GROUNDWATER INFLUENCES SUMMER STREAMFLOW RESPONSE TO CLIMATE CHANGE IN THE OREGON CASCADES

Climate models project continued warming for the Pacific Northwest, which will result in a further reduction of snowpacks and decreased summer streamflow. Using historical records from Oregon Cascades watersheds with contrasting geologic properties and drainage efficiencies, we examine how groundwater storage filters regional climate signals and controls summer discharge. In groundwater-dominated watersheds, aquifer storage and the associated slow summer recession are responsible for sustaining summer discharge even when the seasonal or annual water balance is negative, while in runoff-dominated watersheds, subsurface storage is exhausted every summer. Warmer winters and earlier snowmelt over the past ~60 years have shifted regional hydrographs, resulting in summer drought periods lasting 2 weeks longer. In runoff-dominated watersheds, subsurface storage is exhausted every summer, resulting in little change to low flows over the last 60 years, while in groundwater-dominated watersheds, autumn minimum discharges have declined >25%. The slow recession rate of groundwater-dominated hydrographs makes them more sensitive than runoff-dominated hydrographs to changes in snowmelt amount and timing. These results suggest that water managers in the mountainous western United States should be prepared for significant changes to hydrographs even in watersheds with extensive groundwater.