Hydrologists have recognized for more than a century that ground water and surface water are closely linked, but for most of that time studies of their interaction were carried out largely by single disciplines. This is slowly changing, however, as the need for integrated studies involving many disciplines is becoming more evident.
For many years, development of water resources for water supply drove hydrologic research. Surface-water hydrologists developed analytical methods and statistical tools to determine streamflow characteristics. These tools were then used to design projects to develop streams for water supply. Hydrologists knew that baseflow in streams was ground-water discharge, but they generally were unconcerned about understanding the ground-water flow paths that carried the water to the streams-they just wanted to know if the water was going to be there for human use. Ground-water hydrologists developed analytical methods and statistical tools to determine aquifer characteristics. These tools were then used to design projects to develop aquifers for water supply. To groundwater hydrologists, surface water was a potential source to ground water. They knew that if a well was placed near enough to a surfacewater body, surface water could be drawn to the well, with the added benefit of having it filtered along the way. Indeed, in the case of wetlands, it was common practice to suggest that water could be "salvaged "by withdrawing enough ground water near wetlands to lower the water table below the root zone of aquatic plants, thus gaining "beneficial use "of water that would otherwise be "lost "to evapotranspiration. There was little concern for what these groundwater development practices might do to stream, lake, and wetland ecosystems. With this discipline-oriented thinking, in which surface water and ground water were considered separate resources, it was not surprising that many states issued, and continue to issue, surfacewater and ground-water permits separately. As a result, the total water resource has been overallocated in many states. In addition to the water allocation problem, two other scientific developments have occurred over the past few decades that have affected how scientists and water managers viewed the interaction of ground water and surface water. One was the increased appreciation of ground-water flow systems as an important link in the hydrologic cycle, and the other was the increased appreciation of ground water as an important component of some aquatic ecosystems.