Development of a hydrochemical model for seasonally snow-covered alpine watersheds: Application to Emerald Lake watershed. Sierra Nevada, California

We have developed and tested a model to assess the hydrologic and biogeochemical responses of seasonally snow covered alpine areas to changes in inputs of water, chemicals, and energy. This alpine hydrochemical model (AHM) is capable of incorporating a detailed understanding of watershed processes in order to simulate events critical to biota such as the ionic pulse associated with spring snowmelt, which is only a few days long and may involve only a portion of the catchment. The model computes integrated water and chemical balances for multiple terrestrial, stream, and lake subunits within a watershed, each of which can have a unique and variable snow-covered area. Two years of data from the Emerald Lake watershed in the southern Sierra Nevada were used for fitting and testing by comparing observations with modeled daily output. To the extent possible, model parameters were set on the basis of independent physical or chemical measurements, leaving only a few fitted parameters. In its current application, model capabilities include (1) tracking of chemical inputs from precipitation, dry deposition, snowmelt, mineral weathering, flows external to the watershed, and user-defined sources and sinks; (2) tracking surface and subsurface water and chemical movements through vegetation canopy, snowpack, soil litter, multiple soil layers, streamflow, and lakes; (3) calculating chemical speciation, including precipitates, exchange complexes, and acidneutralizing capacity; (4) simulating nitrogen reactions; (5) using a snowmelt optimization procedure to aid in matching observed watershed outflows; and (6) modeling riparian areas. Using one year of stream data for parameter estimation and a second for evaluation, the agreement between model and data was judged to be quite good. AHM is a flexible, precise algorithm for simulating watershed hydrochemistry and can readily be adapted to other alpine catchments using the Emerald results as a guide. Application of AHM to forested catchments should also be feasible.