An Investigation into Groundwater Recharge Dynamics and Hydrologic Connectivity in a Subalpine Mountainous Headwater Catchment, Colorado, USA

During years of low snow accumulation, groundwater can supply a substantial portion of annual streamflow that agricultural irrigation, recreation, domestic water supply, and riparian and aquatic ecosystems rely on, yet groundwater influences on streamflow generation in mountainous areas are poorly understood.  Isotopic and geochemical surface and groundwater data were used to examine groundwater recharge dynamics and hydrologic connectivity in the dominantly subalpine-forested Como Creek headwater catchment within the Boulder Creek Watershed in the Colorado Front Range.  Streamwater chemistry along an elevational gradient of Como Creek showed strong responses to variations in precipitation and snowmelt inputs spanning 2011-2014.  Elevation effects on δ¹⁸O were apparent, with 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, situated right below treeline, was significantly different from the lower three sites with regards to DOC, δ¹⁸O, and Ca²⁺ (p < 0.05) over May-October 2011-2014.  Additionally, the second highest site in elevation was found to be significantly different from all other sites with respect to Ca²⁺ concentrations.  Soil moisture sensor and geochemical data from soil tension lysimeters co-located with subalpine groundwater wells revealed a disconnect between soil and surface water chemistry during snowmelt and that of deeper, underlying groundwater.  The initial results of this study indicate that higher elevation sites along Como Creek exhibit statistically significant differences with regards to streamwater chemistry relative to lower sites, providing insight on where groundwater recharge and discharge may be occurring in the catchment.  These results help us to evaluate the large September 2013 rain event in the Colorado Front Range, a once in a 1000 year event.  Water isotopes were enriched, Ca²⁺ decreased, and DOC was enriched, showing that new water had flowed through near-surface soils but not deeper recharging groundwater, with all values returning to normal within six months.  The event was also observable compared to a long-term geochemical and stream stage record at the stream site near the catchment outlet, with the most enriched δ¹⁸O value on record of -13.41‰ corresponding to the flood.  Remaining questions concerning groundwater dynamics in the catchment include constraining the tradeoffs between forest ET, groundwater recharge, and streamflow under a warming climate in order to forecast future water yield from the catchment.  Further research quantifying where and when groundwater is recharging in the catchment will be conducted to determine how both groundwater dynamics and hydrological resilience of the catchment will change under varying climatic conditions.