Global change implications on long-term water supply and demand forecasts in the Columbia River Basin

Abstract

The Columbia River Basin (CRB) encompasses parts of seven US states and British Columbia in Canada over a land mass approximately the size of France. It is a vital part of the ecosystem and economies of the entire region. The CRB, like many watersheds around the world, is experiencing increased pressure on water resources and ecosystems, due to population growth, threatened and endangered species, economic development, and climate change. Irrigation is responsible for the majority of consumptive use in the watershed so the implications of climate change are a tremendous concern as we strive to feed a growing population. To facilitate strategic planning and investment, the State of Washington requires a long-term water supply and demand forecast (the Forecast) every five years. An interdisciplinary WSU research team integrated three biophysical models with an agricultural economics model to conduct a system-wide assessment of how future environmental and economic conditions are likely to change water supply and demand by 2030. Timing of supply changes will shift water away from the Sustainable Irrigation and Drainage IV 77 www.witpress.com, ISSN 1743-3541 (on-line) WIT Transactions on Ecology and The Environment, Vol 168, © 201 WIT Press 2 doi:10.2495/SI120071 times when demands are highest. Unregulated surface water supply at the mouth of the watershed will decrease an average of 14.3% between June and October, and increase an average of 17.5% between November and May. The model predicts an additional 141,000 acre-feet (17.3 MCM) of water will be required just to maintain existing irrigation in Washington. The average gross revenue per acre for irrigated land in Washington was estimated to be $3,500 and average water use is 3.1 acre feet per acre. Applying these two values to the 141,000 acre foot shortage translates into a decrease in value of production of $159 million per year although actual losses would likely be less as farmers could reduce water demand by favoring less water intensive crops such as wine grapes or chose to reduce production of lower value irrigated crops first. Within the entire CRB, an estimated 298,500 acre-feet of additional water will be needed. Localized impacts could be more severe than the broader overall averages. The Forecast will help the State of Washington’s Office of Columbia River strategically fund water supply projects by improving understanding of where additional water supply is most critically needed, now and in the future.