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Dailey, Kelsey R ¹ ; Hughes, Holly ² ; Williams, Mark ³

¹ Environmental Studies, INSTAAR
² INSTAAR
³ Geography, INSTAAR

Geochemical surface and groundwater data were used to examine groundwater recharge dynamics and hydrologic connectivity in the dominantly subalpine Como Creek headwater catchment within the Boulder Creek Watershed, in conjunction with the Niwot Ridge Long-Term Ecological Research Program. Previous work through 2012 has shown subalpine groundwater to be very important in sustaining streamflow in the snowmelt-dominated catchment during a low snow year, thus improving the resilience of the catchment to variations in precipitation (Cowie, 2014). By examining data spanning years with variable precipitation regimes (e.g. low vs. high snow years, large events; 2011-2014), this research aims to characterize the Como Creek subalpine groundwater system, which has not been done extensively before, in order to provide insight on future catchment and ecosystem response under a changing climate. Streamwater chemistry along an elevational gradient of Como Creek showed strong responses to variations in precipitation inputs, with apparent elevation effects on d18O and more depleted values indicative of snowmelt influence observed at the higher elevation sites. Results from one-way ANOVA indicated that the highest elevation stream site right below treeline was significantly different from the lower three sites with regards to DOC, d18O, and Ca2+ (p < 0.05) over May-October 2011-2014, providing insight on where groundwater recharge and discharge may be occurring in the catchment. Interannual variability observed in subalpine groundwater well levels and Como Creek hydrographs motivated end-member mixing analysis on streamwater at the catchment outlet for individual water years of 2011-2014 in order to delineate the composition and varying sources of exported waters, with results to be presented. Subalpine wells showed increases in water levels during snowmelt corresponding to the size of the annual snowpack, as well as recharge during the large September 2013 rain event in the Colorado Front Range, a once in a 1000-year event. Also in response to the 2013 event, Como Creek water isotopes were enriched, Ca2+ decreased, and DOC was enriched, showing that new event water had flowed through near-surface soils but not deeper, recharging groundwater, with all values returning to normal within six months. The results of this study will help us to evaluate other hydrologic phenomena, such as the high 2014 peak flows observed at the catchment outlet (Figure 1), and can inform future research regarding groundwater dynamics in the catchment including constraining the tradeoffs between forest ET, groundwater recharge, streamflow, and downstream water resources.

Cowie, R., Surface water and groundwater interactions in natural and mining impacted mountain catchments, Ph.D. Dissertation, Department of Geography, Â鶹ÊÓƵ, 2014.

Knowles, J.F., A.A. Harpold, R. Cowie, M. Zeliff, H.R. Barnard, S.P. Burns, P.D. Blanken, J.F. Morse, and M.W. Williams, The relative contributions of alpine and subalpine ecosystems to the water balance of a mountainous, headwater catchment, Hydrological Processes, doi: 10.1002/hyp.10526, 2015.