Hill, Kenneth听1听;听Williams, Mark听2听;听Caine, Nel听3听;听Janke, Jason听4听;听Hartman, Melannie听5
1听Univeristy of Colorado, INSTAAR
2听University of Colorado, INSTAAR
3听University of Colorado, INSTAAR
4听Metropolitan State College of Denver
5听Colorado State University, NREL
Extreme climate events play a key role in alpine hydrochemistry by altering source waters and flowpaths. Persistent drought conditions from 2000-2002 at Green Lakes Valley resulted in precipitation and streamflow about 60% of normal for the last 25 years. Surprisingly, both concentrations and fluxes of geochemical weathering products and nutrients increased during the drought at alpine sites. Niwot Ridge LTER has continuously monitored streamflow, precipitation chemistry, and water quality for 25 years in Green Lakes Valley at 8 sites representing an elevation gradient extending from 3250 meters at the valley outlet to 4000 meters at the continental divide.
Comparing continuous 5-year blocks of above-average precipitation (1993-1997) vs. below-average years (2000-2004), both concentrations and fluxes were significantly higher during drought for base cations (p<0.05) and Nitrate (p<0.10) throughout upper Green Lakes Valley. DAYCENT modeled predicted discharge correctly during the period of above-average precipitation but underpredicted discharge during drought conditions, suggesting an additional source of water.
End Member Mixing Analyses (EMMA) conducted during a wet year constrains streamflow as a mixture of snowmelt, talus water, and groundwater with subsurface flowpaths contributing more than 50% of streamflow, even during snowmelt (Liu, 2004). During drought, baseflow contributions increase by 15%. EMMA recreates streamwater chemistry with high predictive power during both wet and dry years. However, EMMA results during drought years require a unique baseflow end member suggesting an additional, unidentified source of streamwater.
One possible end member is melting permafrost within the basin. We downscaled a qualitative, regional permafrost distribution model of the Colorado Front Range to investigate the potential role of melting permafrost on hydrochemical characteristics in Green Lakes Valley. Model results indicate that increasing mean annual air temperature by 1掳 C results in a 33% decrease in probable permafrost by area. Future monitoring and research efforts will examine the potential irreversible effects of extreme climate events and permafrost melt on alpine ecosystems.
Liu, F., M. W. Williams, and N. Caine (2004), Source waters and flow paths in an alpine catchment, Colorado Front Range, United States, Water Resour. Res., 40, W09401, doi:10.1029/2004WR003076.