On National Forests and Grasslands, All Droughts are not Created Equal

During the "top five" droughts, researchers found the highest reductions in water yield on national forests in the West and Southeast. Photo by U.S. Forest Service.
During the “top five” droughts, researchers found the highest reductions in water yield on national forests in the West and Southeast. Photo by U.S. Forest Service.

While some may remember 2002 as the year the United States hosted the Winter Olympics in Salt Lake City, celebrated A Beautiful Mind as the year’s best film, and created the Department of Homeland Security, others may remember it for another reason: drought. That year, more than half of the country was in a state of moderate to severe drought.

In the decade following, drought touched increasing numbers of national forests and grasslands and land area within them. In the future, droughts may not occur more frequently, but U.S. Forest Service researchers say today’s droughts are setting in more quickly and becoming more intense. How does this affect the productivity of national forests and grasslands and their ability to provide fresh water to millions of Americans? What does this mean for the management of these lands in a variable and changing climate?

In a study recently published in Forest Ecology and Management , Forest Service Southern Research Station (SRS) and university researchers modeled the impacts of the five most extreme droughts between 1962 and 2012 across the conterminous United States and estimated their potential impacts on each of 170 national forests and grasslands. They used the Water Supply Stress Index (WaSSI) model (which they evaluated in a companion study) to simulate monthly balances of water and carbon in watersheds during those five decades. The model estimated average ecosystem water use, water yield, and productivity and the changes that might have occurred during the worst cases of drought.

Results varied across the landscape because of land surface differences, including climate and vegetation, as well as drought severity. But researchers found that the “top five” droughts, on average, resulted in a 22 percent reduction in annual precipitation on national forests and grasslands. During these droughts, potential impacts include reductions in ecosystem water use by 8 percent, water yield by 37 percent, and productivity by 9 percent. The highest potential reductions were found in the West and Southeast. By comparison, estimates for the 2002 drought suggest a nation-wide reduction in water yield by 32 percent and productivity by 20 percent.

“Among all the climate extremes, drought is one of the most common and costly disasters,” says Ge Sun, a research hydrologist with the Eastern Forest Environmental Threat Assessment Threat Center who co-authored the study with SRS research hydrologist Peter Caldwell, Eastern Threat Center research ecologist Steve McNulty, and resource information specialist Erika Mack, and partners from North Carolina State University, the Nanjing University of Information Science & Technology and the University of New Hampshire. “Even though future drought conditions may differ from the past, this study highlights potential drought impacts on watershed hydrology and productivity.”

In addition to forest products, wildlife habitat, and recreation, researchers estimate that 14 percent of the national water supply originates on national forests. Drought impacts on these lands have important implications for land managers who must ensure that ecosystem services — especially water — keep flowing from vulnerable watersheds.

“Achieving a ‘win-win’ for both protecting and enhancing forest health and satisfying human needs for water, timber, and other services requires a balanced approach in active forest management,” says Sun. “Our study results provide much-needed information to help managers identify national forests and grasslands to prioritize for special management and restoration.”

For more information, email Ge Sun at gesun@fs.fed.us.

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