The knowledge of parameters, defining interaction of a robotic manipulator with environment, is crucial when robots execute contact-tasks involving tracking of trajectories while desired forces are applied on the environment. For contact tasks on planar surfaces, the inclination and stiffness of the surface are key parameters since the first one defines the direction of the desired force and trajectory, which are typically defined relative to a frame attached to the environment, and the second one is required to compute the control signal. There exist methods for estimation of inclination and stiffness, whenever they are constant. The estimation of time-varying stiffness and inclination is less studied. In this paper, we propose a method to estimate on-line the stiffness and inclination of the planar surface, when they are varying during the task execution. The method is based on adaptive observers that ensure asymptotic or finite-time convergence of the estimates to the real values of the parameters.