Seeing the forest for the trees: Interpreting and quantifying emergent catchment hydrology behavior

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A unifying theory describing primary controls on mountain catchment hydrologic response continues to elude hydrologists despite valuable advances in quantitative modeling and process understanding from overland flow, to throughflow, to variables source area dynamics. This may be attributed to the near infinite complexities of natural systems, our inability to measure necessary variables adequately in space and time, and difficulties in quantifying non-linear watershed processes and threshold mediated runoff generation. We acknowledge the advances to date in catchment process understanding and modeling and are engaged in activities focused on both expanding the limits of our system behavior understanding and developing hybrid conceptual (mathematical) models internally validated and tested with synthesized process observations. We are conducting a suite of empirical and modeling studies performed iteratively at the Tenderfoot Creek Experimental Forest (TCEF) in the Little Belt Mountains of central Montana, across seven adjacent diverse watersheds and more than five years of variable climatic forcings. We specifically address the following challenges in catchment hydrology:1) What is the role of landscape structure (shape) in headwater mountain catchment hydrologic response and source water contributions? Follow-on: Does the relative importance of structure change across variable forcing (climatic variability)?2) Can we develop new quantifications (models) of watershed behavior based on hypotheses about the role of landscape structure in watershed response? Follow-on: How do field observations help infer specific model structures and parameters, and constrain predictive uncertainty?3) Does the effectiveness of catchment models based on landscape structure vary across multiple watersheds or under different climatic forcing? Follow-on: How do we compare the transferability of new model conceptualizations to traditional models based on topographic controls, variable source area concepts, or simple water balance representations?To address these challenges we are working to build tools and infrastructure for research and education. We focus on themes critical to watershed science to improve transferability of watershed understanding and predictability of watershed response via new process conceptualizations and model structures.