OPPA and the Art of Physics-Based Distributed Hydrologic Model Calibration

Physics-based distributed (PBD) hydrologic modeling differs from conceptual or empirically based models because conservation laws differential equations are used to generate hydrologic quantities. Calibration of PBD models show predictable response and expected parameter interaction. Within the ordered physics-based parameter adjustment (OPPA) calibration strategy, scalar multipliers adjust the magnitude of the spatially distributed parameter while preserving its spatial pattern. Leveraging the ODEs involved in production of runoff and routing helps advance our understanding of parameter and input sensitivity, and inherant uncertainty in continuous modeling of rainfall-runoff processes. Early work published on event-based OPPA has been extended to calibration with continuous simulation taking into account evapotranspiration, soil moisture and parameter interactions. This approach demonstrates the scientifc basis (and the ‘art’) of calibration, especially, when seeking physcially realistic parameter maps used in unattended hydrologic forecasting. Application of OPPA calibration to a gridded high resolution watershed model demonstrates how hydrologic predictions can be made more reliable.