- Increasing temperatures in western North America are expected to result in a decline in winter snowpacks, earlier snowmelt, and a shift in the timing of streamflows, with an increasing fraction of streamflows occurring earlier in the water year and drier conditions during the summer. However, few streamflow datasets have associated climate and vegetation records adequate to interpret changes in climate, forest processes, and their consequences for streamflows. This study examined long-term streamflow records from three headwater watersheds in old growth forest at the H. J. Andrews Experimental Forest, Oregon, to seek evidence of trends and investigate possible explanations for these changes. The three small (8.5-60 ha) watersheds (WS2, WS8, and WS9) range in elevation from 432-1182 m and have streamflow records dating back to 1953, 1964, and 1969 respectively. Spring and summer average minimum temperatures have increased over the study period, while April 1st SWE at Santiam Junction has declined significantly. Although precipitation has remained unchanged over the period of study, runoff ratios have declined in spring (March through May) at rates ranging from 25.4% (WS2) to 41.2% (WS8) over the 40- to 50-year periods of streamflow record. Total declines in spring runoff equate to 1.13, 2.13, and 1.23 mm day-1 at WS2, WS8, and WS9, respectively. However, neither winter nor summer runoff ratios have changed significantly, nor has there been a significant shift in the date of water year median flow over the period of record. Winter and spring runoff ratios are negatively related to vapor pressure deficit and/or temperature in the corresponding season, while summer runoff
ratios are positively related to vapor pressure deficit and/or temperature. Overall, the findings of this study most strongly support the hypothesis that warming temperatures have resulted in a reduction in spring snowpacks, with an earlier onset of evapotranspiration in the spring and a reduction in evapotranspiration during the summer, due to stomatal closure when VPD becomes sufficiently large or soil moisture becomes limited.