Water Supply Stress Index (WaSSI) Ecosystem Services Model

Overview & Applicability

WaSSI is an integrated, process-based model that can be used to project the effects of forest land cover change, climate change, and water withdrawals on river flows, water supply stress, and ecosystem productivity (i.e. carbon dynamics). WaSSI operates on a monthly time step at the HUC-4 (8-digit HUC) watershed scale (see more on HUCs) and across Mexico at the 0.5 degree scale. For the conterminous U.S., the model can also be run at the HUC12 scale for water and carbon balances from 1960 to 2012. As water yield and carbon sequestration are tightly coupled, WaSSI can be used to evaluate trade-offs among management strategies for these ecosystem services.

The web application for WaSSI allows users to define a custom simulation scenario, view/download model inputs and outputs in tabular and graphical form for a location of interest, and view/export model outputs spatially for a variety of time scales using an interactive map viewer. Users may select their location in the map viewer, select a specific HUC, or input a zip code to view model inputs and outputs

Model Details

The core of WaSSI is a monthly water balance model that is sensitive to land cover and climate, computing the water balance independently for each of ten land cover classes in each watershed. Evapotranspiration, infiltration, soil storage, snow accumulation and melt, surface runoff, and base flow processes are accounted for within each basin, and discharge is conservatively routed through the stream network from upstream to downstream watersheds. Evapotranspiration and ecosystem productivity are computed based on empirical relationships derived from multisite eddy covariance measurements. Net monthly population-adjusted anthropogenic surface water withdrawals based on USGS estimates are subtracted from the accumulated flow at the outlet of each watershed. The water Supply Stress Index (WaSSI) is computed as the ratio of total water demand across all sectors to the total water supply from surface and groundwater sources.

Map of modeled water supply, using WASSIMap of modeled water supply, using WASSI


WaSSI was developed by a team of US Forest Service Southern Research Station & Eastern Forest Environmental Threat Assessment Center scientists and resource specialists (Ge Sun, Steve McNulty, Peter Caldwell, Erika Cohen, Jennifer Moore Myers). Early versions of the model evaluated water supply stress at the annual time-step across the 13 southern states. In its current form , the model operates at the monthly time step, simulating terrestrial carbon dynamics in addition to water supply stress across the conterminous U.S. and in Mexico. A web-based interface for WaSSI was released in July 2012 enabling users to define and evaluate the impacts of custom climate and land use scenarios. Planned future developments include completing the suite of future climate scenarios (16 general circulation models and ensemble, A1B, A2, and B1 SRES emission scenarios), increasing spatial resolution from HUC-4 watershed to HUC-6 (12-digit HUC), accounting for flow regulation and diversions, and linking climate change, flows, water quality, and aquatic ecosystem health.

Inputs and outputs


All WaSSI inputs have been collected, rescaled to the HUC-4/0.5 degree grid, and are available via the web application. Users may select from the range of climate scenarios available and a simulation time period, and may further adjust precipitation and temperature, land cover distribution, human population, and water use by sector to define a custom global change scenario.

Specific model inputs include STATSGO-based soil properties, 2006 NLCD land and impervious cover, MODIS leaf area index by land cover, 2005 USGS water use estimates, population projections from 2010-2060, 1961-2010 PRISM historical monthly precipitation and temperature, and 1961-2099 climate projections derived from several general circulation models (CMIP3 downscaled and bias corrected CGCM3.1, GFDL-CM2.0, and UKMO-HadCM3) under the A1b (mid), A2 (high), and B1 (low) emission scenarios.


Users may view and download model outputs for their custom simulation scenario related to:

  • Water balance (e.g. precipitation, water yield, evapotranspiration, snow water equivalent, and soil water storage),
  • Water supply and demand (e.g. supply, demand, and water supply stress index)
  • Ecosystem productivity (e.g. gross ecosystem productivity, net ecosystem carbon exchange)

These outputs are generated at the monthly, annual, and mean annual scales over the time period and location of interest. The map viewer allows the user to view these outputs spatially with several ancillary map overlays to add context, and to export custom maps of model outputs.

Restrictions and limitations

WaSSI water and carbon predictions are subject to similar uncertainties associated with all process- based models. These include 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. HUC-4 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 (see additional information).

Accessing the tool and additional information

WaSSI can be accessed at: http://www.forestthreats.org/tools/WaSSI

Examples of recent WaSSI applications include the Southern Research Station Southern Forest Futures and Climate Change Adaptation and Mitigation Management Options projects, the Northern Research Station Northern Forest Futures Project, and the Union of Concerned Scientists Freshwater Use by U.S. Power Plants assessment. Read more on these and other projects in the publications below:

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. Available at: http://www.treesearch.fs.fed.us/pubs/31830

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. Available at: http://www.treesearch.fs.fed.us/pubs/38560

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. Available at: http://pubs.usgs.gov/sir/2011/5169/

Lockaby, G., Nagy, C., Vose, J. M., Ford, C. R., Sun, G., McNulty, S., Caldwell, P., Cohen, E., and Moore Myers, J. A.: Water and Forests, in Wear D. N. and Greis J. G. (eds.) The Southern Forest Futures Project: Technical Report, USDA Forest Service, Southern Research Station, Asheville, NC., General Technical Report, 2011. Available at: http://www.srs.fs.usda.gov/futures/

Averyt, K., Fisher, J., Huber-Lee, A., Lewis, A., Macknick, J., Madden, N., Rogers, J., and Tellinghuisen, S.: Freshwater use by U.S. power plants: Electricity's thirst for a precious resource, A report of the Energy and Water in a Warming World initiative, Cambridge, MA, Union of Concerned Scientists, 2011. Available at: http://www.ucsusa.org/clean_energy/our-energy-choices/energy-and-water-use/freshwater-use-by-us-power-plants.html

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, 2012. Available at: http://www.hydrol-earth-syst-sci.net/16/2839/2012/hess-16-2839-2012.html

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 (JAWRA) 49(4): 938-952. DOI: 10.1111/jawr.12075. Available at:http://onlinelibrary.wiley.com/doi/10.1111/jawr.12075/abstract