The water cycle is a large, complex system that encompasses ideas across the K-12 science curriculum. By the time students leave fifth grade, they should understand "that a system is a group of related parts that make up a whole and can carry out functions its individual parts cannot" and be able to describe both components and processes of Earth systems (NGSS Lead States 2013, p. 85). However, elementary students often articulate alternative ideas about water systems (Forbes, Zangori, and Schwarz 2015; Dickerson et al. 2007), partly because some elements of the water cycle are hard to observe (e.g., water vapor and groundwater). Scientific modeling is a productive way students can begin to think about Earth systems and to visualize and represent these unseen water-related processes. In this article, the authors discuss the importance of scientific modeling, present a set of reflective questions for students to use to guide their use of models, and describe one unit investigation focused on groundwater to illustrate effective ways to support students' scientific modeling.