HIGH-ELEVATION MOUNTAIN STREAMFLOW GENERATION: THE ROLE OF DEEP GROUNDWATER

In western states, such as New Mexico, most surface water is derived from high-elevation mountainous watersheds. With growing concerns about declining snowpack, warmer temperatures, and land use changes, it is becoming increasingly important to determine the mountain sources that contribute to this streamflow. While most research on streamflow generation has been conducted at the hillslope scale, there are significant limitations when attempting to upscale these processes to an entire basin. Recent work by Frisbee et al. (2011) in the mountainous Saguache watershed (~1600 km²) of southern Colorado has shown a significant portion of streamflow is controlled by old (>1000 yrs) groundwater inputs. Is the deep-groundwater conceptual model developed for Saguache transferable to other watersheds? We apply the conceptual model developed for Saguache to the Rio Hondo, a mesoscale (~200 km²) watershed in northern New Mexico with crystalline basement rock that has been extensively fractured due to tectonic activity. We hypothesize that the enhanced secondary permeability of the basement rock allows for significant groundwater flow through the mountain block, a portion of which is eventually discharged to the surface water system. Geochemical data collected from the Rio Hondo strongly supports this hypothesis. Surface water solute concentrations for most constituents increase as a function of the drainage area while the stable isotopic signature remains constant (both spatially and temporally), indicating that while nearly all water in the basin is sourced from winter and spring precipitation it has undergone differing degrees of geochemical evolution along different flow paths. Radiocarbon ages for springs and wells within the watershed range from modern to thousands of years old, and are evidence for much longer residence times than previously thought for mountain watersheds. These results suggest that the conceptual model for Saguache is transferrable to at least one other watershed with differing characteristics. Our research provides support for the hypothesis that significant old groundwater flow occurs within cyrstalline bedrock and eventually discharges to the surface water system.