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Baseflow Age Distributions and Depth of Active Groundwater Flow in a Snow-Dominated Mountain Headwater Basin
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  • Rosemary W.H. Carroll,
  • Andrew H. Manning,
  • Richard Gray Niswonger,
  • David W. Marchetti,
  • Kenneth Hurst Williams
Rosemary W.H. Carroll
Desert Research Institute

Corresponding Author:[email protected]

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Andrew H. Manning
United States Geological Survey
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Richard Gray Niswonger
U. S. Geological Survey
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David W. Marchetti
Western State Colorado University
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Kenneth Hurst Williams
Lawrence Berkeley National Laboratory (DOE)
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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.
Dec 2020Published in Water Resources Research volume 56 issue 12. 10.1029/2020WR028161