those made on the ground. 

HULT:

So you're able to use those on-the-ground measurements to sort of check what you can see from space. So you, you kind of need both to be able to do this well?

CAWSE-NICHOLSON:

Yes.

HULT:

How is ECOSTRESS different from the other sensors we've used to measure ET?

CAWSE-NICHOLSON:

One of the major differences is that ECOSTRESS is mounted on the International Space Station, which is in what we call a precessing orbit. Every time it passes over a fixed point, it comes over at a slightly earlier time of day. This means that ECOSTRESS can measure temperature and ET at all kinds of times of day and night, acquiring data at times many other instruments can't, but at very high spatial resolutions or high level of detail.

HULT:

As opposed to something like, say a Landsat, where we're going to see the fields every eight to 16 days, we're talking about multiple times per day? And also with Landsat, we're looking at the same time of day. ECOSTRESS is looking at different times of day. Do you get kind of a different sense of how the plants are reacting at 10 a.m. versus 7 p.m., that sort of thing?

CAWSE-NICHOLSON:

Yes, absolutely. And ECOSTRESS actually works quite well with Landsat. So ECOSTRESS can help to fill in some of those gaps in that 8- to 16-day window between different Landsat observations. And just as you say, we can make observations at different times of day to establish just how representative that Landsat observation is of the way that plants are behaving in that region.

HULT:

And of course, Landsat has a different spatial resolution. It's down to the 30 meters, ECOSTRESS is about 70 meters, so we're kind of filling in those gaps at the field scale then aren't we?

CAWSE-NICHOLSON:

Yes, absolutely right.

HULT:

Tell us a little bit about the estimates that your team has pulled together using ECOSTRESS. We're talking about daily estimates, different times of day, is that correct, like we were just talking about?

CAWSE-NICHOLSON:

Yeah, so we have two major ET products. One of them estimates a daily average ET, and that allows us to look at time series, looking for seasonal patterns. The other is what we call instantaneous ET. In other words, what is the ET at this very moment? So if we look at the same place for several weeks, we can piece together all the ET values acquired at different times of day, to understand diurnal patterns. So how do plants open and close their stomata within the course of a single day? And how does that strategy translate to seasonal behavior?

HULT:

Was there anything like this available before? Or is there anything out there? Let's say if I'm, if I'm a producer, is there any way for me to access this stuff easily?

CAWSE-NICHOLSON:

Before ECOSTRESS, we had nothing like this. We have satellites that are geostationary satellites, which means they acquire data over the same place on Earth very regularly, often in intervals less than an hour. So we can compute ET that way. But because you're trying to acquire data very regularly throughout the day, it means you compromise on your spatial resolution. So those pixels are much coarser, often several kilometers. On the other hand, if you have some of the polar orbiting satellites, like Landsat and others, they acquire those small pixels, they have that fine spatial detail, but they come over much less often. So I mean, a single Landsat instrument often returns to the same place only after 16 days. And so ECOSTRESS is the only way that we've been able to put together this diurnal understanding of plant strategy at the spatial scale of individual farmer's fields. So it's really a unique instrument, and we're really excited to see the kinds of data that we're getting from the mission.

HULT:

Once again, filling in that gap. In the past, you've had to look at 1-kilometer pixels. I imagine there's a little bit of variation in the fields from one place to the next at a kilometer scale. And you're able to sort of fill in the gaps and get that high resolution data that otherwise you would have to wait several days for?

CAWSE-NICHOLSON:

And that spatial resolution is so important, especially in areas with irrigated farmland, surrounded by unmanaged natural vegetation. Because if you average out a single pixel, you can lose a lot of those differences between how the irrigated landscape is changing, versus how the unmanaged landscape is changing. And they often respond in very different ways, as you can imagine, due to human intervention. And a lot of that detail is lost with the very course pixels acquired by some of the other instruments

HULT:

If you're looking at a 1-kilometer pixel, you could have half farmland and half natural land, and it would have the same number, you'd have to give it the same number. 

CAWSE-NICHOLSON:

Absolutely.

HULT:

Let's talk about the paper. You recently released a paper on the accuracy of ECOSTRESS-based ET estimates. How do they stack up to the other ET estimates that are out there?

