Simply active, or contributing too? Investigating the hydrologic connectivity of shallow water tables in a headwater catchment.

Abstract:

Soils in watershed 3 (WS3) of the Hubbard Brook Experimental Forest have been divided into functional groups, called hydropedological units (HPUs), based on the thickness and presence/absence of specific horizons. HPUs had distinct biogeochemical functioning and water tables that developed in the solum after catchment wetness thresholds were exceeded. We were able to predict the spatial distribution of HPUs using digital soil mapping with 80% accuracy using digital elevation model derived topographic metrics and field mapped bedrock outcrop locations. The spatial distribution of HPUs and threshold water table behavior were used to estimate event-scale water table patterns throughout the catchment at different levels of catchment storage. The results of this analysis imply that runoff is generated differently from the classic variable source area (VSA) concept, with shallow soils in channel heads contributing stormflow on an event basis and near stream water tables that rarely expand up adjoining hillslopes. In order to investigate how these active areas in the channel head region may contribute flow to the stream network, we used dissolved organic carbon (DOC) concentrations as a tracer of water sources from shallow organic soils. Due to their proximity to organic soil-covered bedrock outcrops and thick O-horizons, DOC concentrations in the channel head region were often the highest in the catchment. We therefore hypothesized that if shallow water tables in channel heads were connected to the stream on an event basis, their contribution would be detected at the outlet. DOC concentrations at the WS3 outlet were found to vary closely with water table fluctuations in channel heads, suggesting those areas were in fact hydrologically connected to the stream network during events. In contrast, outlet DOC concentrations did not correlate with near-stream water table fluctuations, suggesting the near-stream zone was not a major DOC source to the stream. These results illustrate the importance of understanding the spatial structure of soils distributed throughout a catchment when investigating runoff generation processes, hydrologic connectivity, and solute export.