Projections of Future Climate and Annual Runoff

Shifting roles of climate drivers

Hydrologists have traditionally relied on historic precipitation data to estimate broad-scale runoff. “Rainfall was always number one,” says USDA Forest Service scientist Ge Sun. “But things are changing and getting more complicated.”

davidson-river
Warmer air temperatures provide more energy for water fluxes between the earth and its atmosphere. USFS photo.

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 USGS study that suggested rising temperatures in the past have not affected overall streamflow patterns in the U.S.

Kai Duan, a post-doctoral research hydrologist at Coweeta, led the research team, which included Steve McNulty, Peter Caldwell, Erika 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 thus 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 (WRRs). WaSSI has been applied in the past to understand changes in regional streamflow and how air pollution may affect future climate and water supply.

Listen to a brief audio clip by author Kai Duan describing this publication. • Text Transcript

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. Where WaSSI projected decreases in runoff, rising temperatures were behind those changes. “Temperature increases so much that it becomes more influential,” says Sun.

Where runoff was projected to increase, precipitation remained the dominant climate influence.

In the eastern U.S., increasing humidity also played a significant role. More humidity in the air could partially offset additional evaporative demand caused by warming and mitigate decreases in runoff.

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.

runoff-change-map
Far-future (2080s) projections show that increasing temperature dominates runoff change over much of the U.S. (areas shaded red, with high probability in darker red). Image courtesy of Kai Duan.

More extreme changes were projected for the far-future, but uncertainty also increased for that projection period. Why look so far out? How are those projections useful for natural resource managers?

“Trees are long-lived organisms and ecological research is long-term by nature” adds Sun. “Understanding large, long-range changes in the carbon and water cycles is critical for our science and helps us be better prepared.”

Read the full text of the article.

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

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