Baseflow Age Distributions and Depth of Active Groundwater Flow in a
Snow-Dominated Mountain Headwater Basin
Abstract
Deeper flows through bedrock in mountain watersheds could be important
but lack of data to characterize bedrock properties and link flow paths
to snow-dynamics limits understanding. To address data scarcity, we
combine a previously published integrated hydrologic model of a
snow-dominated, headwater basin with a new method for dating baseflow
age using dissolved gas tracers SF, N, Ar. The original flow model
produces shallow groundwater flow (median depth 6 m), very young stream
water and is unable to reproduce observed SF concentrations. To match
the observed gas data, bedrock permeability is increased to allow a
larger fraction of deeper groundwater flow (median depth 110 m). Results
indicate that interannual variability in baseflow age (3-12 y) is
dictated by the volume of seasonal interflow. Deeper groundwater flow
remains stable (11.7±0.7 y) as a function of the ratio of recharge to
bedrock hydraulic conductivity (R/K), where recharge is dictated by
long-term climate and land use. With sensitivity experiments, we show
that information gleaned from gas tracer data to increase bedrock
hydraulic conductivity effectively moves this alpine basin away from
shallow, topographically controlled groundwater flow with baseflow age
relatively insensitive to water inputs (high R/K), and closer toward
recharge-controlled conditions, in which a small shift toward a drier
future with less snow accumulation will alter the groundwater flow
system and increase baseflow age (low R/K). Work stresses the need to
explore alternative methods characterizing bedrock properties in
mountain basins to better quantify deeper groundwater flow and predict
their hydrologic response to change.