Hydrologic and topographic controls on storm‐event exports of dissolved organic carbon (DOC) and nitrate across catchment scales

Abstract

Patterns of DOC and nitrate (NO3−) concentrations and fluxes were studied for six storm events across four forested, glaciated, subcatchments (1.6 to 696 ha) in western New York, USA. End‐member mixing analysis (EMMA) showed that catchment runoff was composed of shallow groundwater (SGW) discharged at seeps and throughfall (THF) on the rising limb and THF and riparian water (RW) on the recession limb. High THF NO3− contributions produced a pronounced rise and maximum in streamflow NO3−, while low THF concentrations resulted in a dilution trajectory. We did not find any evidence for NO3− flushing by a rising water table. We propose an alternate model for NO3− where the occurrence of seeps and steep slope gradients are the primary determinants of NO3− generation and delivery, and VSAs or saturated areas have a secondary role as loci for interception of NO3− contributions from throughfall. Exports of DOC from the catchments were attributed to THF contributions and the flushing of riparian and hillslope‐bench saturated areas. Concentrations of DOC increased with increasing catchment size and percent‐saturated area whereas NO3− values showed an opposite trend. Concentrations of DOC from a wetland catchment were consistently high, but higher percent increases in event DOC concentration were observed for those catchments with riparian and hillslope‐bench saturated areas. Catchments dominated by riparian zones and catchments with discrete, distal wetlands may have substantial untapped stores of DOC, which become mobilized during high moisture conditions. Thus wetlands in steeply incised valleys may not be effective in buffering hillslope solute loadings.