Evaluating The Utility Of Stable Isotopes Within Environmental Observatories In The Colorado Front Range To Quantitatively Estimate The Age Of Water And Determine The Time And Spatial Dynamics Of Hydrologic Processes
Abstract
Stable isotope observations play an important role in understanding hydrologic and ecologic systems at environmental observatories in the Colorado Front Range. Measurements of δ18O and D in precipitation, surface waters, and groundwater wells as part of the Boulder Creek Critical Zones Observatory at 4 headwater catchments along a 1,500-m elevational gradient provide information on how residence times, source waters, and flow paths change with altitude in mountainous catchments. Application of a convolution algorithm to the δ18O values in precipitation and stream waters produced relatively short mean residence times ranging from 1.12 years in the alpine to 2.08 years in the lower montane ecosystem. Surprisingly, two-component mixing models using δ18O along with natural geochemical tracers show that streams in all catchments streams consisted of greater then 70% old and greater than 50% reacted waters. These results indicate that headwater catchments within the Boulder Creek Watershed, from the alpine to the foothills, have relatively short groundwater residence times, but that groundwater plays an important role in stream flow generation at all sites. Long-term measurements of stable water isotopes by the Niwot Ridge LTER program show that groundwater contributions to streamflow in the alpine Green Lakes Valley of the Colorado Front Range has increased about 15% over the last decade, based on hydrologic mixing models parameterized using a combination of δ18O and natural geochemical tracers. Enrichment of δ18O from -10‰ in the outflow of a rock glacier compared to -20‰ in snow and enrichment of deuterium excess from +17.5‰ in rock glacier outflow compared to +11‰ in snow, suggests that melt of internal ice that had undergone multiple melt/freeze episodes was the dominant source of the increase in base flow. These isotopic values suggest that the source of late-season stream flow at GL4 has shifted towards permafrost melt water in recent warm, dry years. These results demonstrate the utility of stable isotopes within environmental observatories to quantitatively estimate the age of water and determine the time and spatial dynamics of hydrologic processes.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2010
- Bibcode:
- 2010AGUFM.H54B..08W
- Keywords:
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- 1804 HYDROLOGY / Catchment