A catchment in the transitional snow zone: Gordon Gulch, Boulder Creek Critical Zone Observatory

Between areas of intermittent and persistent seasonal snow cover is an understudied transitional snow zone. Gordon Gulch, in the upper montane forest of the Colorado Front Range, lies squarely in this transitional zone. The 2.6 km² headwater, at 2440-2730 m elevation, has been the most intensively monitored field site within the Boulder Creek Critical Zone Observatory. The catchment is close to the 2300 m elevation threshold recognized locally to separate snowmelt-dominated regimes above, and much larger, rain-dominated flood regimes below. Unpacking whether this process switch is associated with elevation-associated changes in climate or with the subsurface architecture of the hydrologic system is a topic of current research.

Monitoring in Gordon Gulch includes 2 stream gages, 6 wells, 2 meteorological stations, 10 manual and 16 automated snow depth measurements, 7 soil moisture and temperature profiles, 6 time-lapse cameras, weekly water sampling in 2 channel locations and 2 perennial springs, and monthly groundwater sampling. Monitoring began in 2009 with a subset, and has developed over time. Airborne LiDAR was collected in 2010, and geophysical surveys have been conducted. In addition, spot or short-duration measurements have been made of hydrologic tracers, sap-flow, ephemeral springs, dust deposition, soil chemistry, soil microbiology, to list a few.

Basic hydrologic parameters have varied considerably over the last decade. Annual precipitation ranged from 530 to 840 mm, with snow fraction ranging from 37 to 62%. Runoff ratios ranged from 0.11 to 0.24. The persistence of snow varies greatly and is strongly structured by slope aspect. North-facing lodgepole pine-dominated slopes develop a thin snowpack, while south-facing Ponderosa woodland-dominated slopes have intermittent snow. Tracer experiments reveal that matrix flow dominates on north-facing slopes, while preferential flow is important on south-facing slopes. Groundwater recharge is most likely in spring, but can also occur in response to summer convective storms. Detailed temporal and dense spatial monitoring is required to document and understand the substantial spatial and temporal variability in hydrologic response in this transitional snow setting, which is likely to be sensitive to climate change and human perturbation.