Deliver Benefits to the Public

Following the trail of rainfall in the Southern Appalachian Mountains

Flowers on the mountain
Leaves are just one intermediary between precipitation and the soil. Other parts of the forest, like tree bark and forest floor litter, also acts as points of rainfall interception. Roan Mountain, North Carolina-Tennessee state line. Photo courtesy of Ken Lane.

NORTH CAROLINA – Millions of people depend on the forests of the Southern Appalachian Mountains for drinking water. As climate, land use and land cover changes alter the forest structure in these mountains, they also alter water budgets.

“The Southern Appalachian Mountains are a humid, montane environment – they are essentially a cooler version of the tropics,” explains USDA Forest Service project leader and research ecologist Chelcy Miniat. “With so much rain coming in, it is important to understand how the forests use the water, especially when the remainder becomes the South’s drinking water supply.”

A forest's water budget can be thought of as the precipitation in and streamflow out, more technically known as the inputs and outputs of the system. It’s influenced by several factors: climate, evapotranspiration and soil infiltration. These factors can vary by tree species, age and overall forest structure.

As researchers attempt to understand the forest water budget, it is necessary to consider how every step of the water cycle is affected by the variation across forests. In a recent study, Miniat and colleagues looked specifically at interception – the water from precipitation that does not make it into the soil because it is evaporated from plant or litter surfaces instead.

The study was led by Steven Brantley, formerly a post-doc with both the Forest Service and the University of Minnesota and was published in the scientific journal Ecohydrology.

“Existing models treat interception estimates very simply. And though it is just one piece of the story, it is important to properly understand how the age and structure of a forest will affect the amount of water intercepted by trees, plants, and floor litter,” said Miniat.

The study included stands from 2 to 200+ years old and used a series of water capture methods and models to approximate interception.

“We tested our models to see how interception could be predicted by the stand age, the basal area of the stand, and the leaf area index,” says Brantley. “The basal area is a measurement of the land area occupied by tree trunks at their base and the LAI is a measurement of canopy cover.”

Stand age was actually the best predictor of interception; interception increased with the age of the stand. In other words, older trees saw more precipitation lost to evaporation than did younger ones.

This research is one step in building the knowledge to develop species-specific models of forest water use within the southern Appalachian Mountains. And this knowledge is an important component of understanding human impacts on natural forests.