Surface-subsurface interactions of the seasonally snow-covered Boulder Creek Watershed at Orodell, Colorado

Abstract:

The hydrology of the western United States and many other semi-arid regions of the world is dominated by snowmelt runoff. An important question is the role of subsurface interactions with snowmelt runoff. Hydrologic mixing models have been used to answer this question at the hillslope and small basin scale. Here we present information on snowmelt/subsurface interactions for the 270-km² Boulder Creek Watershed in the Colorado Front Range using isotopic, geochemical, and hydrometric data along with end-member mixing analysis (EMMA). We measured these parameters at several different elevations in weekly precipitation, the seasonal snowpack, snowmelt before contact with the ground, discharge, springs, soil solution, and groundwater.

At the watershed outlet Orodell, five tracers are selected: Ca²⁺, Mg²⁺, Na⁺, ANC, and d¹⁸O. The first two principal components can explain about 98% of the chemical variance in stream water, and require three end members: groundwater, rain, and snowmelt. The r-squared values of measured and predicted values are higher than 0.95, suggesting that we have identified the correct end-members.

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. On the contrary, contributions of snow to streamflow decreased from GL4 to GG in summer months (Cowie, 2014). Thus, in Boulder Creek Watershed at Orodell, the hydrochemical evolution at headwater catchments is different from that in the main stem.