Groundwater – surface water interactions revisited

Interactions between groundwater (GW) and surface water (SW) have been a focus of hydrologic research for some time. Seminal early work by Toth (1963) and later Winter (1999) had shown the existence of nested GW flow systems and stressed that surface water bodies are integral parts of these flow systems. Despite this early, integral perspective, a simpler perception of GW and SW as two distinct compartments, which interact via some often loosely defined transfer mechanisms, still prevails. This perception can be found in many hydrologic models, but can be misleading, as it implies the existence to two clearly separable compartments, while in fact GW and SW are part of a hydrologic continuum (as a part of the terrestrial hydrologic cycle), in which water dynamically transitions back and forth between surface water bodies (rivers, lakes, wetlands) and shallow aquifers. For example, shallow riparian groundwater may become stream water in one moment and return back to the alluvial aquifer in the next with implications for water and solute exchange and biogeochemical turnover. While simplified conceptualizations of the GW-SW hydrologic continuum may be acceptable for the simulation of catchment streamflow response, they usually fall short, when trying to represent fluxes and dynamics of nutrients and other solutes, which are typically controlled by hydrological and biogeochemical processes in the transition zone between GW and SW. I argue that in our quest to understand coupled hydrological and biogeochemical processes and GW dependent ecosystems at the catchment and landscape scales, we needed to revisit the perception of GW and SW as a hydrologic continuum. I will use the example of dissolved organic carbon (DOC) export from a headwater catchment to stress this point and illustrate how rich field data and an integral numerical model can help to refine and improve a simplified conceptual model for catchment-scale DOC export. Finally an outlook will be given on future requirements for adequate monitoring and modeling of coupled GW-SW ecosystems.