Climate, population, and vegetation cover change impacts on water supply and demand, carbon sequestration, and biodiversity across the conterminous U.S. (WaSSI model)

USDA Forest Service Southern Research Station

SUMMARY: Ecosystems and natural resources across the United States have been increasingly stressed over the past decades, mainly due to population growth and climate change and variability. Quantifying changes in ecosystem processes and ecosystem services under a changing environment is critical for management decisionmaking. Eastern Threat Center scientists are developing an integrated model to estimate ecosystem water and carbon balances and the interactions among ecosystem evapotranspiration (water use), productivity, carbon sequestration, and biodiversity at the continental scale by coupling the key processes of the hydrologic and carbon cycles. This integrated, water-centered modeling system is being built upon previous water supply and demand research that resulted in a Water Supply Stress Index (WaSSI) model, which can be used to examine potential impacts of climate, land use, and population changes individually or in combination.

EFETAC's ROLE: Eastern Threat Center scientists are conducting research and developing the model.

STATUS: Ongoing

PROGRESS: Researchers are synthesizing literature from international carbon flux network data and regional relationships among climate and ecosystem processes to develop a new model, WaSSI-Carbon and Biodiversity (WaSSI-CB), which is being applied in the United States and internationally in Africa, Asia, and North America.

In 2013, researchers updated a web-based version of the WaSSI model to include enhanced features: 1) English and Spanish user guides; 2) geographically relevant maps in user-friendly formats; and 3) expanded climate and land use change options, information, and future scenarios for the United States, Mexico, and east African countries Rwanda and Burundi.

The WaSSI model has been improved in its capability to simulate watershed water balances at a much finer spatial scale, the 12-digit Hydrological Unit Code level (88,000 watersheds across the lower 48 States). Researchers applied the model to examine the impacts of historical droughts and future climate change on water yield and productivity of 170 National Forests and Grasslands as well as potential forest thinning options to sustain water supplies under a changing climate.









Differences in WaSSI modeled mean annual evapotranspiration (a and b) and water yield Q (c and d) between the top-five droughts and the period 1962-2012 for 170 National Forests and Grasslands. Click to enlarge.

The refined WaSSI model was applied to examine the potential effects of forest biomass removal on water yield across the United States. Simulations suggest that all scenarios would have minor impacts on water quantity at the county level due to the small areas of harvesting (<5%) in most counties. However, local impacts can occur.

Sun, G.; Zhang, L.; Duan, K.; Rau, B.. 2017. Impacts of forest biomass removal on water yield across the United States. In: R. A. Efroymson, M. H. Langholtz, K.E. Johnson, and B. J. Stokes (Eds.), 2016 Billion-Ton Report: Advancing Domestic Resources for a Thriving Bioeconomy, Volume 2: Environmental Sustainability Effects of Select Scenarios from Volume 1. ORNL/TM-2016/727. Oak Ridge National Laboratory, Oak Ridge, TN.   doi:10.2172/1338837

Duan, K., G. Sun, S. Sun, P. V. Caldwell, E. C. Cohen, S. G. McNulty, H. D. Aldridge, and Y. Zhang. 2016. Divergence of ecosystem services in U.S. National Forests and Grasslands under a changing climate. Scientific Reports 6:24441. (PDF)

Sun, S.-L., G. Sun, P. Caldwell, S. McNulty, E. Cohen, J.-F. Xiao, and Y. Zhang, 2015, Drought Impacts on Ecosystem Functions of the U.S. National Forests and Grasslands: Part I. Evaluation of a Water and Carbon Balance Model, Forest Ecology and Management, 353:260–268. (PDF)

Sun, S.-L., G. Sun, P. Caldwell, S. McNulty, E. Cohen, J.-F. Xiao, and Y. Zhang, 2015, Drought Impacts on Ecosystem Functions of the U.S. National Forests and Grasslands: Part II Model Results and Management Implications, Forest Ecology and Management, 353:269-279. (PDF)

Sun, G., P.V. Caldwell, and S.G. McNulty. 2015. Modeling the potential role of forest thinning in maintaining water supplies under a changing climate across the conterminous United States. Hydrological Processes 29:5016-5030. (PDF)

