Much attention is directed to assessing how anthropogenic CO2 emissions and climate change impact soil water losses and continental runoff, as reflected in both the Water and Carbon Cycles Science Plans proposed by United States Global Change Research Program (USGCRP) and the 2001 Intergovernmental Panel on Climate Change (IPCC) report. A number of recent studies suggest that continental runoff increased throughout the 20th century despite a rapid increase in water consumption by humans and their activities. The reason for the increase in runoff remains a subject of debate, though it is commonly attributed to either an increase in precipitation (P) or a decrease in evapotranspiration (ET) over the 20th century. While the increase in P can be explained by warming trends, the reduction in ET, especially at sub-continental scales, is more complex. The three plausible explanations for reductions in ET are: (1) Less energy and light input due to solar dimming with lower light levels reducing mean stomatal conductance to water vapor (gc), (2) lower gc due to elevated atmospheric CO2, and (3) land-use change to vegetation that consumes less water. The interplay between these three mechanisms can be explored on a number of scales ranging from the ecosystem level to watershed to sub-continental region. Using a combination of ecosystem models and detailed field experiments, we will investigate how solar dimming, increases in atmospheric CO2, and increases in forested area alter water availability and gross ecosystem CO2 exchange in the Southeastern (SE) U.S., a region that is considered among the most productive in the U.S. in terms of carbon sequestration. The SE provides an ideal case study due to rapid afforestation (and reforestation) over the past 100 years and the minor change in precipitation over the past 50 years. The project's intellectual merit is to elucidate the mechanisms leading to global runoff increases over the past 50 years, and to assess whether runoff time series contain a discernable signal of climate change. Recognizing that carbon sequestration will play an increasing role in regional and national policy in the future, and that water resources currently play a major role, the broader impact of this project is to contribute the scientific foundation, data, and models that can guide ecosystem valuation for C-H2O tradeoffs upon conversion among land cover types. The educational benefit of the project is to support two graduate students, providing them with a unique experience in the state of the art techniques in measurement and modeling of biosphere-atmosphere exchange rates while interacting with a broad interdisciplinary team of physical and biological scientists working on water and carbon cycling at Duke University.
