This paper summarizes the main points of a theoretical study of the general watershed behaviour, on the basis of the known deterministic physics-based approaches for overland flow, underground flow and unsaturated medium phýsics. A parameter z(θ) which describes the soil water movement in the unsaturated zone and accounts for evaporation is introduced. Thus, a differential equation which allows a state-space formulation of the processes in the watershed is obtained, and the solution of the state equation can be given the form of the Volterra Integral series. The obtained results are compared with those of previous studies.

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