In 2016, the State of Washington’s Office of the Columbia River submitted a long-term water supply and demand forecast to the Washington State Legislature. The forecast, due every five years, provides a system-wide assessment of how future economic and environmental conditions will impact water supply and demand by the 2030s. The forecast evaluates three geographic scopes: Eastern Washington’s watersheds, Washington’s Columbia River mainstem, and the entire Columbia River Basin.

The research, coordinated and managed through the State of Washington Water Research Center (WRC), is an analysis of the impacts of climate change, regional and global economic conditions, and state level water management actions on irrigation demands and surface water supplies across the Columbia River Basin, through the use of economic scenarios and state-of-the-art modeling techniques. The research team, led by Jennifer Adam and Jonathan Yoder of the WRC, includes nearly 30 researchers from Washington State University, the University of Utah, Aspect Consulting, and the Washington State Department of Ecology and Department Fish and Wildlife.

“The bulk of our work occurs during the five years between reports,” said Sonia Hall, one of the authors of the legislative report, and Sustainable Systems Analyst for Washington State University.

“All efforts have branched off each other, which led to our use of the modeling tools starting in 2011. We were able to do a much better job by using those tools and we find them to be a representation of science best understanding which have led to significant improvements between forecasts.”

Three modeling tools are developed and integrated together to provide the most accurate results. The Variable Infiltration Capacity (VIC) model allows us to track the water balance across the geography of the Columbia River Basin, including how much evaporates and infiltrates into the soil for crops to use; this shows us how crops react to different weather conditions. The Cropping Systems Simulation (CropSyst) tool determines how much water is needed by crops through irrigation, how the crop will change during the season under a variety of weather conditions. The Columbia River Simulator (ColSim) simulates the movement of water in the reservoirs and provides detailed information of where water is stored for use by irrigators, municipalities, hydropower generation, and for ecological purposes. The tools work together and complement each other. For instance, VIC informs CropSyst about water supply along with daily weather and Cropsyst informs VIC of water stress levels and crop water needs.

Through the use of these integrated modeling tools the research team submitted their latest long-term water supply and demand forecast in December of 2016. They found in the results of their latest forecast that the average annual supply of water for all uses across the Columbia River Basin down to Bonneville Dam is expected to increase around 12 percent by 2035. Their research also concluded that water will be available earlier in the spring than it has been in the past, and the demand for eastern Washington’s irrigated acres will decrease almost 5 percent. If current trends in the proportion of irrigated acres growing different crops continue, we can expect to see more acres growing crops like wine grapes, that demand relatively little water per acre, and less acres growing others like pasture, that demand more water per acre. If this happens, demand for water would decrease close to 7 percent.

“The demand forecast is important because it helps in knowing how drought conditions will impact the various users, enabling managers to better know how to mitigate the effects of drought,” said Hall.

Note: The information in this article was provided by the Agriculture Climate Network, State of Washington Department of Ecology, the University of Washington, The 2016 Columbia River Basin Long-Term Water Supply and Demand Forecast Legislative Report, and the State of Washington Water Research Center.

Link to publication in NIWR