Where does water go when it rains? Moving beyond the variable source area concept of rainfall‐runoff response

Where does water go when it rains? What flow path does it take to the stream? How long does it reside in the catchment? These questions were articulated by John Hewlett within the context of his variable source area (VSA) concept almost 40 years ago (Hewlett and Hibbert, 1967). Today, we still grapple with these often vexing questions—now using new tools and approaches, but, as then, still searching for answers. Rapid progress is being made on the rainfallrunoff modelling front in catchment hydrology vis-a-vis parameter estimation techniques, model uncertainty analysis, examination of parameter identifiability in our models, downward approaches to hydrologic prediction, etc. (Beven 2001; Sivapalan, 2003). However, I wonder if we have somewhat neglected updating our understanding of the rainfall-runoff process and how this informs our needed model structures and response to these three basic questions central to our conceptualization of how catchments work? One could argue that our sharpening perception of water source, flowpath and age in upland headwater catchments is radically different to what the framers of the VSA theory thought a half century ago (i.e. Hewlett in the USA, Cappus in France and Tsukamoto in Japan). Our best models still rely on mechanistic notions underlying the VSA, including saturation excess overland flow and subsurface stormflow (I will avoid mentioning how our operational models often are based on another whole older generation of streamflow generation concepts related exclusively to Horton!). The VSA concept has been distilled into our widely used research model structures by collapsing the process complexity into simple mathematical assumptions of things like the decline in saturated hydraulic conductivity with depth, steady-state catchment water table response, topographically defined water flowpaths and linear wetting and drying from the valley bottom upwards to the ridge (depending upon storm size, intensity and antecedent wetness conditions). Much discussion is now devoted in the modelling literature towards the balance between practical simplifications of the VSA details and justifiable model complexity. This commentary takes a critical look at our process underpinning by discussing new field evidence of where water goes when it rains that directly challenges the status quo. New model structures informed by this new process understanding are then discussed in the context of how data