Forest hydrology is that specialized part of hydrological sciences that emphasizes the influence of trees and forests and their management on the regimen, quality and quantity of water. In the USA the specific role of trees and forests has been elucidated over the past century in early treatises such as Zon's (1927) Forests and Water in the Light of Scientific Investigation and Kittredge's (1948) Forest Influences. The US Forest Service, in an era of concern over the effects of forest management on the hydrologic functioning of watersheds, established many experimental areas with names like Hubbard Brook, Coweeta, Fernow, Fraser, and H. J. Andrews, that have become legendary to students of forest hydrology. Based upon early studies, we have come to regard the natural forested watershed as the ideal in hydrologic functioning with the most moderate flow regime and best water quality (with early emphasis on temperatures and suspended solids), albeit at the expense of some reduction in water yield due to higher evapotranspiration rates. By the mid-to late-20th century, we seemed to achieve the knowledge needed to meet current needs, and forest hydrology and watershed management research was de-emphasized. However, no sooner had we turned away to face hydrologic challenges elsewhere, than some old issues were again being raised.
We have long believed that forest cover by itself only can play a limited role in controlling peak flows due to extreme events. That is, an extreme rain event, spawned perhaps by a hurricane, would produce the same peak flows with or without forest cover, assuming all other conditions, especially soil conditions, were maintained. Thus, it came as no surprise that forest management experiments generally showed that harvesting of trees with minimum soil disturbance did not appear to lead to major increases in annual peak flows or floods, although specific attention was often not given to extreme events. As urban/suburban areas spread and the public increasingly looked to forests less as a source of wood products and more as a source of water, recreation and aesthetic benefits, not to mention home sites, the concern with effects of forest cutting on flooding is being increasingly expressed. On a global basis these flames have been recently fanned by stories linking flooding in lowlands to forest loss in the uplands of China, Pakistan, and India that do not necessarily distinguish between sustainable forest management and forest 'destruction'. Concern over increased flooding due to climate change and urbanization impacts has added fuel to this fire.