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 biog eochemical responses of seasonally snow covered alpine areas to change s in inputs of water, chemicals, and energy. This alpine hydrochemical model (AHM) is capable of incorporating a detailed understanding of w atershed processes in order to simulate events critical to biota such as the ionic pulse associated with spring snowmelt, which is only a fe w days long and may involve only a portion of the catchment. The model computes integrated water and chemical balances for multiple terrestr ial, stream, and lake subunits within a watershed, each of which can h ave a unique and variable snow-covered area. Two years of data from th e Emerald Lake watershed in the southern Sierra Nevada were used for f itting and testing by comparing observations with modeled daily output . To the extent possible, model parameters were set on the basis of in dependent 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, snow melt, mineral weathering, flows external to the watershed, and user-de fined 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 acid-neutr alizing capacity; (4) simulating nitrogen reactions; (5) using a snowm elt optimization procedure to aid in matching observed watershed outfl ows; and (6) modeling riparian areas. Using one year of stream data fo r parameter estimation and a second for evaluation, the agreement betw een model and data was judged to be quite good. AHM is a flexible, pre cise 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 feasib le.