Aspects of the present invention provide a solution for calibrating a model of a complex flow system. In an embodiment, a comparison is made between the output from the model and a set of observed values for each of a plurality of nodes in the complex flow system. An adjoint sensitivity is computed for each of the nodes based on the comparison. These computed adjoint sensitivities are used to adjust a set of coefficients of the models. This calibration process can be performed multiple times, periodically and/or continuously to maximize the accuracy of the model.