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Our ability to measure, understand and model surface and subsurface runoff generation is key to integrated water resources planning, development and management at the watershed scale. Rapid progress is being made on the rainfall-runoff modelling front in catchment hydrology vis-à-vis parameter estimation techniques, model uncertainty analysis, examination of parameter identifiability in our models, downward approaches to hydrologic prediction. However, one could argue that our sharpening perception of water source, flowpath and age in upland headwater catchments is radically different to what the studies conducted during the International Hydrologic Decade thought a half century ago. Our best models still rely on mechanistic notions described in field studies from decades past. The concepts have been distilled into our widely used 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 modeling literature towards the balance between practical simplifications of these “process” details and justifiable model complexity. This presentation 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 and objective discretization of catchment units may be used both to structure and test the model. An argument for considering the watershed as a series of cryptic reservoirs is presented. Flexible box models are then proposed as a way to describe the first order controls on runoff generation as perhaps more a tractable way to represent the first order controls of the age, origin and pathway of storm runoff in watershed management, planning and conservation
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