Caldwell, P., C. Muldoon, C.F. Miniat, E. Cohen, S. Krieger, G. Sun, S. McNulty, and P.V. Bolstad. 2014. Quantifying the role of National Forest System lands in providing surface drinking water supply for the Southern United States. Gen. Tech. Rep. SRS-197. Asheville, NC: U.S. Department of Agriculture Forest Service, Southern Research Station. 135 p. (PDF)

Tavernia, B.G., M.D. Nelson, P. Caldwell, and G. Sun. 2013. Water stress projections for the Northeastern and Midwestern United States in 2060: anthropogenic and ecological consequences. Journal of the American Water Resources Association 49(4): 938-952. (PDF)

Caldwell, P.V., G. Sun, S.G. McNulty, E.C. Cohen, and J.A. Moore Myers. 2012. Impacts of impervious cover, water withdrawals, and climate change on river flows in the conterminous US. Hydrology and Earth System Sciences 16:2839-2857. (PDF)

Caldwell, P.V., G. Sun, S.G. McNulty, E.C. Cohen, and J.A. Moore Myers. 2011. Modeling impacts of environmental change on ecosystem services across the conterminous United States. In: Medley, C.N., G. Patterson, and M.J. Parker (eds). Observing, studying, and managing for change: Proceedings of the Fourth Interagency Conference on Research in the Watersheds: U.S. Geological Survey Scientific Investigations Report 2011:5169, 202 p. (PDF)

Sun, G., K. Alstad, J. Chen, S. Chen, C. R. Ford, G. Lin, N. Lu, S. G. McNulty, A. Noormets, J. M. Vose, B. Wilske, M. Zeppel, Y. Zhang, and Z. Zhang. 2011. A general predictive model for estimating monthly ecosystem evapotranspiration. Ecohydrology 4(2):245-255. (PDF)

Sun, G., P. Caldwell, A. Noormets, S.G. McNulty, E. Cohen, J. Moore Myers, J.-C. Domec, E. Treasure, Q. Mu, J. Xiao, R. John, and J. Chen. 2011. Upscaling key ecosystem functions across the conterminous United States by a water-centric ecosystem model. Journal of Geophysical Research 116:G00J05. (PDF)

Sun, G., S.G. McNulty, J.A. Moore Myers, and E.C. Cohen. 2008. Impacts of climate change, population growth, land use change, and groundwater availability on water supply and demand across the conterminous U.S. Watershed Update, May-August, Vol. 6, No. 2. (PDF)

Sun, G., S.G. McNulty, J.A. Moore Myers, and E.C. Cohen. 2008. Impacts of multiple stresses on water demand and supply across the southeastern United States. Journal of American Water Resources Association 44(6):1441-1457. (PDF)

McNulty, S.G., G. Sun, E.C. Cohen, and J.A. Moore Myers. 2007. Change in the southern U.S. water demand and supply over the next forty years. Book Chapter 5. In: Wetland and Water Resource Modeling and Assessment: a Watershed Perspective. W. Ji (ed). P 43-56. CRC Press.

Sun, G., S.G. McNulty, E. Cohen, J. Moore-Myers, and D. Wear. 2005. Modeling the impacts of climate change, land use change, and human population dynamics on water availability and demands in the Southeastern U.S. ASAE Paper No. 052219. St. Joseph, Mich.: ASAE.

McNulty, S., G. Sun, and J. Moore Myers. 2004. Climate, Population, and Vegetation Cover Change Impacts on Water Yield and Demand Across the Southern US. In: Geographic Information Systems and Water Resources III: AWRA Spring Specialty Conference. Nashville, TN. May 17-19, 2004.


Water Supply Stress Index

WaSSI fact sheet

Related articles from CompassLive:

Ge Sun, Eastern Threat Center Research Hydrologist, or 919-549-4070

Updated June 2017

Document Actions
Personal tools

For the latest up-to-date ag webinars on all things agriculture, visit the Agriculture Webinars Portal