More Runoff - But Why?

August 15, 2007

Water Runoff

Even though humans are using more water than ever, continental water runoff steadily increased in the 20th Century. Competing scientific explanations abound. Some argue that global warming is causing more rainfall than the soil can absorb. Others contend runoff is a result of less overall transpiration by plants due to global change.

Environmental engineering Associate Professor Amilcare Porporato, a specialist in ecohydrology, wants to determine whether evapotranspiration has decreased and why. Using the Southeastern region of the U.S. as his test bed, Porporato is exploring the interplay of three major climate variables: a decrease in sunlight passing through the Earth’s atmosphere called solar dimming that may reduce plant transpiration; increased carbon dioxide which makes photosynthesis more effective (and thus with lower water needs); and significant changes in land use (changes in the fraction of natural, agricultural, and urban areas) that affect how much water is stored in the soil or transferred to streams and water bodies by runoff.

Did you know?

Non pointsource pollution is caused by rainfall or snowmelt moving over and through the ground carrying pollutants to lakes, river, wetlands, coastal waters, and even our underground sources of drinking water. This is the leading cause of water quality problems. Source: U.S. Environmental Protection Agency.

Working in the Duke Forest in collaboration with Professor Gaby Katul from Duke’s Nicholas School of the Environment and Earth Sciences, Porporato is analyzing grasslands, pine forest and old hardwood forests in order to develop a regional model of how rainfall is partitioned into evapotranspiration and runoff. He will then adopt an inverse approach using streamflow data as an integrated measure of runoff over the entire Neuse River Basin in North Carolina. The research will look into the fundamental link between climate change and the water and carbon cycles and help quantify the role of the Southeastern U.S. in carbon sequestration.

Through a better understanding of the relationship between water evaporation, stream flow, evapotranspiration and photosynthesis, Porporato hopes his research will provide insight on how evaporation and stream flow jointly explain adjustments in the water cycle as the climate changes. The quantification of future water availability for ecosystems and society is essential, practical information needed by decision makers charged with managing the tradeoffs inherent in changes to land use.

More information:

Amilcare Porporato Website:

Gaby Katul Website:

Nicholas School for the Environment and Earth Sciences:

Neuse River Basin: