The future of water: A new tool helps managers plan ahead

Photo Credit

USDA Forest Service photo by Joel Nowak.

Clark Creek Falls, Olympic National Forest: The Water Supply Stress Index model can help users better understand how such features, and the ecosystem around them, may change in the coming years. 

When the weather report says it will rain on Wednesday, you might bring a jacket, or you might accidentally leave it at home. Generally, the worst that will happen is you get wet. The stakes are a bit higher when it comes to projecting long-term water yield. Water yield is the long-term difference between the water added via precipitation and the water lost via evaporation and plant transpiration. Water yield is critical to many aspects of everyday life. Farmers consider water yield when deciding what crops to plant. Changes in water yield influence ecosystem change, and how wildfire might behave, and much more.

Our predictions about the future, and particularly about precipitation, have come a long way, but they still are not perfect. However, a model and interactive tool developed by researchers from the U.S. Forest Service's Western Wildland Environmental Threat Assessment Center (WWETAC) and Eastern Forest Environmental Threat Assessment Center (EFETAC); USDA Southeast Climate Hub; and University of New Hampshire provides insights about the future of U.S. water.

A Coupled Model

To be fair, the model cannot say whether it will rain next Wednesday 50 years from now, but it does give an idea of predicted future water-yield trends.

The interactive Water Supply Stress Index (WaSSI) Dashboard, and the publication it draws from, link a series of existing models to produce more accurate results. The dashboard uses two potential greenhouse-gas emission scenarios: (1) a moderate-emission scenario that assumes some emission reductions are implemented (RCP 4.5), and (2) a high-emission scenario that assumes little to no action (RCP 8.5). These scenarios are used by MC2, a dynamic global vegetation model produced and supported by WWETAC. The output is a projection of the likely plant communities and land cover types based on each climate scenario, which in turn influences biomass production and potential evapotranspiration (water lost by evaporation and plant transpiration). The projected future climate and associated vegetation dynamics are then entered in the WaSSI ecohydrological model to project future water yield, which was developed primarily by EFETAC researchers.

These projections are helpful but do not include all environmental factors that affect how water and climate interact. However, the coupling of the models makes for clearer overall projections. The final product of this integration is a series of water-yield projections for the high- and low-emission scenarios that cover the contiguous United States down to roughly county-sized watersheds from 2008 through 2099.

Photo Credit

Duarte et al. 2024

The new dashboard lets users search watersheds and provides a line graph of projected water yield with an adjustable date range.

Projections in Action

To make this model more concrete, imagine you are standing in the shade of a ponderosa pine in eastern Oregon. Current conditions are dry, with hot summers and relatively low annual precipitation. The pines, Douglas-firs, and other drought-tolerant species in this ecosystem provide shade and maintain soil structure, capturing some precipitation in the canopy to evaporate later while the rest has a chance to percolate into the soil. A standard water model can generally take into consideration some of these factors.

As time goes on and climate change progresses, higher temperatures and greater fluctuations in precipitation will be the norm. Most models can also calculate how yield will change given different climate inputs. But this leaves one major gap: If the climate is changing in a model, what happens to all the plants?

When summer rainfall decreases and temperatures rise due to climate change, the open forest we just imagined will be more stressed. At some point, it could even be replaced by a different ecosystem like sagebrush shrubland. This vegetation change would mean a reduction in biomass and canopy cover, with a corresponding loss in soil sheltering and structure retention (to name just a few effects). As a result, the already changing water yield pattern for this watershed would be thrown further from historic norms.

This example only scratches the surface of how complex factors make water modeling challenging. The new model and dashboard, although not perfect, have improved the accuracy of water-yield projections.

The Power of a Dashboard

This new coupled model and the resulting dataset are exciting, but modeling outputs can be opaque. To help make this valuable data accessible, Charlie Schrader-Patton (WWETAC), helped build a tool that allows anyone to visualize the model’s data.

This tool does more than just offer a visually appealing layout, though. As Schrader-Patton pointed out, “From a scientist’s standpoint, I think it provides a very efficient way to look at the data. You often read these papers with graphs in them, and those are just snapshots of one little piece of the data, either in time or space. But with a tool like this, they can surf through the entire dataset, both spatially and temporally, and really get a handle on the breadth of the data.”

But the WaSSI tool isn’t just for scientists. Although no model can tell the future, this tool will allow land managers and communities, as well as scientists, to ask their own questions about the future of water and make informed decisions about how best to prepare for the future.