The method presented combines a black box analysis with a conceptual model approach. To describe the nonlinear behaviour of the systems, a model in the form of a second-order approximation of a cascade of nonlinear reservoirs was used. Such a model is equivalent to the first two terms of the Volterra series. Applications of the Volterra model based on a nonlinear cascade to the modelling of flow in open channels and of surface runoff systems indicate that the proposed model can be used to represent systems with nonlinear dynamic and linear static behaviour.