The Columbia River Basin has grappled with limited water supplies for decades. This was most noticeable during 2015, when we experienced severe summertime drought across large areas of Washington State, which reduced the amount of water available to meet the region’s demands. The 2015 drought and other recent occurrences of lower water availability are representative of a warmer future with lessening snowpack and earlier snowmelt. In fact, Washington is expected to experience drought again this summer due to rapidly melting snowpack and low precipitation forecasts, underscoring the prevalence of water supply issues for the state.

In addition to these climatic changes affecting water supplies, we also anticipate changes in irrigation needs for agriculture and other out-of-stream demands in many parts of the state. Simultaneous decreases in supply and increases in demand can lead to myriad economic and environmental challenges, such as pro-rationed water rights, decreasing crop yields, increased risk of wildfire, and deteriorating aquatic habitat.

To better prepare for a potentially more stressed water future, researchers and decision makers in Washington teamed up to produce the 2021 Columbia River Basin Long Term Water Supply and Demand Forecast (Hall et al., 2022). The 2021 Forecast is the latest iteration in a series of reports assessing changing water supplies and demands across eastern Washington over the next 20 to 50 years. Led by the Washington Water Research Center in partnership with WSU’s Department of Civil and Environmental Engineering and Center for Sustaining Agriculture and Natural Resources and the Washington Department of Ecology’s Office of Columbia River, the Forecast has been completed by a consistent core team since 2011, producing new results every five years.

The 2021 edition used an integrated set of computer models to predict changes in surface water supply, agricultural and residential demands, as well as exploring hydropower production needs, and trends in groundwater levels across eastern Washington. Results are presented for two future time periods (2040 and 2070) and for various spatial scopes across eastern Washington, including the entire Columbia River Basin, the Columbia River mainstem, eastern Washington’s Water Resource Inventory Areas (WRIAs), and eastern Washington’s aquifers.

The 2021 Forecast results focus on when and where eastern Washington will face future vulnerabilities due to the co-occurrence of decreasing water supplies and increasing demands caused by climate change, population growth, and changes in agriculture and other sectors. In the Columbia River Basin, our main concern is how shifting timing in surface water supply drives overall water availability. As temperatures increase, snowpack is predicted to decline as more precipitation will fall as rain, and the snow we do see will melt sooner in the water year.

These changes translate into higher supply during the wet season and lower supply during the dry season, when demands are highest. This is especially apparent in the snowmelt-dominated Cascades watersheds, where supplies could shift earlier in the year by as much as 23 days within the next 20 years. Such changes will leave many watersheds without sufficient water supplies in the summer to meet the high demands for agriculture and residential water use.

The Forecast results suggest that climate change on its own may not lead to increasing agricultural water demand. However, these climate-related changes occur in the context of changes in multiple other factors which we did not model. For example, as planned water development projects in Washington come online, they would make additional water available both for instream and out of stream uses, and the amount expected to be available by 2040 for irrigation could be larger than any expected changes due to climate change.

We expect different demands to increase in specific locations and during specific times of the year. For instance, the Yakima River watershed is expected to experience substantial changes to timing of supply and demand, coupled with overall decreases in supply and increases in out-of-stream demands during summer months. These will all further exacerbate water management challenges in the future.

Overall, many areas in eastern Washington are expecting concurrent decreases in supply and increases in demand, at least during certain times of the year. Combined with instream flow rules that protect rivers by requiring specified amounts of water be left instream, we could see more frequent and deeper curtailments of junior water rights, if additional water supplies are not secured. And even then, rivers may still not have enough water to sustain fish and other aquatic species.

We also quantified trends in aquifers and found that many locations have declining trends. This suggests that switching from surface water to groundwater supplies will not be a viable option to address increasing demand in most areas. Therefore, decision-makers need to explore other options if we are to prepare for and mitigate the impacts of future supply and demand changes. Several municipalities included in the study rely heavily on groundwater for meeting residential demand and exist in areas where groundwater levels are substantially declining. Some of those seeing the greatest declines are already looking at future options to meet demand.

These findings are concerning, but they also give us critical information for future water management. Further, by partnering with Ecology’s Office of Columbia River, the Forecast is tailored to help bridge between results from our analysis and the complex decision making needed to ensure sustainable water supply decades into the future. The 2021 Forecastis therefore an important tool that allows water managers in the Columbia River Basin to plan for future water needs or infrastructure investments at multiple scales that will ultimately create a more sustainable and reliable water supply for the people, farms, and fish of Washington State.

Reference:

Hall, S.A., Adam, J.C., Yourek, M.A., Whittemore, A.M., Yorgey, G.G., Scarpare, F., Liu, M., McLarty, S., Asante-Sasu, C., McClure, S., Turk, J., Haller, D., Padowski, J., Deshar, R., Brady, M.P., Rajagopalan, K., Barber, M.E., Weber, R., Stockle, C.O., Goodspeed, H.L., Gustine, R.N., Kondal, A., Yoder, J., Deaver, B., Downes, M., Tarbutton, S., Callahan, M., Price, P. Roberts, T., Stephens, J., Valdez, W. 2022. 2021 Washington State Legislative Report. Columbia River Basin Long-Term Water Supply and Demand Forecast. Publication No. 21-12-006. Washington Department of Ecology, Olympia, WA. 284 pp. https://apps.ecology.wa.gov/publications/SummaryPages/2112006.html