Eyes on Earth Episode 17 - Evapotranspiration

JOHN HULT

Hello everyone. My name is John Hult and I'm your host for today's episode of Eyes on Earth, a podcast of the U.S. Geological Survey's Earth Resources Observation and Science Center. Managing water use in the face of competing interests and sometimes scarce availability is one of the most intensive of resource management tasks. Modern managers have come to rely increasingly on satellites like Landsat to help them do the job. Satellites can be especially powerful tools for tracking evapotranspiration, or ET. Here to talk with us about evapotranspiration is Gabriel Senay, a Research Physical Scientist at EROS. Gabriel, welcome to Eyes on Earth. 

GABRIEL SENAY

Thank you.

JOHN

Can you tell us what evapotranspiration is, and how it works?

GABRIEL

Evapotranspiration is a process, it's a hydrological process by which water returns back to the atmosphere. So it's basically plants withdrawing water from soil moisture and releasing it into the air. That's really, like we sweat, that's another form of saying sweat - plant sweat or sweat of the landscape. 

JOHN

And the sweat in the landscape scenario, you're talking about evaporation, that's the landscape, and transpiration is the leaves?

GABRIEL

That's correct.

JOHN

Can you explain in simple terms how your model measures ET?

GABRIEL

So, yeah it goes back to really the process of ET or evapotranspiration, it's a sweat. So, it's a water moving through the human body or through the plant system. Water moves, leaves our body and gives us a cooling effect. So we call it evaporative cooling. The same thing, when plants transpire water, theyíre taking energy, so the plant cools down. So, a landscape or a crop field that transpires the most, cools the most. So it's a cooler, like you put a thermometer in a kidís armpit and you can see the temperature of a body. Satellites, the same way, look at the landscape, if it is a transpiring landscape, it becomes much cooler. 

JOHN

So you measure the temperature of the surface. It's almost like a laser thermometer for a charcoal grill.

GABRIEL

That's correct. As a thermometer points at your head, your forehead, and you measure that, same thing, satellites look at the land surface, and the land surface is emitting energy. If it is a cooling surface, it doesn't emit as much energy. 

JOHN 

I see. But this particular thermometer is 438 miles up.

GABRIEL

That's correct.

JOHN

What can a satellite that high up tell us about how much water a farmer is putting on his cornfield?

GABRIEL

Very good. So there's a good farmer, let's say puts water, the right amount of water at the right time, that landscape is going to be much cooler than a farmer who maybe doesn't water the field very well at the right time. The difference between the two could be as high as 40 degrees F. 

JOHN

Really. From one field to the next?

GABRIEL

Yes. It could be as high as that. So the one that is much cooler would be a farmer, a good farmer, putting the right amount of water, but losing water is not a bad thing, so it depends. The farmer which is losing a lot of water, applying a lot of water most likely is going to get more yield. So loss of water or ET is tied to land biomass, and yield.

JOHN

So the healthier your crops are, the cooler they will...

GABRIEL

That's correct.

JOHN

...as cool as a cucumber. And you get more cucumbers when you're cool.

GABRIEL

That's right. Yeah. So now just by looking at the temperature of the landscape, that tells you how much water is being used.

JOHN

Can you talk to us about how your model has been used to help deal with water rights issues in places like the Upper Klamath River Basin?

GABRIEL

We were asked to monitor a management program, which they called "curtailment" which means they are curtailing the amount of water given to the farmers. So they say well because we want more water to stay in the river and to go to the lake, we're going to put in place an order not to irrigate. So they put in place this program in 2013. So they asked us, can you check this? 

JOHN 

Right.

GABRIEL

And can you come with an estimate of how much water is now available in the river? So we compared 2013 through 2016 the ET of the farms, the evapotranspiration of the farms, irrigated areas, and we compared it to years prior to that. Our analysis showed in the target years, when the curtailment was put in place, the amount of ET was much lower than the base years. So this assured them, one, the curtailment has worked. People obeyed. But even with that, the first year was more obeyed than other years. Subsequent years were less. So the satellite data helps you monitor if even a program like this is working. So our end results showed because of that curtailment and lower ET, the river flow has increased into the lake.

JOHN 

So ultimately you were able to sort of verify that this program was doing the job.

GABRIEL

That's correct.

JOHN

That's not the only place where this has been used of course. President Trump signed legislation in 2019 that allowed the Bureau of Reclamation to oversee a water conservation plan along the Colorado River's lower basin, which is a huge area, right? Several states covered there. How is your ET work important to that plan?

