Defining Hyporheic Zones - Advancing Our Conceptual and Operational Definitions of Where Stream Water and Groundwater Meet

There is a growing recognition of the importance of connections between streams and adjacent alluvial aquifers. The exchange of water, mass, and energy between these locations influences stream ecosystem structure and function by facilitating nutrient cycling, respiration, stream temperature buffering, and survival of macroinvertebrates. Near-surface aquifers that interact with surface water have been termed 'hyporheic zones', yet there are several accepted definitions of this term (biological, geochemical, and hydrological definitions), which do not necessarily agree with each other. Biologists refer to it as the subsurface inhabited by hyporheos ('stream' macroinvertebrates observed in the subsurface). Hydrologists support a conceptual model that suggests a 'flow-through' subsurface region containing flowpaths that originate and terminate at the stream. Geochemical definition suggests that the hyporheic zone is a mixing zone between surface water and deep-sourced groundwater, with intermediate conditions between these end members. Temporal and spatial scales over which we seek to define hyporheic zones constrain our ability to do so. It is recognized that stream water exchanges through the subsurface over many timescales. Proposed here is a definition based on the timescale of flow through the hyporheic zone, in which residence times of interest are explicitly distinguished and referred to - e.g. 'the 24-h hyporheic zone'. Thus the 'component' of the hyporheic zone that is of interest can be related to specific rates of processes of interest (e.g. denitrification rate). Adoption of this approach will reduce ambiguity in hyporheic science and hopefully contribute to interdisciplinary scientific advancements.