2. A brief history in regional groundwater flow modeling
Toth (1963) used for the first time analytical solutions to investigate groundwater flow in hypothetical small drainage basins. He found theoretically the existence of hierarchically nested groundwater flow systems: local, intermediate (sub-regional) and regional (Fig. 1). Topography, geology and climate turned out to be major factors for the formation of three sub-flow systems of gravity-driven flow in a homogenous and isotropic groundwater basin. These flow systems can be identified in the field by investigating recharge and discharge areas, changes of groundwater levels with depth, hydrochemistry patterns, environmental isotopes, vegetations and surface water networks (Toth, 1970, Toth, 1971 and Toth, 1972).