[dramatic inspiring music] - Water is fundamental to everything that we do as a society, as a planet.

- [Narrator] Most people would agree with Dr. Tamlin Pavelsky.

Water is important.

We need it to drink, we need it for food, and we need to know when it just might be a threat to us and where we live.

- But surprisingly, we don't know very well how much water we have in different parts of the world.

And maybe just as importantly, we don't know how that's changing.

- [Announcer] Two, one, engine ignition, and liftoff.

Liftoff of SWOT, our first global service... - [Narrator] In December of 2022, NASA, in partnership with the French Space Agency CNES, and other nations launched a satellite to answer that basic question about water.

It's called the Surface Water and Ocean Topography mission, or SWOT.

For three and a half years, SWOT will measure all the water on the planet.

- [Tamlin] SWOT is gonna give us a completely new picture of where the world's water is and how it changes over time.

- [Narrator] To find out how much water there is on earth, SWOT will measure its topography.

That's right, it's gonna study the height of those bodies of water.

And scientists don't have a full understanding of how much water is flowing or where it's going.

And it's not just rivers and lakes, it turns out the ocean has topography too.

- There are places in the ocean that are a little bit higher, places that are a little bit lower, and that actually has a lot to do with ocean currents.

So for example, here in North Carolina, we have the Gulf Stream offshore, and you can have a difference in elevation easily of a meter from one side of the Gulf Stream to the other side of the Gulf Stream.

That's ocean topography that we can hopefully detect from space.

- [Narrator] For the mission to succeed, SWOT must take lots of very precise pictures.

That's a challenge.

Pavelsky says the measurements on the satellite must be just as exact as those on the ground.

- [Tamlin] So SWOT is a radar satellite mission.

So it's sending radar waves down from space, bouncing them off of the earth, measuring how much comes back and how long it takes to come back.

And that in combination can tell us the topography of all of the world's surface water.

[casual contemplative music] - [Narrator] He and other scientists have been validating data since launch, measuring water height on the ground in spots like New Zealand.

- [Tamlin] Over a certain area of river, we have to have a certain accuracy in terms of the the elevation of the water surface in terms of how well we measure that, and that accuracy is 10 centimeters, right?

So that's on the order of four inches.

What we have to do is we have to go into the field and use really high precision GPS equipment to make those same kinds of measurements.

- [Narrator] You could say Pavelsky started preparing for the SWOT mission as a child.

He grew up just north of the Denali National Park in Alaska with no electricity, running water, or even a phone.

He says he had an acute awareness for change, particularly in Arctic rivers.

So he went to school to learn about it and found SWOT.

- I went to my first meeting about SWOT in 2004 when I was a graduate student.

So from 2004 to 2022, that's 18 years.

So this is almost like raising a kid.

And then we, you know, rather than sending it off to college, we send it off to space.

It's, you know, please phone home, please send data.

[dramatic inspiring music] - [Narrator] Pavelsky says SWOT is the most advanced satellite of its kind.

And when the mission is completed, SWOT will have measured the water height of around 6 million bodies of water on the planet.

But it's not just about telling how much water we have.

Data from SWOT will be able to tell us about hazards as well, like flooding, easily the deadliest natural disaster we have.

- [Tamlin] We want to do a good job of being able to understand where we have flooding, how much flooding we have, how deep it is, and do a better job of predicting the future.

We need to have exactly the kind of data that SWOT is gonna provide.

- [Reporter] More than 17,000 people are being evacuated in the region.

- [Narrator] SWOT got its first test in early June of 2023 when news outlets worldwide were reporting on the sudden failure of a major dam in southern Ukraine.

It had just unleashed flood waters, inundating large swaths of urban and rural areas.

Darker purple colors mean lower elevation.

Yellow and green mean higher.

- [Tamlin] In early June of 2023, the Kakhovka dam, which is a big dam on the Dnieper River in Ukraine, collapsed catastrophically.

We were lucky enough that the SWAT satellite was passing over this area every day during this time period.

And so we actually captured what this river looked like beforehand, and then what the flood wave looked like on multiple different times.

This is an image from June 4th, which was before.

You can see that the river is relatively flat, and there's definitely some wetlands.

A few days after the dam broke, on June 8th, it looks pretty different.

You can really see a lot of the flooding going on here.

These green colors are about five to 10 meters higher.

- [Narrator] A new United Nations report points to the importance of SWOT.

The study shows 1/4 of the world's population lack access to safe drinking water, and nearly half don't have access to basic sanitation.

[dramatic inspiring music] Pavelsky hopes SWOT can make a difference.

- [Tamlin] We depend on it for drinking.

We depend on it for irrigating agriculture, we depend on it to keep our ecosystems healthy.

Satellite data has been so incredibly important in making discoveries about our planet.

[dramatic uplifting music]