What Is a Groundwater Flow System?

Abstract:

Characterizing the spatial organization of groundwater flow is one of the most fundamental aspects of hydrogeology and critical for water resources management. At first order, within a study area, hydrogeologists typically identify different groundwater flow systems. However, the notion of a groundwater flow system is often subjective. Tóth proposed a formal definition of a groundwater flow system as a set of flow lines in which any two flow lines adjacent at one point of the flow region remain adjacent through the whole region [Tóth, J., 1963. A theoretical analysis of groundwater flow in small drainage basins. Journal of Geophysical Research, 68(16): 4795-4812]. Although highly influential, Tóth’s study was only based on the analysis of steady two-dimensional groundwater flow in vertical cross sections. Later, the applicability of Tóth’s definition in three dimensions has been challenged, and new definitions of a groundwater flow system have been proposed. However, the different definitions have not been rigorously compared. In this contribution, we first prove that Tóth’s definition is generally ill-posed in three dimensions, as well as in two-dimensional groundwater flow other than steady flow in vertical cross sections. We then propose that a groundwater flow system is best defined as a flow region bounded by separating streamlines (in two dimensions) or separating stream surfaces (in three dimensions), which emanate from stagnation points. We demonstrate that this definition is equivalent to Tóth’s definition in steady two-dimensional groundwater flow in vertical cross sections, and that it provides the most rigorous and comprehensive extension under more general flow conditions. By clarifying the notion of a groundwater flow system, this study advances our fundamental understanding of the spatial organization of groundwater flow, and provides a robust framework for future studies.