Are all runoff processes the same?

Copyright © 2013 John Wiley & Sons, Ltd. Introduction Research on runoff processes to date has focused on the differences between the main divisions of runoff partitioning. Indeed, our major advancements in runoff theory have come with new differentiations of different forms of overland flow and subsurface stormflow. These studies of ‘how runoff processes are different’ have resulted in our current summaries of runoff regimes conceptually [e.g. the variable source area (VSA) concept] and codified in our models (e.g. TOPMODEL and its derivatives). Such summaries are captured in iconic figures in textbooks that we teach new generations of hydrologists – the most popular of which is shown in Figure 1, from Dunne (1983) and reproduced more recently in Wagener et al. (2007) and Mirus and Loague (2013). Although such process differentiation was useful as new dominant forms of runoff were ‘discovered’ in different climates with different soils, slope morphologies and vegetation cover, continued differentiation does not appear helpful for improved understanding of runoff dynamics and streamflow generation. We seem to have exhausted the main list of runoff classes (infiltration excess overland flow, saturation excess overland flow and subsurface stormflow) some decades ago, with perhaps the last wave of minor updates to these processes coming in the 1980s and early 1990s in response to isotope tracing demonstration of the importance of stored water and clarification of differences between hydrologic and hydraulic time scales (see reviews by Bonell, 1993, 1998 and in Bachmair and Weiler, 2011).* In the spirit of Sivapalan (2009), I wonder if it is more useful now to change our organizing question from ‘how are runoff processes different?’ to ‘how are runoff processes similar’? In many ways, I am simply building upon and restating what others have said in recent, useful statements; on the importance of boundary conditions and flux closures (Beven, 2006), the need for new theory (e.g. Troch et al., 2008) and new ways of considering runoff systems (Spence, 2010). Asking if all runoff processes are the same, conceptually, is a possible new way to come at runoff process research to aid improved process measurement, understanding and prediction (through new, flexible model structural approaches similar to Fenicia et al., 2011) across diverse regions. It opens up new research questions such as: What can we learn about subsurface stormflow from overland flow (and vice versa)? Can we recognize things on the surface (where boundary conditions are visible) that may help guide new theory for the subsurface where such boundary controls are hidden? Here, I present a simple analogical reasoning that shows how all runoff processes are similar. This comes following the viewing of many dozens of hillslopes and catchment sites across the world for 25 years, especially Although I do note that pioneering work in the 1990s in deglaciated landscapes particularly in Canada has shown new behaviours outside of these three classes, largely linked to groundwater and wetland processes (see Branfireun and Roulet, 1998; Peters et al., 1995 and others).