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.