Using automatic calibration to improve the physics behind complex numerical models: An example from a 3D lake model using Delft3d (v6.02.10) and DYNO-PODS (v1.0)

Abstract. Models are simplified descriptions of reality and are intrinsically limited by the assumptions that have been introduced in their formulation. With the development of automatic calibration toolboxes, finding optimal parameters that suit the environmental system has become more convenient. Here, we explore how optimization toolboxes can be applied innovatively to uncover flaws in the physical formulations of models. We illustrate this approach by evaluating the effect of simplifications embedded in the formulation of a widely used hydro-thermodynamic model. We calibrate a Delft3D model based on temperature profiles for a case study, Lake Morat (Switzerland), through the optimization tool DYNO-PODS. Results show that neglecting the fraction β of shortwave radiation absorbed at the water surface can be compensated by higher values of the light extinction coefficient. This leads to unrealistic values of the latter parameter, as the optimization pushes the coefficient towards the limit of no transparency, consistent with the need to reproduce a significant absorption at the surface. While it is well-known that β is significantly larger than zero, its absence in the model was never noticed as critical. The extensive use of automatic calibration tools may offer similar outcomes in other applications.