Hydrologic flowpaths in foothill catchments of the Colorado Front Range
MA: University of Colorado Boulder, 2019.
Climatic shifts, disturbances and land-use change have the potential to alter hydrologic flowpaths, water quality and water supply to downstream communities. Identifying the hydrological processes responsible for the transport of water to streams is important for understanding current and future hydrologic regimes but our knowledge of these processes in foothill and montane ecoregions of mountainous areas is limited. Prior research investigating streamflow generation processes has largely focused on high-elevation catchments and less is known about how lower elevation catchments in the western US respond to summer storms and how flowpaths sourcing streamflow change seasonally. Using two-component hydrograph separations, constituent concentration – discharge relationships and constituent loads calculations, we inferred flowpaths to streams from April to August 2018 and during summer storms in three small (< 10 km2) foothill catchments, and one large catchment (63.2 km2) extending from the foothills to the subalpine in the Colorado Front Range. We selected catchments with varying land-use and disturbances such as historical mining to investigate the relationship between these characteristics and hydrologic flowpaths. In general, constituent concentrations increased as seasonal runoff decreased in the three foothill catchments, reflecting a transition from shallow subsurface flowpaths to deeper subsurface flowpaths. Streamflow during storm responses in the foothill catchment with imperious surfaces and historic mines was sourced from faster, surficial flowpaths compared to its more natural neighboring catchment, holding implications for long-term storage and infiltration rates. Elevated SO42- and Cl- concentrations during low flow periods at two of our catchments suggests discharge from historical mines alters stream chemistry, or the application of road salt during winter has contaminated local groundwater. Results from this study provide a baseline framework for understanding how hydrologic regimes in foothill catchments operate and how they may function in the future with human development, precipitation shifts and disturbances.