Groundwater connections between the boreal landscape and its headwater streams: the role of discrete riparian inflow points (DRIPs)

Abstract: River networks connect the Swedish boreal landscape with the Baltic Sea. Groundwater provides a majority of the river water, and therefore it is important to understand the mechanisms of groundwater-stream interactions. The riparian zone, or near stream area, is an important terrestrial interface where groundwater becomes stream water. This thesis focused on riparian areas where subsurface flow paths converge, referred to as discrete riparian inflow points (DRIPs). DRIPs connect a large part of the landscape with a narrow section of the stream, and therefore represent landscape connectivity between hillslope and catchment scales. Results showed that DRIPs have near-surface groundwater levels and organic-rich groundwater chemistry. Combined with flow path convergence, this facilitates high mobilization rates of organic-rich groundwater to local points along stream reaches, which affects local stream ecosystems as well as downstream transport of carbon. Moreover, the response of DRIPs to changing hydrological conditions indicated that hydrological processes deviate from the rest of the riparian zone. Interactions between groundwater, peat-rich soil, vegetation and biota can be attributed to the contrasting characteristics of DRIPs compared to the rest of the riparian zone. This thesis demonstrated that DRIPs play an important role in both terrestrial and aquatic ecosystems in the Swedish boreal landscape. Therefore, DRIPs need to be explicitly considered in sustainable forest management.