BIOGEOMON Journal of Conference Abstracts, Vol 2(2), p. 143, 1997

Hydrologic flowpaths at scales from eight hectares to seven kilometers in a continental alpine catchment, Green Lakes Valley, Colorado.

Mindia Brown
INSTAAR and Department of Geography University of Colorado Boulder, CO

Mark W. Williams
INSTAAR and Department of Geography University of Colorado Boulder, CO

Nel Caine
INSTAAR and Department of Geography University of Colorado Boulder, CO


We are conducting a study in in the Green Lakes Valley of the Colorado Front Range to understand the processes that determine the sources and flowpaths of solutes in high-elevation catchments. Hydrologic pathways were evaluated in 1990 using a sampling protocol that included the quantity and quality of atmospheric deposition, the snowpack, snowpack meltwaters, soil interstitial water, groundwater, and stream waters. Approximately 400 samples were analyzed for their isotopic ratios of H/D and o18/o16, and for concentrations of reactive silicate (Si) and major ions. Our sampling protocol permits evaluation of the sources, residence time and pathways of streamflow in this system throughout the year and at scales that vary from 8 ha to 10 km2.

Isotopic separation was significant among potential water sources, based on delta o18 ratios of -19.8 (sd = 1.1) for the snowpack, -12.3 (sd 2.4) for rainfall, and from -16.1 to -12.5 in interstitial waters. Average delta o18 values for stream waters decreased from -19.2 at the head of the Green Lakes Valley to -15.7 at a low elevation sampling site in the Green Lakes Valley; analytical precision of all samples was +-0.1 delta o18.

Our preliminary analysis using a two-component mixing model indicates that at any time the percentage of antecedent water in streamflow increases with the area of the catchment. For example, on 21 June, 1990, the delta o18 of streamflow in headwater basins was indistinguishable from that of meltwater, indicating that less than 1% of streamflow was from antecedent water in catchments of about 8 ha. The precentage of antecedent water in streamflow increased to 23% with a basin area of 42 ha (Navajo site), to 50% with a basin area of 135 ha (Green Lakes 5), to 57% with a basin area of 221 ha (Green Lakes 4), and to 77% with a basin area of 355 ha (inlet to Lake Albion).

A three-component mixing model was used to partition streamflow into: i) meltwater that becomes overland flow; ii) meltwater that infiltrates surficial deposits and is then discharged back to streamflow as interflow or saturated return flow; and iii) antecedent water stored in vadose and groundwater reservoirs prior to snowmelt. The partitioning was conducted using Si and delta o18 values as independent tracers. As in the two-component model, delta o18 values were used to discriminate meltwater versus antecedent water. The Si values are used to discriminate meltwater that has infiltrated surficial deposits versus classic Hortonian overland flow. Application of the three-component model to headwater basins shows that subsurface flow contributed about 50% of streamflow during snowmelt. This subsurface component is composed primarily of snowmelt that infiltrates surficial deposits and is then discharged as return flow where the rising water table intersects the ground surface. Antecedent water contributed relatively little to streamflow. The remaining portion of stream flow is primarily from water traveling laterally through the snowpack to channels or across the snow-soil interface to channels.