Abstract:
Despite the importance of rain and snowmelt in causing peakflows in the Pacific
Northwest, the interactive effects of a snowpack and watershed physiography on
streamflow are largely undocumented. This study investigated the influence of soil and
snowpack moisture on peakflow hydrograph shape in three small (< 60 ha) control sub-watersheds
and the containing Lookout Creek Watershed (6200 ha) in the western
Cascade Mountains of Oregon with three main objectives: 1) determine the statistical
correlation between antecedent conditions and peakflow hydrograph shape within each
small watershed; 2) determine how these correlations differed between small watersheds;
and 3) determine the correlation between sub-watershed and Lookout Creek peakflow
hydrograph shapes. A partial-duration set of peakflows was selected from at least 21
years of continuous discharge data for each small watershed using a linear quickflow vs.
baseflow separation technique. Peakflows were then classified into three significantly
different types based on estimated antecedent soil and snowpack moisture storage: "dry-soil
rain" (e.g., soil moisture < 90 percent field capacity), "wet-soil rain" (e.g., soil
moisture> 90 percent field capacity), and "wet-soil rain-on-snow". These three types
accounted for over 70 percent of peakflows and over 80 percent of quickflow volume in
each watershed. Generally, wet-soil rain peakflows began sooner after precipitation,
lasted longer, and had larger instantaneous peaks than did dry-soil rain peakflows, while
wet-soil rain-on-snow peakflows had the longest durations and largest instantaneous
peaks. However, effects of soil wetting and snowpack on peakflow hydrographs differed
significantly between watersheds. Lookout Creek peakflows occurred during several
combinations of small watershed peakflow types, though the largest Lookout Creek
peakflows were coincident with large, synchronized wet-soil rain-on-snow peakflows in
Watersheds 2 and 8. Very little attenuation of the peakflow hydrograph was observed
between the small watersheds and Lookout Creek. Results from this study indicate that
by altering snowpack dynamics, and therefore the frequency distribution of peakflow
types, climate change or land-use may differentially affect peakflow hydrographs in small
watersheds. One consequence of this in Lookout Creek Watershed may be to
desynchronize small watershed peakflow responses and decrease downstream peak size.
