The terrestrial hydrologic cycle: an historical sense of balance

In this article, I explore the evolving historical views of the terrestrial hydrologic cycle through a lens that might be called 'a model of balance.' The notion of balance in western literature, seems to me to capture a central theme for how scholars of natural history, medieval mathematicians, designers of scientific instruments, and early waterworks engineers viewed the complexities and evaluated the evidence for how water moves in the terrestrial landscape. Using historical images and text, this study attempts to document how we arrived at the modern interpretation, its sources, origins, and relationships within the terrestrial earth. By comparing ancient models for the hydrologic cycle one can begin to construct a plausible story of its evolution as well as the important actors. I pay particular attention to the origins and sources of springs, rivers, and groundwater, the kinds of evidence proposed, and its relation to the sense of balance that emerges. Finally, I argue that at by the end of the 18th and into the early 19th century, the modern view of the terrestrial hydrologic cycle that emerged became a necessary foundation for the geophysical, ecological, and cultural Earth science themes that still flourish today. (C) 2017 The Authors. WIREs Water published by Wiley Periodicals, Inc.