Subsurface Flow and Transport Through a Snowmelt-recharged Hillslope
Constrained with Multiyear Water Balance
Abstract
Quantifying flow and transport from hillslopes is vital for
understanding surface water quality, but remains obscure because of
limited subsurface measurements. A recent combination of water mass
balance over a single year with the transmissivity feedback model for a
lower montane hillslope in the East River watershed (Colorado) left
large uncertainties in transmissivities and predicted fluxes. Because
snowmelt drives subsurface flow on this hillslope, improved constraints
on the transmissivity profile were obtained by optimizing flux
predictions over years having large differences in precipitation minus
evapotranspiration. The optimized field-scale hydraulic properties
combined with water table elevations predict groundwater discharges that
are consistent with wide ranges of snowmelt. As snowmelt rapidly raises
the water table, solutes released primarily through bedrock weathering
are largely transported out of the hillslope via its highly transmissive
soil. Such pulsed water and solute exports along the soil are minimized
during snow drought years. Although solute concentrations generally are
lower in soils relative to the underlying weathering zone, solute
exports during high recharge occur predominantly via soil because of its
enlarged transmissivities under snowmelt-saturated conditions. In
contrast, this shallow pathway is negligible when recharge and water
table elevations are low. The multiyear calibrated subsurface properties
combined with updated pore water chemistry continue to show that the
weathering zone is the primary source of base cations and reactive
nitrogen released from the hillslope. Subsurface export predictions can
now be obtained for wide ranges of snowmelt based on measurements of
water table elevation and profiles of pore water chemistry.