Historical Climate Trends in the Bull Run Watershed

Blog Post
Stream flows through forest
How analysis of climate trends informs our management of the Bull Run Watershed and the water system as a whole.
Man with glasses stands in front of white wall
Benjamin Beal, Water Resource Modeler

The Bull Run Watershed is a great primary water source. The temperate rain forest gets as much as four times the precipitation of notoriously rainy Portland. However, despite the abundant and reliable rainfall, the watershed has experienced the impacts of a warmer climate.

Benjamin Beal, a Water Resource Modeler in Resource Protection, analyzed environmental data to determine long-term climate and hydrology trend in the Bull Run Watershed. Ben’s analysis shows significant trends of increasing air temperatures, decreasing summer streamflow, and increasing water temperature in streams throughout the Bull Run.

Maximum and minimum air temperatures have increased by 2°F to 3°F since the early 1900s, with winter and summer having the highest seasonal rates of rising air temperature. Bull Run water temperatures have increased at an annual rate of 0.01°C to 0.02°C, with a summertime increase of 0.03°C .

The plots below show decreasing summer streamflows over the past 40 to 50 years. Lower summer  flows have implications for summer water supply availability. A definitive cause of these reduced streamflows remains a question of interest to bureau researchers, but climate change is likely an important factor.

“To me, this sort of analysis is a local reminder that the Pacific Northwest is not immune to the effects of climate change and that we have all already been impacted, whether we realize it or not,” says Beal.

Although the population of Portland is growing, the demand for water has been declining. Over the last decade, demand has fallen 16% from 109 gallons per capita per day in FY 2008–09 to 90 gallons per capita per day in FY 2018–19.

While the average customer being more water-wise helps stretch water supplies during the summer months, continued investments with our groundwater system are needed to make our entire water system more adaptable. The bureau pumped significant amounts of groundwater to meet water demands in 2015 and 2018 in response to an earlier than usual dry season in both years.

“The decline in summer streamflows has significant implications for our future summer supply,” says Kavita Heyn, the bureau’s Climate Science and Adaptation Program Manager. “We need to recognize the vulnerability of the Bull Run supply to warmer and drier conditions, and ensure we have a reliable groundwater supply that, together with Bull Run, can meet water demands.”

4 graphs of tributaries that feed into Bull Run showing a decreasing streamflow trend
The plots show a decreasing streamflow trend in four of the Water Bureau’s primary stream flow gages for the hydrologic summer, which spans July, August, and September. The y-axis shows flows in cubic feet per second, and the x-axis shows the average monthly flow for a given year.