GABRIEL

So yeah, we worked both the lower Colorado and the upper Colorado. Specifically the lower Colorado river basin researchers/managers contacted us to regenerate water use for the lower basin irrigated area.

JOHN

Similar to the Klamath Basin.

GABRIEL

That's right. Before they had it compiled by hand using different methods, but now what they wanted is, well can we use this approach? So they asked us to do this since 1985, actually going back to 1975. 

JOHN

But you weren't doing this in 1975.

GABRIEL 

No...

JOHN

Seems like you probably wouldn't have had your training at that point.

GABRIEL

Exactly. That's very true. That's the nice thing about the historical archive of Landsat, that you can go back and reprocess, and that's exactly what the lower Colorado was interested in, they asked us "can you go back and reprocess historical water use?" So that they understand trends in water use. 

JOHN

Right. And do you have on the tip of your tongue how many hectares or how many acres you're talking about for that entire...

GABRIEL

At least several millions.

JOHN

Several millions.

GABRIEL

Yeah. Two, three million acres of land under irrigation. 

JOHN

I shudder to think how many ground stations you would need to do that job accurately.

GABRIEL

Yeah, that's the question. It's almost impossible to cover this with ground stations. The cost is prohibitive to do this. 

JOHN

Let's move on to another example here. Can you talk at a large scale about how ET work is important in places like the western United States, and in Africa. Dry areas.

GABRIEL

ET is what they call sometimes the biggest part of the water budget. Much of the water globally, 65-75% of that rain, goes back as ET. It's not available for human consumption. So it's very important, especially in the west, dryer land, how much water is used by ET because that tells you, not only its usefulness for crop production, but how much is available for human and industrial water use.

JOHN

Right. So you can decide, we can use this much or that much and we'll still have enough, but if you don't know what you have, you can't make those decisions.

GABRIEL

That's right. It would help you plan, or maybe should we cut down in irrigation, by how much? How many hectares of land should we take out so that so much water would be available for other uses? It's the biggest component of the water budget, which is any water available as a form of rain; most of it will go back. If it is like for example evaporating from a lake, that's useless! That's not available for you. Much of it is unavoidable loss for example in shrub lands and forests, in these are areas Ö we need to have forests, we need to have shrublands, what we're worried about is that fringe, that small amount of water, we have to balance, these are competing needs. Agriculture for growing foods, but also for industry, and for domestic water supply. We can still play with that 5-10% number, which is very critical for domestic water supply. 

JOHN

But can you also work to capture more of this? I'm thinking of ground water, aquifer recharge... is that something that this helps you do?

GABRIEL

That's a good point. So what you do is, rainfall is your source, so you minimize unnecessary losses. Water can be captured by harvesting ground water, in time, put in the ground, in aquifers, or put in reservoirs, deep reservoirs with less surface area so you don't lose it evaporating, if you just put it in small ponds everywhere, within a couple of months you will lose it. So being wise where to capture it, by itself, is very important. You don't want to capture it in the desert in a huge, expansive surface area where you're going to be losing it.

JOHN 

And again, knowing the ET rate gives you, I mean, 100 years ago you could have built a check dam and sort of tried to capture this water, but knowing the ET rate helps you to decide, let's do that here and not there because we know the rate.

GABRIEL

That's correct. Because ET is such an invisible, we don't know until we start understanding, and this is how science has been going for the last 50 years. Think of rainfall, we have a way to measure it. Put a bucket, you can see how much rain, you know, it says 1 inch, 2 inch rain. The same thing with river flow. It's a liquid. You can measure it. Both rain and river flow they are liquid. You know we know how to handle it. This is a gas, and it's invisible to the eye. So satellite data is really helping us understand such an important component of the water budget.

JOHN

Do you work much with other countries on their water use and management? Do you have some examples that you might share?

GABRIEL

Yes we do. ET can be used for drought monitoring purposes, because really the response of the landscape, you may have a lot of rain, but if that rain comes in one day, it really doesn't say much about how crops and vegetation are growing. ET tells you how vegetation is *using* the rain. So we use it for drought monitoring all around the world actually, for much of Africa, Central America, Afghanistan. So this is one of the important tools to monitor drought around the world.

JOHN

We hope you come back for the next episode of Eyes on Earth. This podcast is a product of the U.S. Geological Survey, Department of Interior. 

GABRIEL

Thank you for having me.