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Baseflow age distributions and depth of active groundwater flow in a snow‐dominated mountain headwater basin

November 18, 2020

Deeper flows through bedrock in mountain watersheds could be important, but lack of data to characterize bedrock properties limits understanding. To address data scarcity, we combine a previously published integrated hydrologic model of a snow‐dominated, headwater basin of the Colorado River with a new method for dating baseflow age using dissolved gas tracers SF6, CFC‐113, N2, and Ar. The original flow model predicts the majority of groundwater flow through shallow alluvium (<8 m) sitting on top of less permeable bedrock. The water moves too quickly and is unable to reproduce observed SF6 concentrations. To match gas data, bedrock permeability is increased to allow a larger fraction of deeper and older groundwater flow (median 112 m). The updated hydrologic model indicates interannual variability in baseflow age (3–12 years) is controlled by the volume of seasonal interflow and tightly coupled to snow accumulation and monsoon rain. Deeper groundwater flow remains stable (11.7 ± 0.7 years) as a function mean historical recharge to bedrock hydraulic conductivity (R/K). A sensitivity analysis suggests that increasing bedrock K effectively moves this alpine basin away from its original conceptualization of hyperlocalized groundwater flow (high R/K) with groundwater age insensitive to changes in water inputs. Instead, this basin is situated close to the precipitation threshold defining recharge controlled groundwater flow conditions (low R/K) in which groundwater age increases with small reductions in precipitation. 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.

Publication Year 2020
Title Baseflow age distributions and depth of active groundwater flow in a snow‐dominated mountain headwater basin
DOI 10.1029/2020WR028161
Authors Rosemary W.H. Carroll, Andrew H. Manning, Richard G. Niswonger, David W Marchetti, Kenneth H. Williams
Publication Type Article
Publication Subtype Journal Article
Series Title Water Resources Research
Index ID 70219111
Record Source USGS Publications Warehouse
USGS Organization Geology, Geophysics, and Geochemistry Science Center