The Future of Streams: Using Air Temperature to Model Stream Warming

Cosby Creek runs through the Great Smoky Mountains National Park. Stream temperatures in the Appalachian ecoregion have increased, and will probably increase in the future. Photo by Stewart Tomlinson, US Geological Survey.
Cosby Creek runs through the Great Smoky Mountains National Park. Stream temperatures in the Appalachian ecoregion have increased, and will probably increase in the future. Photo by Stewart Tomlinson, US Geological Survey.

Stream temperatures affect the health of aquatic animals as well as many other biological and ecological processes. However, finding out whether – or how much – streams are warming has been difficult, as long-term temperature data does not exist for many waterways.

A new U.S. Forest Service Southern Research Station (SRS) study shows that long-term historic air temperature data can be coupled with short-term stream temperature to predict future warming in streams.

The study was led by Peter Caldwell, a research hydrologist at the SRS Coweeta Hydrologic Laboratory, and recently published in the journal Hydrological Processes. The SRS Eastern Forest Environmental Threat Assessment Center provided funding for the study.

“Predicting climate change impacts on stream temperature is extremely uncertain,” says Caldwell. “However, we demonstrated that simple, site specific, linear regression models can be developed using short-term stream temperature observations and can be useful for estimating historical and future changes in stream temperature.”

Caldwell and his colleagues showed that the models developed using as few as 18 months of stream temperature observations could explain variability in stream temperature for up to 37 years. They then modeled historical stream temperatures at 61 sites in the Southeast from 1961 to 2010, as well as future temperatures for 2011 to 2060.

Stream temperature is affected by the amount and speed of water flowing through the stream, the amount of groundwater feeding into the stream, the canopy cover over the stream and nearby areas, and other factors. Many of these factors are relatively permanent, although streams can be affected by urbanization and dams. All 61 streams Caldwell and his colleagues studied were undammed, and represented both large and small watersheds from across the southeastern U.S.

“Our study suggests that stream temperatures over the historical period have already increased at many sites in the southeastern U.S.,” says Caldwell.  Streams in the Appalachian ecoregion, which includes parts of Alabama, Georgia, North Carolina, Tennessee, Kentucky, and Virginia, were predicted to be amongst the most vulnerable to climate change.

“This could have significant consequences for coldwater fish species endemic to this region, such as Eastern brook trout,” says Caldwell. “However, predictions of habitat loss are thought to be overly pessimistic because some suitable habitats may persist under climate change in localized landscape conditions.” The study results suggest that streams will continue to warm through 2060. The magnitude of change will also increase with sites in the Appalachians most impacted and sites in the Southeastern Coastal Plain least impacted by climate change.

“Because of the stable relationship between stream and air temperature, stream temperature models can be developed with minimal stream temperature observations,” says Caldwell. “Until now, it has been very difficult to evaluate potential climate change impacts on stream temperature because there are few long-term regional stream temperature datasets available, but we think short-term stream temperature observations and readily available air temperature estimates can be used by resource managers to develop stream temperature models for rapid assessment of potential climate change impacts.”

Read the full text of the article.

For more information, email Peter Caldwell at pcaldwell02@fs.fed.us

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