Hydrologic flowpaths and biogeochemical cycles in the subalpine Como Creek catchment, Colorado Front Range, USA

An outstanding question for snowmelt-dominated watersheds of the western US are the responses of biogeochemical processes to two major drivers of environmental change: directional changes in climate and increasing dissolved inorganic nitrogen (DIN) deposition in wetfall. In the Colorado Front Range, atmospheric deposition of DIN has increased several-fold in the last 25 years. In response, nitrate concentrations at the alpine Green Lakes 4 (GL4) catchment have increased from 1985 to 2009 by 0.27 μeq L^-1 yr^-1. In contrast, we see no directional change in either nitrate concentrations or fluxes in the subalpine Como Creek catchment. We hypothesize that differences in surface/groundwater interactions result in the differing behavior of stream nitrate between the alpine and subalpine catchments that are receiving similar amounts of DIN from atmospheric deposition.

For both basins we sampled precipitation, snowpack, snowmelt, surface water, and subsurface waters. All water samples are analyzed for geochemical, nutrient and isotopic (δ18O, δD) composition.  Stream chemistry data from the last ten years at Como Creek show increases in nitrate concentration during baseflow conditions and then a sharp decline during snowmelt. In contrast, in the alpine basin there is sharp increase in surface water nitrate during snowmelt. Hydrograph separation at the alpine GL4 using end member mixing analysis (EMMA) shows that stream flow is a mixture of three components, groundwater, talus, and new snowmelt that each contribute to roughly a third of discharge, with talus flow supplying the majority of nitrate. In contrast, and somewhat surprisingly, EMMA shows that for the subalpine Como Creek basin, annual streamflow is primarily a mixture of just two components, groundwater and new snowmelt, with minimal contributions from summer rain. During snowmelt the groundwater and snow contributions are nearly equal and subsurface flows dominate the remainder of the year.  Newly installed piezometers at Como Creek provide evidence that the basin is largely a losing reach during snowmelt, with water levels in the piezometers increasing 5-7 m.  After peak snowmelt however, Como Creek becomes a gaining stream, with piezometer levels dropping. Thus, both EMMA and piezometers show that surface-groundwater interactions are tightly coupled during snowmelt, with snowmelt at Como first replenishing the subsurface water deficit and increasing groundwater levels before contributing to discharge.  Thus, in contrast to the alpine GL4 basin, DIN released in snowmelt is assimilated belowground as snowmelt infiltrates the subsurface in the subalpine basin.