Multidecadal hydrochemical response of a Sierra Nevada watershed: sensitivity to weathering rate and changes in deposition

To address the responses of the very dilute waters in the Sierra Nevada, California, to acidic atmospheric deposition, the Alpine hydrochemical model (AHM) was used to simulate 47 years of runoff and solute concentrations in the Emerald Lake catchment. The AHM is a semi-distributed model of alpine watersheds that incorporates representations of the major hydrologic and biogeochemical processes that control stream chemical composition. Proxy data of discharge and snowfall were used to develop the necessary inputs for the 47-year runs. The long-term simulations were stable, but conflicts in the simulation of base cation and silica concentrations indicate that the model has a missing process or misrepresents mineral weathering. Sensitivity analysis of the weathering parameters indicates that a weathering rate of approximately 80% of the value fitted based on a one-year calibration would match the observed base saturation and the initial one year estimate had incorrect stoichiometry. Additionally, comparison of annual modeled mass flux to observed mass flux indicates that the model overestimates cation and silica export in dry years and underestimates export in wet years. Our results indicate that the Emerald Lake watershed, as represented by AHM, is not sensitive to chronic acidification with atmospheric deposition at current levels and that there would be little episodic acidification with a doubling in atmospheric deposition. However, in the simulations climate variability had an impact on stream water pH and this sensitivity should be taken into account in assessing alpine catchment sensitivity to changes in atmospheric deposition.