Hydrologic forcing of nitrogen (N) biogeochemistry is poorly understood due to difficulties in coupling biogeochemical and hydrologic models. Long-term catchment studies in the Sierra Nevada and Rocky Mountains demonstrate that N cycling in alpine systems is strongly influenced by snowpack dynamics, but mechanisms underlying this control remain undefined. General circulation models indicate that increasing air temperature over the coming century will cause substantial changes in the amounts and rates of snow accumulation and melt in mountainous regions. At the same time, mountains are ?hot-spots? for localized impacts from anthropogenic N emissions in upwind, lower-elevation areas. Given both changing climate and N deposition rates, impacts to mountain ecosystems and water supplies are likely to be non-linear and impossible to predict without detailed mechanistic models. This study will fill this critical gap in knowledge by quantifying relationships among variability in snowmelt, hydrologic pathways and residence times, and N cycling. By merging satellite observations of snow properties with models that couple hydrological and biogeochemical processes we will gain broader understanding of the sensitivity of these processes and feedbacks to climate variability and change. Fifteen-year retrospective analyses and future climate scenarios will be used evaluate the following questions:
1) How does climate variability influence snow-atmosphere energy exchange and the rate and spatial patterns of snowmelt? 2) How does inter-annual variability in climate impact hydrologic flow routing and hydrochemical fluxes? 3) How will linkages between hydrologic and elemental fluxes change under future climate scenarios?
These questions will be addressed in two of the best-studied mountain research sites in the United States - the Tokopah watershed in the Sierra Nevada, California and the Green Lakes Valley in the Rocky Mountain Front Range of Colorado. The spatially explicit representation of snowmelt within flow-path models will improve understanding of the processes that control hydrochemical fluxes of alpine systems. Future climate scenarios will leverage these advances to determine the susceptibility of alpine systems to episodic and chronic acidification for the coming century.
