With some exceptions, precipitation, water yield from forests, and forest growth and productivity generally increase from west to east across the United States. Shifts in temperature and precipitation associated with climate change may not necessarily alter these general west-to-east trends, but U.S. Forest Service and university researchers do anticipate great variability in how watersheds respond to increasing temperatures and the extreme precipitation and drought events expected in the future.
For a study recently published in the journal Hydrology and Earth System Sciences, researchers compared past (1979-2007) and future (2031-2060) watershed dynamics using a combination of two computer models—the Water Supply Stress Index Model, a tool that predicts water and carbon balances from the watershed to national scale, and the Weather Research and Forecasting (WRF) Model, which simulates atmospheric conditions and allowed researchers to “downscale” global climate change data to make them relevant to U.S. regional areas.
Using a background scenario that assumes that high levels of climate-warming greenhouse gas emissions and related climate impacts will continue into the future, the researchers ran the models for nearly 83,000 relatively small watersheds as well as 18 much larger regions across the U.S.
“When we looked at the U.S. as a whole, we found that future precipitation and temperature could follow a similar west-to-east pattern as the 1979 to 2007 baseline period,” says Ge Sun, a research hydrologist with the Eastern Forest Environmental Threat Assessment Center and a co-author of the study. “But for each watershed, these two climate variables will likely increase or decrease by different magnitudes in the future, leading to different responses in terms of water yield and ecosystem productivity.” Additional co-authors include scientists from the Eastern Threat Center, North Carolina State University, Nanjing University of Information Science & Technology, and Coweeta Hydrologic Laboratory.
The model results project future annual precipitation increases in 82 percent of the watersheds and decreases in others, these last mainly in the Southeast and coastal West. Consistent temperature increases are projected across the nation, with the highest increases in the northwestern and north central United States.
Water yields are projected to increase in 52 percent of the watersheds, mainly in the South and the Northeast, with decreases elsewhere. In 98 percent of the watersheds, ecosystem productivity is projected to increase, especially in the Northeast, while decreases could be seen in northwestern coastal areas.
Researchers caution that the study incorporates land cover data from the year 2006 and that tree mortality is not accounted for, so results may underestimate future impacts to water yield and productivity influenced by human development and tree loss due to natural disturbances. The journal article includes maps and charts of specific findings.
“Climate change patterns and their impacts on water cycles and ecosystem productivity will not be uniform across space or time. Increased water yields in wet months can intensify flooding risks, and decreased yields in dry months can worsen water shortages and impact water quality, for example,” says Sun. “Information from this modeling study can be useful to policy makers and land managers who must prioritize watershed restoration activities and develop measures to mitigate negative impacts of climate change.”
For more information, email Ge Sun at email@example.com.