Streamflow generation of the Boulder Creek Watershed at Orodell

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The mountainous watershed, Boulder Creek Watershed at Orodell, has a significant source of streamflow from snowmelt, which is similar to other high elevation watersheds. Several headwater catchments are located in this watershed. The streamflow contributions and flow paths at these headwater catchments have been investigated, but the main stem in the watershed has not yet been explored. An understanding of the processes that control snowmelt runoff generation in the main stem of this basin is critical for the sustainably use, and the prediction and management of water resources.

EMMA method was applied to the main stem of Boulder Creek. At the watershed outlet (Orodell), three end members can explain most of the variations of stream water samples: groundwater samples from Gordon Gulch well 2 or 3 (they have similar soil type, well depth), rainwater above Sugarloaf, and snowpit samples from subalpine to alpine areas. A re-evaluation of the predicted versus measured concentrations of the selected tracers gives high correlations (r-square > 0.95). At a higher elevation (Nederland), the three end members are: groundwater from Gordon Gulch well 1, rainwater above sugarloaf, and snowpit from subalpine to alpine areas. The r-squared values for predicted versus measured values of tracer concentrations are higher than 0.96. The main difference between end members at these locations comes from groundwater source. Groundwater samples from Gordon Gulch well 1 have lower solute concentrations, but greater depth than that of well 2 or 3, suggesting the possibility of preferential flow paths or rock fractures that result to the lower solute concentrations.

Because measurements of stable water isotopes at Nederland were only available during the summer months in the year 2009 and 2011, the average contributions from these months were compared with that from Orodell. It is concluded that in summer months, contributions from groundwater to stream flow decreased from high to low elevations along the Boulder Creek main stem, while contributions from rain and snow increased. Whether this trend represents the general contributions for streamflow on a yearly basis is uncertain, and needs further investigation.