Climate Change and Streamflow Trend Analysis

The Spokane Aquifer Joint Board (SAJB) is a regional consortium of water purveyors that collectively operates 122 wells to supply drinking water to more than 500,000 people in the area. These wells draw groundwater from the Spokane Valley-Rathdrum Prairie (SVRP) aquifer, the primary source of drinking water in the region.

GSI created a high-resolution groundwater flow model that encompasses the entire footprint of the SVRP aquifer in Washington and Idaho. Using the model, GSI conducted a three-phase study to examine the degree to which seasonal redistribution of pumping might be able to increase the magnitude of summertime groundwater discharges to the Spokane River, and to examine potential causes of continued declines in the seasonal low flow of the river. The study found that the continued decline in summer low-flows in the Spokane River could not be explained by the changes that have occurred during the past half century in urban growth, groundwater pumping, stormwater management, and water supply and wastewater management.

To further analyze potential causes of summer low flows, GSI conducted a forensic study of the Spokane/Coeur d’Alene area. This involved retracing the history of surface water and groundwater use, urban and agricultural water demands, streamflows, and climate dating back to 1900. The study found that while water practices during the first half of the 20th century likely contributed directly to declines in river flows during those years, further declines in streamflows now strongly appear to be the result of a climate-change influence. This has affected snow accumulation volumes since the year 2000 and the timing of snowmelt runoff. These river flow declines are persisting in spite of significant improvements in water supply management and stable (if not rising) groundwater levels during the past 2 to 3 decades—indicating that the climate-change influence appears to be overriding any potential gains from improved water supply management and stable or rising groundwater levels.

This study was published in the December 2017 issue of The Water Report, a monthly publication focused on water rights, water quality, and water solutions in the western United States.

GSI’s work involved:

• Developing and applying a groundwater flow model.
• Analyzing historical surface water and groundwater use, water demands, streamflows, and climate.
• Identifying that climate change is the likely cause of continued low summer flows, overriding gains from better water management and improved groundwater levels.