You know when we talk about the water cycle,

particularly when you teach the water cycle,

say, in school, you think of that diagram

and it has the clouds dropping snow onto the

mountains and the water flowing off into the

rivers and going into a lake maybe and then

into the ocean. And in many depictions, humans

are absent. We don't see that actually there's

a giant dam next to that lake, and that dam

actually controls how much water is flowing

into the ocean. So in this study we used data

from NASA's new ICESat-2 satellite, which launched

in October of 2018, to try to better understand

how surface water varies around the world.

And so we intersect data from the satellite,

which tells us the water levels. So how much

water is in the lake essentially, with a global

dataset of where surface water bodies are,

and which ones are reservoirs and which ones

are not. Try to better understand how natural water

bodies and how reservoirs vary seasonally

and to be able to understand the impact of

human management on total seasonal variability

of water storage. In my view this study represents

kind of a first quantification of the impact

that humans have on the surface water storage component

of the water cycle. No one has ever been able

to quantify this value before. And what we

find is that reservoirs are significantly

more variable. So the amount of water stored

in them varies a lot more than natural water

bodies. And in particular they're responsible

for 57 percent of the total seasonal water

storage variability on Earth. So in other

words, human management. So you know, humans

modulating the water levels in reservoirs

is responsible for the majority of surface

water variability on Earth. Even though reservoirs

actually account for a very small percentage,

only about 3.9% of the total water bodies

that we observed. And the reason for this

is that ICESat-2 is a laser altimeter, which

means that it provides very high resolution,

really highly accurate observations of surface

water level. This means that we can now observe

the height of very small water bodies, as

small as several hundred or a thousand square

meters. And so in doing this now we can look

at hundreds of thousands of water bodies instead

of a couple hundred water bodies, and also

we know that those observations of water level

are quite accurate. In the future, as we have,

you know, increasing population, we have growing

development around the world and growing industrialization,

we have growing demands on water through agriculture

and we also have climate change, which is

changing the way water is moving around the

world. So when you have all those factors,

it's clear that there's going to be greater

demands on water usage in the future. And

so a study like this can tell us, okay, here

is how water is currently being managed. Here

is how humans are currently controlling the

water cycle.