HYDROCHEMICAL MODELING OF EMERALD LAKE WATERSHED, SIERRA-NEVADA, CALIFORNIA - SENSITIVITY OF STREAM CHEMISTRY TO CHANGES IN FLUXES AND MODEL PARAMETERS

We applied a detailed hydrochemical model of watershed processes to as sess the response of the Emerald Lake watershed in the southern Sierra Nevada to changes in inputs of water, chemicals, and energy and to ch anges in model parameters. Observations during the 1986 and 1987 water -years were used as base conditions. In 1986 snowfall in the Sierra Ne vada was well above average, although chemical fluxes were lower than in 1987, which was a low-accumulation year. The chemical composition o f stream water during the early part of snowmelt was sensitive to chem ical loading in the snowpack and to snowmelt rate, both of which are s ubject to year-to-year natural variations and to anthropogenic perturb ations. Stream-water chemical composition was also sensitive to uncert ainties in the model, including rate of chemical release from the snow pack, nitrogen reactions, and flow routing in the catchment. The large st effects were limited to a period lasting up to a few days, coincidi ng with the ionic pulse in spring snowmelt. Some significant effects w ere also apparent during summer rains. Stream waters in late summer an d fail were also affected by a faster snowmelt rate, primarily associa ted with soil drying and lower late-season streamflows. Overall, the a nalysis suggests that the Emerald Lake basin may be particularly sensi tive to acidic loadings in precipitation and to faster snowmelt and th us earlier runoff associated with climate warming.