Adaptive requirements are system requirements that change either internally, or externally in the environment, where environment includes the users and other systems interacting with the subject system. Modeling and analyzing adaptive requirements are important because change is a ubiquitous phenomenon in social-technical systems. Engineering adaptive requirements is also difficult because many contextual factors need to be taken account of, and some of these factors (e.g., human intentional changes) are hard to quantify. This paper investigates an approach to modeling adaptive requirements by integrating system dynamics modeling into i* goal-oriented modeling. We illustrate that each modeling method has its advantages that are difficult or impossible to achieve using the other method, and each has its limitations that can be overcome using the other method. We then propose a goal-oriented adaptive requirements engineering (GARE) approach that integrates desirable features of system dynamics into i*, taking advantage of the goal-oriented modeling initiatives in i* and the built-in constructs in system dynamics for modeling time-based system evolution and adaptation.