Role of discrete landscape units in controlling catchment dissolved organic carbon dynamics

Abstract

The spatial sources and delivery mechanisms of dissolved organic carbon (DOC) to streams are poorly understood. We examined the relationship between storm DOC dynamics, catchment landscape units, and catchment scale to elucidate controls on DOC export dynamics at the Maimai watersheds, a group of highly responsive, steep, wet catchments located on the west coast of the South Island of New Zealand. Specifically, we address the controls on the characteristic hysteresis in DOC export dynamics (i.e., DOC concentrations higher on the rising than falling limb of the discharge hydrograph) previously ascribed to a flushing mechanism. We found that during the storm event, the proportion of riparian runoff was larger on the rising than falling limb of the hydrograph, while the proportion of hillslope runoff was smaller on the rising than falling limb of the hydrograph. The delayed response of hillslope runoff resulted in a disconnection between hillslope and riparian areas early in the event and higher DOC concentrations on the rising limb than the falling limb of the event hydrograph. Later in the event, hillslope and riparian areas became connected once the hillslope soil moisture deficits were satisfied. We suggest that the relative timing of riparian and hillslope source contributions and the connections and disconnections of dominant runoff contributing areas are the first-order catchment controls on stream DOC concentrations and mass export.