TYPOLOGY AND METRICS IN HYPORHEIC HYDROLOGY

This paper evaluates the perceptions and conceptual basis in hyporheic flow (HF) modeling. The combination of flow processes at several scales may currently not be well described by mechanistic models which are often limited by the calibration data and the modeler's understanding of the system. For example, transient storage models calibrated on tracer tests may underestimate long residence time (days) flow paths; the Darcy approach based on hydraulic heads tends to underestimate the shorter ones (hours). This has important implications for predicting contaminant fluxes through the hyporheic zone at the catchment scale.

A set of standard measurements (metrics) in the hyporheic physical environment, combined with a typology of HF systems are discussed as promising lines to predict the relative magnitude of changes of residence time, flow budgets and spatial extent of flow paths. Since models do not need to be strictly realistic in terms of structure, the potential for hyporheic metrics to inspire simplified models is stressed. Such an approach may be appealing to ecologists who must integrate the hydrological component in more complex systems. These metrics are discussed on the basis of both experimental evidence and theoretical studies. Because some metrics may only apply to specific environments, a typology of hyporheic systems is presented which is a function of the physiographic setting, including scale and dynamics, and the biogeochemical problem addressed.

There has been speculation that stream morpho-dynamic controls on HF deal more with shallow exchange, which is often easier to measure. Such a bias may be addressed by building on the hydrogeological experience and concepts in dealing with heterogeneous media, in particular in groundwater contaminant literature. More widely, environmental modelling principles such as equifinality and predictive uncertainty, which are often ignored in hyporheic studies, are discussed on the basis of generic HF models.

The scope for hyporheic metrics and a physical typology of hyporheic systems is illustrated by the study case of a groundwater-fed river in the UK. A range of methodological tools, such as electrical resistivity profiles, environmental and artificial tracers, piezometric network development, has been applied from the bedform to the kilometre scale.