Hydrological controls on stream water chemistry in alpine catchments, Colorado Front Range, United States

Identification of flow sources and pathways is crucial in understanding the links between terrestrial and aquatic ecosystems at the watershed scale. Hydrologic models and hydrologic tracing using isotopic and geochemical measurements were used to understand source waters, flowpaths, and biogeochemical cycles in the alpine, seasonally snow-covered Green Lakes Valley, Colorado Front Range, U.S.A.; Using XTOP_PRMS, a semi-distributed hydrologic model, simulation of streamflow discharge was successful at the 8-ha Martinelli catchment, but relatively poor at the 225-ha Green Lake 4 (GL4) catchment because of the poor performance of the flowpath routines in TOPMODEL.; Stable water isotopes in 2-component mixing models indicated that streamflow was dominated by new water at the Martinelli catchment and, somewhat surprisingly, by old water at the GL4 catchment. Using both isotopic and geochemical tracers in end-member mixing analysis (EMMA), it's shown that subsurface flow, including talus water, was a major component controlling stream water quantity.; Uncertainty of hydrograph separations was examined based on temporal variations of isotopic and geochemical compositions in end-members. Sensitivity of mixing models to snow definition (snowpack vs. snowmelt) was also tested to understand how isotopic enrichment over time and ionic pulse in snowmelt affect hydrograph separations. The results indicated that uncertainty and sensitivity of mixing models to snow definition depend not only on catchment size but also on tracers used in the mixing models.; A procedure was proposed to test if end-members measured at one site/period can be used at different sites/periods for end-member mixing analysis. It's shown that end-members measured at GL4 in 1996 can be used not only at GL4 catchment for different periods, but also can be used at GL5 of the Green Lakes Valley and can be partially used at the Andrews Creek and Icy Brook watershed at Loch Vale, Rocky Mountain National Park, for qualitative understanding of flow generation.; The fate and transport of atmospheric nitrate was then elucidated using EMMA in combination with dual-isotopic analysis of both the nitrogen and oxygen atoms in the nitrate molecule. These two approaches showed that the snowmelt period begins with a dominance of atmospheric nitrate in stream water and then is gradually replaced by microbial nitrate primarily originating from groundwater in talus fields. It's suggested that surface water and groundwater interactions are much more important to the quantity and quality of surface water in high-elevation catchments than previously thought.