This model treats interplanetary structures as a two-time scale problem, a small-scale incompressible Alfv¿nic fluctuation superposed on a large-scale background flow, and uses the complete magnetohydrodynamic equation and the full energy equation to describe the interplanetary structures. The small-scale fluctuation is governed by a system of linear equations, and the large-scale background flow by a system of nonlinear partial differential equations in a heliocentric coordinate system corotating with the sun. For steady state corotating structures the model assumes the background flow to be field aligned and proposes to treat the nonlinear system as composed of two subsets: one dealing with the expansion of the solar wind in stream tubes, the other with the interaction between neighboring stream tubes. The interactjion process governs the variations in the cross-sectional area and the direction of each stream tube. This process is required to satify the divergence-free condition of the magnetic field and a dynamical equilibrium of momentum transverse to the streamline direction. The interaction subset has a singularity at the Alfv¿nic point. The expansion process is required to satisfy the equations of mass and energy conservation, and the equation of motion tangential to the streamline direction. The expansion subset has its singularity at Parker's critical radius. The two subsets should be solved simultaneously for numerical solutions. We obtain the characteristic equation of the interaction subset, and discuss the method of solutions for three-dimensional corotating interplanetary structures. The viscous attenuation of the small-time-scale incompressible Alfv¿nic fluctuation is also studied by using the WKB approximation.