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.
In many alpine regions of the world snowmelt provides the dominant input of water and the spring snowmelt transition influences nutrient fluxes and stream water quality. In the Western U.S. wet and dry deposition of atmospheric pollutants accumulates in the mountain snowpack and enters the terrestrial and aquatic ecosystems via snowmelt water. Given that the timing and magnitude of snowmelt is highly sensitive to climate warming, the objectives of this research were to evaluate the hydrologic impacts of both changing climate and Nitrogen (N) deposition rates on mountain ecosystems. The processes governing these impacts are likely to be non-linear and impossible to predict without detailed mechanistic models and detailed quantification of the relationships between variability in snowmelt, hydrologic pathways and residence times, and N cycling. Thus, this project merged satellite observations of snow properties with models that couple hydrological and biogeochemical processes to obtain broader understanding of the sensitivity of these processes and feedbacks to climate variability and change. Determining these relationships is essential for predicting future impacts of climate change and atmospheric deposition on alpine aquatic ecosystems. The guiding questions for this effort included: 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? Through an evaluation of these science questions at two alpine study sites in the Sierra Nevada and Rocky Mountains the following three outcomes were obtained: 1) The spatial variability in maximum snow water equivalent was significantly greater at the Rocky Mountain site with a spatial coefficient of variation three times greater than at the Sierra Nevada site; these differences likely result from significantly higher wind speeds and lower snowfall densities at the Rocky Mountain site. 2) Streamflow and snowmelt are tightly coupled in the Sierra Nevada site but largely decoupled at the Rocky Mountain site. Snowmelt and streamflow timing ranged by nearly 2 months from year to year at the Sierra Nevada site. At the Rocky Mountain site, however, snowmelt timing ranged by 42 days over the 12 year period but streamflow timing ranged by only 9 days. 3) Nitrate concentrations at the Sierra Nevada site were strongly correlated with snowmelt contributing area with correlation coefficient exceeding 0.6 in 7 out of 12 years compared. Conversely, in the ground-water dominated Rocky Mountain site we see much lower correlations between stream nitrate and snowmelt contributing area which is consistent with longer residence times and reduced flushing of biologically produced nitrate to streams. These results indicate that a dichotomy may exist in which streamflow at the Sierra Nevada site may be more sensitive to climate variability than the Rocky mountain site but the stream chemistry of the Rocky Mountain site may be more sensitive to climate variability than the Sierra Nevada site. These outcomes provide important insights into how alpine ecosystems may change in the future with increases in regional air temperature.
