Estimating Evapotranspiration Trends in the Eastern United States: Genevieve Noyce Mount Holyoke College Research and Discover Program Dr. Lahouari Bounoua Biospheric Sciences Branch (614.4) NASA Goddard Space Flight Center

Evapotranspiration, Vegetation and Climate Evapotranspiration cools air and changes surface water balance Vegetation affects climate through transpiration Variability of vegetation/climate interaction over time Transpiration related to atmospheric CO 2

Physiological Responses Downregulation: decrease in leaf conductance to conserve water Observed in some experiments with elevated CO 2 (Sellers, Bounoua et al. 1996) Model incorporating downregulation predicts less evapotranspiration and higher temperatures (Sellers, Bounoua et al. 1996, Bounoua et al. 1999) Runoff increase more than precipitation Other models showed similar results (e.g. Betts et al. 1997, Nohara et al. 2006) Results confirmed for large basins (Gedney et al. 2006)

Nohara et al., 2006: 19 climate models participating in IPCC AR4 Precipitation: 5.0% increase Runoff: 8.9% increase

Are these models accurately replicating observations? Determine if observed historical trends match predictions Estimate 20 th century evapotranspiration using water budget method “Provide information about future changes in global carbon cycling and terrestrial and aquatic ecosystems for use in ecological forecasting and as inputs for improved climate change projections.” -NASA Carbon Cycle & Ecosystems focus area

Precipitation Runoff Evapotranspiration Aquifer recharge Surface storage

Study Area and Data Sources Precipitation: Dai et al Runoff: USGS WaterWatch Recharge: Wolock 2003, USGS USGS

Runoff Aquifer recharge Withdrawal Base flow Aquifer Recharge Wolock 2003

Precipitation and Runoff

Evapotranspiration

Bounoua et al. Climate Implications CVIBIB-CV Global All land Eastern U.S Surface Temp. °C

Thanks to: Research and Discover Program NASA GSFC Betts, R.A., P.M. Cox, S.E. Lee, and F.I. Woodward Contrasting physiological and structural vegetation feedbacks in climate change simulations. Nature 387: Bounoua, L., G.J. Collatz, P.J. Sellers, D.A. Randall, D.A. Dazlich, S.O. Los, J.A. Berry, I. Fung, C.J. Tucker, C.B. Field, and T.G. Jensen Interactions between vegetation and climate; Radiative and physiological effects of doubled atmospheric CO 2. Journal of Climate 12: Dai, A., I.Y. Fung, and A.D. Del Genio Surface observed global land precipitation variations during Journal of Climate 10: Gedney, N., P.M. Cox, R.A. Betts, O. Boucher, C. Huntingford, and P.A. Stott Detection of a direct carbon dioxide effect in continental river runoff trends. Nature 439: Sellers, P.J., L. Bounoua, G.J. Collatz, D.A. Randall, D.A. Dazlich, S.O. Los, J.A. Berry, I. Fung, C.J. Tucker, C.B. Field, and T.G. Jensen Comparisons of radiative and physiological effects of doubled atmospheric CO 2 on climate. Science 271: Nohara, D., A. Kitoh, M. Hosaka, and T. Oki Impact of climate change on river discharge projected by multimodel ensemble. Journal of Hydrometeorology 7: Slayback, D.A., J.E. Pinzon, S.O. Los, and C.J. Tucker Northern hemisphere photosynthetic trends Global Change Biology 9: United States Geological Suvery WaterWatch. Accessed 16 June Wolock, D.M Base-flow index grid for the conter­minous United States. U.S. Geological Survey Open-File Report 03–263, digital dataset. Accessed 16 July