Longitudinal patterns of concentration-discharge relationships in stream water draining the Hubbard Brook Experimental Forest, New Hampshire

Longitudinal variations of concentration-discharge relationships and chemical fluxes were evaluated in two headwater streams at the Hubbard Brook Experimental Forest, New Hampshire. At high elevations changes in subsurface flow paths explained variations in H⁺, inorganic Al and Si concentrations, whereas variations of DOC concentration were inconsistent with this mechanism. Flow responses of middle and low elevation subcatchments were influenced by variable contributions of hydrologic source areas and the elevational concentration gradient which exists in these catchments, but in most cases were not consistent with responses predicted by changes in flow paths. Spatial patterns of chemical fluxes indicate that, in general, catchment neutralization processes increased in effectiveness in the downslope direction. However, this pattern can be interrupted by secondary tributaries, both ephemeral and persistent, which originate in variable source areas that contribute acidic surface runoff during high flow conditions. Current models of catchment acidification need to incorporate spatial variations of biogeochemical processes and flow responses to improve predictions of short-term variations in surface water chemistry.