SNOWPACK TRENDS AND STREAMFLOW IN SMALL, FORESTED WATERSHEDS, WESTERN CASCADES, OREGON

Snowpack and streamflow have been measured in small (9 to 100 ha) and large (60 km2) watersheds in the western Cascades of Oregon since 1925 (snow) and 1950 (streamflow), providing an opportunity to examine long-term trends in streamflow and their relationship to snowpacks. We examined long-term trends in monthly streamflow, precipitation, temperature, and snowpack from transient snow zone (400-700 m), the transient to seasonal znow zone (500-1000 m) and the seasonal snow zone (>800 m) at the H.J. Andrews Experimental Forest and four Snotel sites (Santiam Jct, Hogg Pass, McKenzie, and Three Creeks Meadow, 1000-1400m) in the nearby Oregon Cascade Range over the period 1925 (snow) or 1950 (streamflow) to 2008.

Spring streamflow has declined over the past 50 years in small watersheds. The annual runoff ratio, annual baseflow, annual runoff ratio, and spring (March to May) runoff ratios have declined significantly in transient and seasonal snow zones. Streamflow in other seasons has not changed significantly. Declines in spring streamflow are steepest in the seasonal snow zone, with runoff ratios declining from 1.1 to about 0.7 over the 40-year period of record. Air temperatures in January, March, and April have increased by approximately 2°C since 1958. Controlling for the effect of the Pacific Decadal Oscillation, a measure of sea surface temperature, and month, snow water equivalent declined significantly by 0.5 to 1% of average snowpack in Jan-May from 1930-2007 at Santiam Jct and Three Creeks Meadow, but not at McKenzie or Hogg Pass.

Alternative explanations for these trends include climate warming effects on evapotranspiration, climate warming effects on snowmelt, and cyclical ocean temperature effects on snow accumulation and melt. PDO explains about half of interannual variation in snow water equivalent (swe) at the five snow sites. At the four western Cascades sites, swe varies by 100 to 200 mm for every unit of PDO. Contrary to model-based predictions, changes in air temperatures and snowpacks have not led to increases in winter streamflows or to declining summer flows at this site. The future response of water yield to changing climate will depend on ocean temperatures and forest water use response to altered snow inputs.