Relationships between stream nitrate concentration and spatially distributed snowmelt in high elevation catchments of the western U.S.

Abstract

This study compares stream nitrate ( urn:x-wiley:00431397:media:wrcr21185:wrcr21185-math-0001) concentrations to spatially distributed snowmelt in two alpine catchments, the Green Lakes Valley, Colorado (GLV4) and Tokopah Basin, California (TOK). A snow water equivalent reconstruction model and Landsat 5 and 7 snow cover data were used to estimate daily snowmelt at 30 m spatial resolution in order to derive indices of new snowmelt areas (NSAs). Estimates of NSA were then used to explain the urn:x-wiley:00431397:media:wrcr21185:wrcr21185-math-0002 flushing behavior for each basin over a 12 year period (1996–2007). To identify the optimal method for defining NSAs and elucidate mechanisms underlying catchment urn:x-wiley:00431397:media:wrcr21185:wrcr21185-math-0003 flushing, we conducted a series of regression analyses using multiple thresholds of snowmelt based on temporal and volumetric metrics. NSA indices defined by volume of snowmelt (e.g., snowmelt ≤ 30 cm) rather than snowmelt duration (e.g., snowmelt ≤ 9 days) were the best predictors of stream urn:x-wiley:00431397:media:wrcr21185:wrcr21185-math-0004 concentrations. The NSA indices were better correlated with stream urn:x-wiley:00431397:media:wrcr21185:wrcr21185-math-0005 concentration in TOK (average R2= 0.68) versus GLV4 (average R2= 0.44). Positive relationships between NSA and stream urn:x-wiley:00431397:media:wrcr21185:wrcr21185-math-0006 concentration were observed in TOK with peak stream urn:x-wiley:00431397:media:wrcr21185:wrcr21185-math-0007 concentration occurring on the rising limb of snowmelt. Positive and negative relationships between NSA and stream urn:x-wiley:00431397:media:wrcr21185:wrcr21185-math-0008 concentration were found in GLV4 with peak stream urn:x-wiley:00431397:media:wrcr21185:wrcr21185-math-0009 concentration occurring as NSA expands. Consistent with previous works, the contrasting urn:x-wiley:00431397:media:wrcr21185:wrcr21185-math-0010 flushing behavior suggests that streamflow in TOK was primarily influenced by overland flow and shallow subsurface flow, whereas GLV4 appeared to be more strongly influenced by deeper subsurface flow paths.