How is climate change impacting the water cycle?

How is the water cycle affected by change in climate over time? To find out, scientists at the Coweeta Hydrologic Laboratory have been recording data from a western North Carolina watershed for more than 80 years. And the data provides valuable information on the effects of climate change

TRANSCRIPT

Most elementary-school students learn about the hydrologic cycle -- the circulation of water from the atmosphere to the Earth and back again.

How is this cycle affected by change in climate over time?

To find out, scientists at the Coweeta Hydrologic Laboratory have been recording data from a western North Carolina watershed for more than 80 years, and the data provides valuable information on the effects of climate change.

Let's take a look.

The rain that falls outside your window today is the same rain that watered the grounds and filled the streams and rivers millions of years ago, during Earth's earliest days.

That's right -- the rain that was still is.

So, we're interested into quantifying that, seeing how much it changes, depending on the environmental conditions.

That's because rain -- actually, water -- is vitally important to every living thing on the planet.

And water is constantly cycling from Earth to sky and back again.

So, water in streams important, obviously, right, for not only the fish and the critters that live in the stream but also our water supplies.

It's called the hydrologic cycle.

Let's follow this drop to see what happens.

When the little water droplet becomes heavy enough... ...it falls from the clouds as snow or rain.

Some of the water washes over the ground and flows into streams and rivers and eventually into lakes and even the ocean.

It evaporates back into the atmosphere, turning back into a cloud.

Water droplets can also fall to Earth and soak into the ground and then be taken up by trees and plants.

It's later released into the atmosphere through transpiration.

But we're only now beginning to understand the specifics of the hydrologic cycle, thanks in part to the Coweeta Hydrologic Laboratory in western North Carolina.

The U.S. Forest Service bought the 5,600-acre forest and set it aside as an experimental forest back in 1934.

The primary focus is on rainfall, stream flow, and how the forest uses water.

One of the main research tools is a weir.

Again, it's called a weir, and this is one of the most precise ways you can use to measure how much water is coming down a stream.

A weir resembles a dam, but it's more than that.

It's a stream-gauging station built across a watershed.

32 weirs are built on various watersheds throughout the Coweeta Basin.

So, this wall extends all the way down to bedrock and extends all the way into the hillslope on one side of the stream.

And we can see it crosses the road and goes into this other hillslope.

And so the idea is that you're taking all the water uphill of this weir, whatever's coming down the stream and whatever's moving in the shallow groundwater, and you're forcing that water to come over this what we call a blade -- this opening of the weir -- to quantify every bit of water leaving this watershed.

You see the pond upstream of the wall.

That pond is connected through pipes to a well inside of our gauge house.

And so the height of the water inside this well is exactly the same as the height of the water in this pond.

So, in this well, we have a float sitting on top of that water that's going up and down with the level of the water, and we record that with a time chart in the gauge house.

And knowing the height of that water and the geometry of this weir, there's been experiments done to calculate flow as a function of the height of that water.

So it's an equation -- a statistical equation that's calculating flow as a function of the height of the water.

Measuring stream flow gives a more accurate view of the water flowing through a watershed because rainfall varies over an area.

And the weir system allows the watershed to be measured day and night, through storm and sunshine.

But here, we're measuring the height of this water to within 1 millimeter of its actual value.

Readings have been taken every five minutes since 1934.

That's roughly 200 million bits of data.

And that consistency is important.

The strength in what we do here is that we've got continuous measurements from one place over time with the same method.

It's very consistent, very robust, and what we've seen over time is not that it's getting -- not that our mean is shifting up or down -- so it's not getting uniformly wetter or uniformly drier -- but our dry years are getting drier, and our wet years are getting wetter.

The long trend of data shows the Coweeta Basin still receives about the same amount of rain each year it's been getting for the past 80 years.

But it also reveals that the dry years are getting drier, which means periods of drought are becoming more common and more severe.

It also shows that wet periods are getting wetter, what many people call a torrential downpour with flooding.

All of that will affect what plants, trees, and eventually animals live in the forest.

So, we expect, in the future, as our climate becomes more variable, we have more drier and more prolonger droughts, that we will see some species suffer as a result of that.

So our species composition is gonna be continuously changing.

To understand what those changes mean for the forest, dozens of experiments are studying sap flow in individual trees, soil moisture, and how the mineral content in the soil is changing.

This instrument-laden tower measures just how much carbon the forest is taking in and releasing.

The leaves, during the daytime, are doing photosynthesis.

So, carbon dioxide enters the leaf, it gets transformed into a carbohydrate, into sugar, but at the same time, those leaves need to metabolize.

They're like all of our cells.

They basically have a certain cost of doing business.

So they're breaking down a certain amount of carbon, releasing that back as carbon dioxide.

It takes a long time for these changes to manifest.

Especially given the variability we have in rainfall, you have to have a lot of data to be able to tease out a trend.

The forest moves slowly.

It takes a long time for a forest to change, and so you can't do this in 10, 20 years.

You have to have a long-term record to do it.