Long-term Streamflow Trends from Old Growth Catchments in the Western Cascades

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Kathleen Moore
Julia Jones - Principal Advisor

Climate warming in western North America is expected to result in reduced snowpack, earlier melt, and increased evapotranspiration.  Consequently a shift toward a greater proportion of streamflow earlier in the water year with diminished spring and summer streamflows is anticipated.  However, few datasets exist of streamflow with associated climate and vegetation records adequate to interpret changes in climate, forest processes, and their consequences for streamflows.  This study examines trends in long-term streamflow records from three headwater catchments in old growth forest at the H. J. Andrews Experimental Forest, Oregon, to seek evidence of declining streamflows and investigate possible explanations for these changes. The three small (8.5-60 ha) catchments (WS2, WS8, and WS9) range in elevation from 432-1182 m and have streamflow records dating back to 1953, 1964, and 1969 respectively.  Linear regression analysis was used to assess both annual and seasonal trends in streamflow, baseflow, and precipitation. Initial findings indicate that while precipitation has remained unchanged over the period of study, runoff ratios (discharge/precipitation) and baseflows have declined over time.  All three catchments exhibit significant declines in spring (March-May) runoff ratios, ranging from -0.004 to -0.01 per year (p-value<0.05). This equates to a maximum decline of 0.43 in spring runoff ratio over the period of record.  WS2 and WS8 also exhibit significant declines in spring baseflows, ranging from -1.82 to -2.60 mm per year (p-value<0.05), which equates to a decline of 39-42% over the study period.  Continued research will 1) employ wavelet analysis to assess trends in daily streamflows, and 2) investigate changes in snowpack, temperature, and evapotranspiration as potential causal mechanisms.

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