The natural dynamics of the hydrologic cycle involves the exchange of water, substances, and energy between the atmosphere, soil surface, biota, interflow zone, and aquifer subsystems. The dynamics is governed by physical laws and is powered by the sun; however, today it is also necessary to consider the action of man on the natural dynamics. The purpose of this paper is to present a conceptual physical framework for the study of the component subsystems of the hydrological cycle, which allows the development of integrated mathematical models. It is assumed that each subsystem is governed by its own field equations of balance of mass, energy and constitutive relationships, and that these are related by coupling terms, all of them following the maximum entropy principle, that drives the processes and subsystems to equilibrium thermodynamic states.