Daily Precipitation Patterns Are Changing at Coweeta

Since 1950, heavy rains have become more common in the southern Appalachians. U.S. Forest Service researchers have witnessed such changes at Coweeta Hydrologic Laboratory.

Coweeta was established in 1934. Its location in the mountains of western North Carolina is no accident – early Forest Service researchers strategically selected it.

“Coweeta receives as much as 90 inches of rain each year,” says SRS researcher Chelcy Miniat. “The topography is steep, and the bedrock is solid. We can account for most of the rainfall that enters the basin.”

These traits make Coweeta an ideal place to study forested watershed hydrology. The current study builds on a long research history, but is closely tied the more recent past – a study published in 2012.

“In 2012, we showed that summers have become drier, while autumns are now wetter,” says Miniat. “That study showed that average annual rainfalls are stable over time, but wet years are getting wetter, and dry years are getting drier.” SRS biological scientist Stephanie Laseter led the 2012 study, which was published in Hydrology Research.

Storm Cell over Southern Appalachians
Storm cell over the southern Appalachians. Intense storms have become more common. NASA.

Tim Burt, an emeritus professor at Durham University in the UK, led the new study. It builds upon the previous one in several important ways: it explores the timing of intense storms and the effect of higher altitudes, and it ties precipitation trends at Coweeta to global atmospheric patterns, or teleconnections. The study was published in the International Journal of Climatology.

Teleconnections are atmospheric patterns that span the oceans and connect distant countries. Teleconnections are often measured in pressure differentials, and they can affect the weather.

The scientists used indices describing several teleconnections, such as the Bermuda High Index (BHI). The Bermuda High drifts over the Atlantic Ocean, migrating east and west with the seasons, and the BHI describes this. Over the past few decades, the location and intensity of the Bermuda High have changed.

“We compared the indices to long-term precipitation trends at Coweeta,” says Miniat. The precipitation trends were calculated with 75 years of daily precipitation data collected at Coweeta. The data were collected with four rain gauges, located at various altitudes.

Miniat and her colleagues focused on precipitation trends over time. A separate study, which was also published in the International Journal of Climatology, focused on spatial trends.

“The Bermuda High strongly influences precipitation at Coweeta,” says Miniat. “Its effect is strongest in the fall. Long-term rainfall trends at Coweeta reflect changes in the Bermuda High and other global circulation patterns,” says Miniat.

Miniat and her colleagues found that the North Atlantic Oscillation also affects weather at Coweeta. Its effects are strongest in the summer.

Mountains also affect precipitation patterns. “At Coweeta, as you go from the valley floor to the top of the ridge, there’s a 30 percent increase in precipitation due to elevation alone,” says Miniat. “Rainfall at higher elevations is characterized by higher rainfall intensity.” The phenomenon is called orographic enhancement.

Coweeta weir
Precipitation patterns at Coweeta have shifted, and the changes could affect forest productivity. Photo by Sarah Farmer.

Double orographic enhancement refers to an amplified mountain effect, caused by variations in the North Atlantic Oscillation. The researchers documented double orographic enhancement, although some relationships were not as strong as expected.

“Our results reflect Coweeta’s relatively southern location, its exposure to warm, moist tropical air masses, and the steep topography and high elevation,” says Miniat.

The results also confirm the trend towards more frequent and intense fall storms, along with drier summers. Trees need water in the growing season, and at Coweeta, research has shown that the number of small storms per year is the best predictor of forest growth.

Scientists are concerned about how shifts in precipitation will affect forest productivity. “We know there will be an effect,” says Miniat. “We just don’t know how big the effect will be.” Whatever happens, researchers at Coweeta will be watching and learning, as they have been for the past 84 years.

Read the full text of the study.

For more information, email Chelcy Miniat at cfminiat@fs.fed.us.

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