CAWSE-NICHOLSON:

So we compared ECOSTRESS ET with measurements made on the ground, over many different land cover types. And we found that ECOSTRESS ET is extremely accurate, to within about one millimeter per day, which is in line with ET estimates made from other satellites. And importantly, it's within the accuracy that agricultural users and water resource managers typically find acceptable in order for them to make informed water management decisions using this data.

HULT:

Well, you're already getting into this, but I wanted to ask you what would make ET from ECOSTRESS so valuable, and where you see it being the most useful?

CAWSE-NICHOLSON:

So ECOSTRESS ET is available nearly globally for free through the LP DAAC. And it can see all kinds of detail, with each pixel a little smaller than a football field. It acquires data really often, depending on the ISS orbit, sometimes even more than once in a day. And it gives us insight into plant strategies during times of stress. We've tried our hardest to really make us data easily accessible to anyone who wants it, and we're working on developing new data formats so that people can easily just drag and drop the data into a GIS tool of their choice. And we really want people to be able to use this. 

HULT:

Let's talk about that a little bit. What are the next steps for ECOSTRESS-based ET? It sounds like you're doing a lot of work trying to make it more easily accessible. What are we looking for on the horizon?

CAWSE-NICHOLSON:

Soon we'll release the next collection of ECOSTRESS ET and other products. We're trying to produce easy-to-use data formats, and we'll also make available a lot of the other data that we use to compute ET, so that other people can calculate ET on their own if they'd like or compare different methods for computing ET. Or they can start understanding how surface temperature changes relative to air temperature, or how ET relates to plant greeness. These aren't all things that we derive from ECOSTRESS, but they are things that we use. And so if we put them at the same resolution as our ECOSTRESS image, other people can start doing that direct comparison much more easily. So we really are aiming to make this useful and easy to use, and we welcome new users. So I really, really encourage folks to reach out to us for more information if they'd like to start using this data. 

HULT:

Do you need to be a GIS specialist to understand this? Is there a middle ground where you can hit that you would be able to learn something from this if you're relatively new? Or are there places that you can go to learn how to use this?

CAWSE-NICHOLSON:

Yes on both of those points. The LP DAAC, based at EROS, as you said, has made available a number of tutorials on how to use the data. But they also have tools that are available. So there is a tool called AppEEARS, which means you could select an individual point, say a farm, or any other area of interest, and it could return to you the ET values acquired over that one pixel over time in a CSV file, something that you could very easily read without any knowledge of GIS. And so we're really trying to make this usable to everyone, not just the experts.

HULT:

Right, so you've got all the data and all the ancillary data if somebody really wants to go dig deep and figure out what went into this. You're looking at releasing that stuff, so you have everything that you need if you're a GIS person, but you also have a more sort of simplified version if you're maybe not someone who's super familiar with Arc or the cloud.

CAWSE-NICHOLSON:

We're really trying to hit all levels of expertise.

HULT:

Kerry, do you have any closing thoughts? Anything you want to leave us with on the topic of ET or ECOSTRESS?

CAWSE-NICHOLSON:

Just that ECOSTRESS is something that is really exciting to me because it started off as being a very, very small demonstration product, and we have a very small team. The initial mission was supposed to only last for one year. And now we're heading on four years, and we still anticipate going for much longer. And so ECOSTRESS is really just a great example of how developments in science can benefit the community, and how small teams can make available really useful data. So it's been a mission that I've really been very excited to work on. And I also wanted to mention LP DAAC and EROS, because they've been so great in helping us reach that user community and making this data easily accessible. So it's been a great collaboration.

HULT:

We've been talking with Dr. Kerry Cawse-Nicholson about her work with ECOSTRESS and water balance. It's been a fascinating conversation. Kerry, thank you so much for joining us. 

CAWSE-NICHOLSON:

Thank you so much for having me, John.

HULT:

And a big thank you to the listeners as well for joining us. Be sure to join us next time, to learn more about satellites, remote sensing, land change, and much, much more. You can find all our shows on our website. That's usgs.gov/EROS, U-S-G-S dot G-O-V, forward slash E-R-O-S. You can also subscribe on Apple podcasts or Google podcasts. You can also find us on social media to stay up to date on the latest episodes. This podcast, this podcast, this podcast, is a product of the U.S. Geological Survey, Department of Interior.