Shifting Hydrologic Pathways in Temperate Forest Headwaters Under a Changing Climate

ABSTRACT Shifts in climate are fundamentally altering hydrologic processes in temperate forested headwater catchments, which serve as sensitive indicators of ecosystem‐scale water balance. In the Northeastern United States, rising temperatures and increases in both total and extreme precipitation have been observed over recent decades. While these changes have been linked to altered soil moisture dynamics and evapotranspiration regimes at sites like Harvard Forest, the role of groundwater and its connection to streamflow generation remains underexplored. We use regression analysis and interaction models to evaluate shifts in streamflow generation processes in headwater catchments. Following a climatic shift in 2016 that was magnified by a prolonged drought, we observed a decrease in average streamflow response time, indicating more rapid water movement through these systems. Our findings suggest a transition toward more shallow‐dominated flow paths, including intermittent shallow subsurface flow through preferential flow paths, particularly since 2016 in Massachusetts. Increasingly frequent extreme precipitation events appear to activate shallow subsurface pathways more often, resulting in faster hydrologic responses. However, we also find that wetland‐connected streams show more stability in response time and flow dynamics, acting as buffers against these rapid shifts.