The subject matter: definition, history, study methods
Gravity-driven regional groundwater flow is induced by elevation differences in the water table and its pattern is self-organized into hierarchical sets of flow systems. Tóth (1963, p. 4806) defined 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; they can be intercepted anywhere by an uninterrupted surface across which flow takes place in one direction only.’ While flow is generated by the relief of the water table, its patterns are modified by heterogeneities in the rock framework's permeability.
Topographic effects are ubiquitous and may cause water to move at depths of several kilometres beneath the Earth's terrestrial areas. Most of people's needs for subsurface water are met with water obtained from this depth range. However, in addition to satisfying this need, gravity-driven groundwater also generates and affects a wide variety of economically important natural processes at or below the land surface. It is of both economic and environmental importance, therefore, to understand the properties, controlling factors, effects and manifestations of this type of flow, as well as to develop methods and techniques for its study and possible modification.