Hydrochemical Response To Drought Conditions At An Alpine Watershed, Colorado Front Range
Hill, Kenneth 1 ; Williams, Mark 2 ; Caine, Nel 3 ; Janke, Jason 4 ; Hartman, Melannie 5
1 Univeristy of Colorado, INSTAAR
2 University of Colorado, INSTAAR
3 University of Colorado, INSTAAR
4 Metropolitan State College of Denver
5 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° 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.