Classics in physical geography revisited

Hewlett and Hibbert’s (1967) ‘Factors affecting the response of small watersheds to precipitation in humid areas’ (hereafter referred to as ‘Factors’) is one of the most important papers published in the field of catchment hydrology. Despite its grey literature appearance as a proceedings paper from the fi rst International Symposium on Forest Hydrology at Penn State University, the work outlined a manifesto for change in catchment hydrology and the basis for modern rainfall-runoff concepts in upland humid areas. The paper was transformative in that it presented a compelling alternative – the Variable Source Area (VSA) concept – to the then status quo concept of infi ltration excess overland fl ow. It also ushered in a perception of catchment runoff behaviour that would be codifi ed in many topographically based rainfall-runoff models used today. The paper also introduced a new quickfl ow hydrograph separation approach that has been used to classify and organize watersheds with diverse sets of hydrological behaviour from different parts of the globe. This short review of the classic paper attempts to set the work in the context of the fi eld, then and now, and to explore how the variable source area concept links to modern catchment hydrology. Hewlett and Hibbert were field-based, forest hydrologists working at U.S. Department of Agriculture Forest Service Coweeta Hydrologic Laboratory in the southern Appalachian mountains near Asheville, North Carolina. John Hewlett (Figure 1) was then an Associate Professor of Forest Hydrology at the University of Georgia and Alden Hibbert was a Research Forester for the Forest Service based in Ashville. Through their field studies at Coweeta they were struck by the fact that headwater streams responded quickly to rainfall inputs – but with seemingly no overland fl ow (except for areas around the stream margin). The prevailing assumption in catchment hydrology at the time of ‘Factors’ was that direct runoff was a ‘product of overland flow and that other types of fl ow were mere exceptions to that general rule’ (p. 277). Their philosophy, based on basic observations at Coweeta during storm rainfall events, was that ‘the opposite approach is more logical in the case of forest land; that is, to begin with the assumption that all flow is subsurface flow until there is evidence otherwise’ (p. 277). Clearly, Hewlett and Hibbert’s ideas on these issues were very much infl uenced by their predecessors at Coweeta, especially C.R. Hursh (Hursh and Brater, 1941; Hoover and