Organic carbon in the boreal spring flood from adjacent subcatchments

A large portion of runoff and associated hydrochemical transport in boreal regions occurs during the spring flood. It has been hypothesized that the dynamics of total organic carbon (TOC) concentrations during the spring flood can be explained by new catchment source areas of TOC activated during periods of high flow that are subsequently depleted. This hypothesis was examined in the 1993 spring flood on the Svartberget Catchment in northern Sweden where the runoff from three subcatchments-two forested and one mire-could be isolated. Twenty-eight percent of the 1993 runoff occurred during the two-week period of the spring flood. A similar proportion of the annual TOC output came from the forested subcatchments, but less TOC (20% of the annual output) came from the mire. Snowmelt comprised about half of the runoff from the Mire Subcatchment, but only a third of the runoff from the forested subcatchments. The TOC concentrations in runoff from the Mire Subcatchment decreased about 50% from later winter values to a minimum of 15 mg/L, The TOC concentrations in runoff from the forested subcatchments, however, increased markedly during the early phase of the spring flood before starting to decline. These patterns are consistent with superficial flow pathways activated in the forested areas during spring flood, and a superficial flow pathway in the mire that is active both before and during the spring flood. Without more information on the hypothesized superficial flow paths, the possibility of a progressive change in flow paths as spring flood recedes can be considered an alternative explanation of the observed TOC dynamics.