OTTO — Rain may be inconvenient; no one likes to get drenched and be in cold, wet clothes; but most people find at least the sounds of rain shower soothing. And it’s hard to beat that just-rained clean smell when you walk outside. The sounds, smells, inconveniences—they're all things that come to mind when it rains. But something we don't often think about, is that the rain that falls outside your window today are the same rains that watered the grounds and filled the streams and rivers millions of years ago during the Earth’s earliest days.
That’s right, the rain that was, still is.
“And we’re interested in quantifying what happens to all of that rain and seeing how it changes over time, depending on the environmental conditions,” explains Chris Oishi, Ph.D., a Research Ecologist with the Southern Research Station, Coweeta Hydrologic Laboratory, United States Forest Service.
And rain is constantly cycling from Earth to sky and back again.
“Water is obviously important not only for the fish and critters that live in the stream, but also for our water supply,” adds Peter Caldwell, Ph.D., Research Hydrologist with the Southern Research Station. “So we need to know what happens to it.”
Water's journey is called the hydrologic cycle.
Water falls from the clouds as snow or rain. Some of the water flows into streams and rivers and then the ocean. It then evaporates back into the atmosphere. Some water is taken up from the ground by trees and plants and then released into the atmosphere through transpiration, while other water molecules are not taken up, but filter through the soil and into the groundwater. It will take hundreds of years for this water to return to the cycle.
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 56-hundred acre forest and set it aside as an experimental forest in 1934. The primary focus at the lab is studying rainfall, streamflow and how the forest uses water.
The most precise way to measure how much water is coming out of a watershed is by using a weir. It resembles a dam, but it’s more than that. It’s a stream gauging station.
“This wall goes all of the way down to bedrock and it also stretches from the hillside on one side of the stream across the road to the hill slope on the other side," explains Caldwell, as he points to the wall and then gestures, showing the general outline of the rest of the structure. “So the idea is you’re forcing all the water uphill of the weir, what is coming in the stream, and in the shallow groundwater to go over the blade, which is the opening of the weir.”
The goal is to quantify all of the water leaving the watershed by forcing it through the opening in the wall. But the genius of the weir lies in the pond, which sits in just upstream of the wall.
“The pond is connected through pipes to a well in the gauge house, which is the small building that sits next to the pond,” Caldwell continues his explanation, walking from the edge of the opening in the wall towards the gauge house. The house is made of wood and is no larger than a porta-john. In fact, it looks a bit like on old school outhouse.
“That connection means the water level in the pond is the same as the well,” Caldwell continues. “And there is a float in the well, that goes up and down, and we record that with a time chart in the gauge house.”
Caldwell opens the door to the gauge house and sure enough, there is a wire leading from a graphing device across a small table and it drops down into a well in the floor.
“Knowing the height of the water and the geometry of the weir, there have been experiments done to calculate flow as a function of the height of the water,” explains Caldwell. “What makes it work so well is that the height of the water can be measured to within one millimeter of its actual value.”
Measuring streamflow gives a more accurate view of the water flowing through the watershed because rainfall varies over an area. And the weir system allows the watershed to be measured day and night, through storm and sunshine. Those measurements have been taken every five minutes since 1934. That’s roughly 200 million bits of data. That consistency is important.
“The strength of what we do here is that we’ve got continuous measurements from one place, over time, with the same method,” says Chelcy Miniat, Ph.D., Project Leader at the Southern Research Station. “Because of that consistency, it doesn’t fall under criticism that you’ve moved your climate station, or you’ve changed methods. It’s consistent and it’s robust.
And what that long trend of data shows is that the 5,600 acre Coweeta watershed is still getting roughly the same amount of rain each year that it has been getting during the past 80 years. But it also reveals that the dry years are getting dryer, 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.
“So we expect that in the future, as our climate becomes more variable and we have longer and deeper droughts, we will see some species suffer,” adds Caldwell. “The species mix in the forest will be changing.”
To understand those changes, dozens of experiments are spread throughout the forest measuring the flow of sap in individual trees, soil moisture and how the mineral content in the soil is changing. There is also a huge tower poking up through the tree canopy. The instrument-laden structure measures how much carbon the forest is taking in and releasing.
When it is all compiled and analyzed, the long stretch of data provides a window into the Coweeta watershed’s history, health and future.
“It takes a long time for these changes to manifest, especially given the variability of rainfall, so you have to have a lot of data to tease out a trend,” says Caldwell, who is justifiably proud and amazed at the amount of high quality information the lab has recorded. “That long term data is invaluable because you can’t look at climate change without those long records. The forest moves slowly, it takes a long time for a forest to change, and you have to have a long term record to do it.”