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Shifting roles of climate drivers

Photo: stream running through a forest.
Warmer air temperatures provide more energy for water fluxes between the earth and its atmosphere. Forest Service photo.


—Hydrologists have traditionally relied on historic precipitation data to estimate broad-scale runoff. “Rainfall was always number one,” said research hydrologist Ge Sun. “But things are changing and getting more complicated.”

Sun co-authored a recent modeling study that investigated how other climate factors might influence future changes in runoff. The researchers were intrigued by a U.S. Geological Survey study that suggested rising temperatures in the past have not affected overall streamflow patterns in the United States.

Kai Duan, a post-doctoral research hydrologist at Coweeta Hydrologic Laboratory, led the research team, which included Steve McNultyPeter CaldwellErika Cohen Mack and other partners. Their results were published in Hydrology and Earth System Sciences.

According to Duan, their central question was, “Will climate change affect how temperature influences runoff and water availability?”

Warmer temperatures could increase water loss by evapotranspiration. Without a comparable increase in precipitation, runoff and water available for humans and ecosystems could be reduced.

The researchers used the WaSSI model to estimate annual runoff across the continental U.S. and for large Water Resource Regions. WaSSI has been applied in the past to understand changes in regional streamflow and how air pollution may affect future climate and water supply.

This new analysis with the WaSSI model incorporated a number of climate variables: precipitation, temperature, solar radiation, wind speed and specific humidity. Climate data projections came from the Coupled Model Inter-Comparison Project, or CMIP5.

The team compared a historic baseline (1970–1990) with near-future (2030s) and far-future projections (2080s).

Runoff was projected to decrease across much of the central U.S. and for several WRRs in the South. Areas with rising temperatures were found to be areas where WaSSI projected decreases in runoff. “Temperature increases so much that it becomes more influential,” said Sun. Where runoff was projected to increase, precipitation remained the dominant climate influence.

In regions where less runoff is projected, water conservation strategies may be needed. Decreased water availability could bring increased drought stress, along with higher risk for wildfires and insect and disease outbreaks.