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You know when we talk about the water cycle,
particularly when you teach the water cycle,

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say, in school, you think of that diagram
and it has the clouds dropping snow onto the

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mountains and the water flowing off into the
rivers and going into a lake maybe and then

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into the ocean. And in many depictions, humans
are absent. We don't see that actually there's

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a giant dam next to that lake, and that dam
actually controls how much water is flowing

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into the ocean. So in this study we used data
from NASA's new ICESat-2 satellite, which launched

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in October of 2018, to try to better understand
how surface water varies around the world.

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And so we intersect data from the satellite,
which tells us the water levels. So how much

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water is in the lake essentially, with a global
dataset of where surface water bodies are,

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and which ones are reservoirs and which ones
are not. Try to better understand how natural water

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bodies and how reservoirs vary seasonally
and to be able to understand the impact of

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human management on total seasonal variability
of water storage. In my view this study represents

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kind of a first quantification of the impact
that humans have on the surface water storage component

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of the water cycle. No one has ever been able
to quantify this value before. And what we

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find is that reservoirs are significantly
more variable. So the amount of water stored

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in them varies a lot more than natural water
bodies. And in particular they're responsible

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for 57 percent of the total seasonal water
storage variability on Earth. So in other

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words, human management. So you know, humans
modulating the water levels in reservoirs

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is responsible for the majority of surface
water variability on Earth. Even though reservoirs

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actually account for a very small percentage,
only about 3.9% of the total water bodies

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that we observed. And the reason for this
is that ICESat-2 is a laser altimeter, which

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means that it provides very high resolution,
really highly accurate observations of surface

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water level. This means that we can now observe
the height of very small water bodies, as

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small as several hundred or a thousand square
meters. And so in doing this now we can look

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at hundreds of thousands of water bodies instead
of a couple hundred water bodies, and also

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we know that those observations of water level
are quite accurate. In the future, as we have,

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you know, increasing population, we have growing
development around the world and growing industrialization,

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we have growing demands on water through agriculture
and we also have climate change, which is

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changing the way water is moving around the
world. So when you have all those factors,

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it's clear that there's going to be greater
demands on water usage in the future. And

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so a study like this can tell us, okay, here
is how water is currently being managed. Here

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is how humans are currently controlling the
water cycle.