Landscape scale controls on dissolved organic carbon flux in a mountainous catchment

The spatial distribution of source areas and residence times of water in the catchment are significant factors controlling the annual cycle of dissolved organic carbon (DOC) in Deer Creek (Summit County, CO). During spring snowmelt, stream DOC concentrations increased with the rising limb of the hydrograph, peaked before maximum discharge, then declined rapidly as melting continued. To investigate catchment sources of DOC to streamflow, DOC was measured in groundwater wells, vadose zone lysimeters, the snowpack, and in streamflow. Concentrations of DOC in the lysimeters decrease rapidly during the melt period, supporting the hypothesis that hydrological flushing of near-surface soil horizons is the primary mechanism affecting the temporal variation of DOC in Deer Creek. Aerial photos of snow covered area were used in conjunction with the lysimeter data to show that the distribution of snow exerts important controls on the DOC flushing responses. Asynchronous melting of the snowpack across the landscape caused the flush to be initiated at different times throughout the catchment. Streamflow integrates the staggered flushing responses, yielding a protracted response of elevated DOC concentrations in the stream.; By coupling a catchment-scale hydrological model (TOPMODEL) with a simple DOC mixing model, the extent of saturated soils contributing discharge and DOC to streamflow was estimated. The simulations suggest a well-distributed, variable-source area contributing DOC to the stream, with 88% of the landscape contributing solutes during peak discharge. Although groundwater accounted for the greatest volumetric contribution of water to the stream from the catchment (75% of total annual flow), this flow path was responsible for delivering only 31% of the annual mass flux of DOC to streamflow. Subsurface flow through the upper soil horizons (21% of total annual flow) is the most important flow path for DOC export, contributing 67% of the annual mass flux of DOC to streamflow during the snowmelt period.