The ionic pulse, snowmelt flowpaths, and surface water chemistry in two alpine basins, Colorado Rocky Mountains, United States

The goal of this research is to understand how snowmelt flowpaths are influenced by frozen ground in mid-latitude alpine basins (Colorado Rocky Mountains) and how these flowpaths dictate surface water chemistry in relation to snowmelt. Two alpine headwater basins, the Martinelli and Saddle watersheds at Niwot Ridge, were investigated to compare snowmelt chemistry, steam chemistry, frozen soils and flowpaths. It is suggested that where infiltration is low, stream chemistry will reflect the chemistry of snow meltwater, while meltwater that travels through subsurface pathways will undergo predictable chemical transformations. The overarching questions addressed by this investigation include, (1) How does chemical concentration of snowmelt vary? (2) How is the chemistry of the ionic pulse transformed between the snowpack and alpine surface waters during the start of the melt and runoff season? and (3) Is infiltration limited due to frozen ground?; Results demonstrated the existence of a concentrated ionic pulse of snowmelt at the start of the melt season that is strongly related to snow depth. The chemistry of the ionic pulse is greatly transformed in the Saddle basin due to vegetation and soils. The Saddle stream buffers acidic inputs, exports high concentrations of base canons and damps inter-annual variations in snowmelt inputs. Solute concentrations in the Martinelli stream, in contrast, are more similar to snowmelt concentrations and are more variable both intra- and inter-annually.; A conceptual model suggests that early in the morning, meltwater draining through near-surface soils is the primary contribution to streamflow. As snowmelt increases later in the day, meltwater both pumps pre-melt water from the sub-surface to streams and travels overland as surface flow due to an increased water table and because infiltration capacities of soils have been exceeded. Over several weeks, both sub-surface and meltwater contributions to streamflow increase due to an increasing active layer and the replacement of pre-melt storage. Approximately 50% of melt during a two-week period in 2001 was used to fill liquid storage in the Saddle basin.; Frozen soils can limit infiltration at Niwot Ridge. Flat areas downslope of contributing meltwaters had the highest occurrence of saturated soils, standing water and basal ice and may serve to channel meltwaters more directly to streams.