Integrated hydrogeochemical modeling of an alpine watershed: Sierra Nevada, California.

Abstract

Seasonally snow covered alpine areas play a larger role in the hydrologic cycle than their area would indicate. Their ecosystems may be sensitive indicators of climatic and atmospheric change. Assessing the hydrologic and bio-geochemical responses of these areas to changes in inputs of water, chemicals and energy should be based on a detailed understanding of watershed processes. This dissertation discusses the development and testing of a model capable of predicting watershed hydrologic and hydrochemical responses to these changes. The model computes integrated water and chemical balances for watersheds with unlimited numbers of terrestrial, stream, and lake subunits, each of which may have a unique, variable snow-covered area. Model capabilities include (1) tracking of chemical inputs from precipitation, dry deposition, snowmelt, mineral weathering, baseflow or flows from areas external to the modeled watershed, and user-defined sources and sinks, (2) tracking water and chemical movements in the canopy, snowpack, soil litter, multiple soil layers, streamflow, between terrestrial subunits (surface and subsurface movement), and within lakes (2 layers), (3) chemical speciation, including free and total soluble species, precipitates, exchange complexes, and acid-neutralizing capacity, (4) nitrogen reactions, (5) a snowmelt optimization procedure capable of exactly matching observed watershed outflows, and (6) modeling riparian areas. Two years of data were available for fitting and comparing observed and modeled output. To the extent possible, model parameters are set based on physical or chemical measurements, leaving only a few fitted parameters. Thc effects of snowmelt rate, rate of chemical elution from the snowpack, nitrogen reactions, mineral weathering, and flow routing on modeled outputs are examined.