The Water Supply Stress Index Model (WaSSI) is a web-based tool that can be used to project the effects of land use change, climate change, and water withdrawals on river flows, water supply stress, and ecosystem productivity (i.e. carbon sequestration dynamics) across the conterminous United States, Mexico, Rwanda, and Burundi. As water yield and carbon sequestration are tightly coupled, WaSSI is useful for evaluating trade-offs among management strategies for these ecosystem services.
How does WaSSI Work - The core of WaSSI is a water balance model that is sensitive to land cover and climate, and operates on a monthly time step at the 8-digit Hydrologic Unit Code (HUC) watershed scale across the conterminous U.S, at the 0.5 degree grid resolution in Mexico, and at the watershed scale in Rwanda and Burundi. Gross ecosystem productivity, ecosystem respiration, and net ecosystem carbon exchange are estimated using actual evapotranspiration. For the U.S., annual US Geological Survey (USGS) water demand estimates are adjusted for population, disaggregated to the monthly scale, and compared to groundwater and surface water supply to assess water supply stress. Consumptive water use is subtracted from stream flow throughout the river network.
Who should Use WaSSI - Resource managers for making informed management decisions in light of changing environmental conditions; educators, researchers, NGOs, and the general public for gaining insight on the effects of global change on water and carbon at both local and continental scales.
Who is behind WaSSI - USDA Forest Service scientists from the Eastern Forest Environmental Threat Assessment Center developed the model algorithms. This web application was developed with programming support from Praecipio Consulting and Photo Science Inc. through a partnership between the Eastern Forest Environmental Threat Assessment Center and the USDA Forest Service International Programs to facilitate transfer of the science to all users. The application of the WaSSI model in Rwanda was possible through colloboration with the Wildlife Conservation Society and the Rwanda Agricultural Board.
Getting Started with WaSSI - After reading the background information, select the Options tab to define a region for simulation (U.S. or Mexico). Once a region is selected, explore the model inputs by clicking on the Input Viewer tab. A simulation scenario can be created and executed from the Simulation Tool tab. Please be patient!!! The model is simulating the water balance, water supply and demand, ecosystem productivity, and biodiversity for the entire region (U.S. or Mexico) over as many as 100 years and may take several minutes to complete. After a simulation, explore model outputs for your watershed in the Output Viewer or view outputs spatially in the Map Viewer. Try making changes to the simulation scenario to assess the impact of climate, land use, and population change on water resources and ecosystem productivity. Click to download a PDF version of the WaSSI user guide v1.0 in English or Spanish, which provides an overview and illustrates, step-by-step, how to access, navigate, and run the WaSSI model.
Appropriate Use - WaSSI water and carbon predictions are subject to similar uncertainties associated with all ecosystem models, including uncertainty in input data, uncertainty in the representation of the physical processes that govern the watershed water balance and ecosystem productivity, and uncertainty in how people use and manipulate water resources at the continental scale. WaSSI is designed to provide large-scale global change impact assessments using readily available data collected using consistent methodologies across broad regions (e.g. water use and return flow). As a result, model results at the local scale (e.g. HUC8 watershed) may vary from actual conditions and users are cautioned to consider this uncertainty in interpreting results for their watershed of interest. WaSSI users have the capability of evaluating an unlimited number of climate, land use, and water use scenarios, and are ultimately responsible for determining whether the scenarios they define are realistic or not. For evaluating impacts of future climate scenarios, users are encouraged to consider several general circulation model and emission scenario combinations to cover a range in possible impacts. Users may refer to peer-reviewed WaSSI documentation for further information regarding model assumptions and limitations.
Supported Web Browsers - The recommended web browsers to run WaSSI web in are Internet Explorer 8 or higher and FireFox.
- Caldwell, P. V., Sun, G., McNulty, S. G., Cohen, E. C., and Moore Myers, J. A.: Impacts of impervious cover, water withdrawals, and climate change on river flows in the conterminous US, Hydrol. Earth Syst. Sci., 16, 2839-2857, doi:10.5194/hess-16-2839-2012, 2012.
- Caldwell, P.V., Sun, G., McNulty, S.G., Cohen, E.C., and Moore Myers, J.A. 2011. Modeling Impacts of Environmental Change on Ecosystem Services across the Conterminous United States, in Medley, C.N., Patterson, Glenn, and Parker, M.J. (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.
- Sun, G., Caldwell, P., Noormets, A., Cohen, E., McNulty, S., Treasure, E., Domec, J. C., Mu, Q., Xiao, J., John, R., and Chen, J., 2011, Upscaling key ecosystem functions across the conterminous United States by a water-centric ecosystem model, Journal of Geophysical Research, 116, G00J05.
- 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.
- 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.