For example, at the coarsest level, a beam is just a 1D curve whose torque is a function of local curvature.
As the field progresses and our understanding of the underlying mechanisms deepens, governing equations may be replaced or refined by new, more accurate models that better represent the system's behavior.
These new governing equations can then be considered the deepest level of phenomenological model at that point in time.
Darcy's law of groundwater flow has the form of a volumetric flux caused by a pressure gradient.
This is the case for a model of an oil production plant which on the average operates in a steady state mode.