Depth‐ and Time‐Resolved Distributions of Snowmelt‐Driven Hillslope Subsurface Flow and Transport and Their Contributions to Surface Waters

Abstract

Major components of hydrologic and elemental cycles reside underground, where their complex dynamics and linkages to surface waters are obscure. We delineated seasonal subsurface flow and transport dynamics along a hillslope in the Rocky Mountains (USA), where precipitation occurs primarily as winter snow and drainage discharges into the East River, a tributary of the Gunnison River. Hydraulic and geochemical measurements down to 10 m below ground surface supported application of transmissivity feedback of snowmelt to describe subsurface flow and transport through three zones: soil, weathering shale, and saturated fractured shale. Groundwater flow is predicted to depths of at least 176 m, although a shallower limit exists if hillslope‐scale hydraulic conductivities are higher than our local measurements. Snowmelt during the high snowpack water year 2017 sustained flow along the weathering zone and downslope within the soil, while negligible downslope flow occurred along the soil during the low snowpack water year 2018. We introduce subsurface concentration‐discharge (C‐Q) relations for explaining hillslope contributions to C‐Q observed in rivers and demonstrate their calculations based on transmissivity fluxes and measured pore water specific conductance and dissolved organic carbon. The specific conductance data show that major ions in the hillslope pore waters, primarily from the weathering and fractured shale, are about six times more concentrated than in the river, indicating hillslope solute loads are disproportionately high, while flow from this site and similar regions are relatively smaller. This methodology is applicable in different representative environments within snow‐dominated watersheds for linking their subsurface exports to surface waters.

Differential C-Q Analysis: A New Approach to Inferring Lateral Transport and Hydrologic Transients Within Multiple Reaches of a Mountainous Headwater Catchment

“…This implies that lateral contributions are significant in this focus floodplain section and add to the stream concentrations of these solutes. Tokunaga et al (2019) confirm that hillslope contributions to PH are significant and typically contribute to 54-57% of solute exports. In comparison to these solutes, DOC, P and Si showed a gain in concentration at all times except the base flow period.…”

Direct Observation of the Depth of Active Groundwater Circulation in an Alpine Watershed

“…All rights reserved. also shallow compared to the two prior estimates of which we are aware from mountain sites underlain by gently dipping clastic sedimentary rocks: ~150 m for an area of the Wasatch Plateau in Utah (Mayo et al, 2003) and ~180 m for a site only ~10 km east of Redwell Basin in the Upper East River watershed (Tokunaga et al, 2019). The former area is composed of mixed shale and sandstone, and the latter is composed entirely of shale.…”

“…(2019) for two different sites in the nearby Upper East River watershed. Our lower bedrock recharge rate is probably due to the shallow bedrock unit in our model not including an uppermost highly weathered zone, which commonly have K values up to 10 −5 m/s (Tokunaga et al., 2019; Welch & Allen, 2014). Inclusion of such a shallow high‐K zone in our model was not justified given our lack of discrete data at depths <5 m outside of areas of groundwater discharge, but if included would presumably allow considerably higher bedrock recharge rates (though flow at depths >5 m would remain the same).…”

“…A clear need exists to directly observe and more rigorously constrain the active circulation depth in mountain watersheds by measuring hydraulic conductivity (K) and groundwater-flux-dependent variables, such as age and temperature (T), at discrete levels in the subsurface sufficiently deep to penetrate the inactive zone. An increasing number of hillslope nested wells are being installed at Critical Zone Observatories and other research watersheds (e.g., Salve et al, 2012;White et al, 2019;Tokunaga et al, 2019), and these have provided…”

“…A clear need exists to directly observe and more rigorously constrain the active circulation depth in mountain watersheds by measuring hydraulic conductivity ( K ) and groundwater‐flux‐dependent variables, such as age and temperature ( T ), at discrete levels in the subsurface sufficiently deep to penetrate the inactive zone. An increasing number of hillslope nested wells are being installed at Critical Zone Observatories and other research watersheds (e.g., Salve et al., 2012; Tokunaga et al., 2019; White et al., 2019), and these have provided important information on the flow at different depths within the active system and the relative importance of different subsurface water stores in streamflow generation. However, to date these lack sufficient depth and/or adequate groundwater‐flux‐dependent data collection and interpretation to permit direct, robust delineation of the active circulation depth.…”