Abstract

Water availability during snowmelt allows montane forests to serve as the primary water source and carbon sink in the southern Rocky Mountains, but seasonal snowpacks are threatened by a changing winter climate.    We investigated three adjacent catchments (~3 km2) in the Valles Caldera, N.M. during the ‘average’ 2010 and ‘dry’ 2011 snowmelt seasons to answer two questions: 1. How does catchment topography interact with climatic variability to control hydrochemical response, 2. What are the relative roles of terrestrial and aquatic processes in controlling catchment nutrient chemistry.  A Si-Cl mixing model captured 90% of the variability in streamwater concentrations with three end-members: precipitation (P), shallow groundwater (SGW), and deep groundwater (DGW).   Streamflow generation was dominated by DGW in all catchments during the 2010 snowmelt.  There were relatively few differences in the dissolved concentrations and fluxes of N (total dissolved nitrogen (DN) and NO3), P (total dissolved phosphorus (DP) and PO4), and C (DIC and DOC) in 2010.  Likewise, the catchments all had observed to predicted ratios (O:P) indicating elevated DN and DOC concentrations relative to the Si-Cl mixing model predictions.  Following the smaller 2011 snowmelt, streamflow generation differed based on catchment aspect, the most south-facing catchment had no observed snowmelt discharge and the north-facing catchment had a clear hydrograph response and larger DGW contributions.  The timing and magnitude of DGW contributions demonstrated the importance of snowmelt for recharge.   Our results suggest that catchments with less topographical shading and smaller DGW contributions will be more hydrochemically sensitive to reduced snowpacks.  Decreasing O:P ratios with greater DGW contributions for C and N suggests the development of near-stream ‘hot-spots’ at peak hydrological connectivity during snowmelt.  Production of DOC and consumption of DIC, NO3, and DN is indicative of more in-stream nutrient cycling in 2011.  Overall, these results indicate that stream nutrient chemistry is more variable and depends on in-stream processes and catchment orientation following poor winter snowpacks.  Prediction of nutrient chemistry under a changing winter climate would therefore benefit from relationships describing how hydrochemical sources and flowpaths co-vary with catchment topography.

Citation

Harpold A. A., Brooks P. D., Perdrial J. N., McIntosh J. C., Meixner T., Zapata X., Chorover J. (2012): Quantifying Variation in Solute Sources and Nutrient Cycling in Montane Headwater Catchments. Abstract B22D-06 presented at 2012 Fall Meeting, AGU, San Francisco, Calif., 3-7 Dec (Talk).