Water is a crucial topic that spans the K-12 science curriculum, including the elementary grades. Students should engage in the articulation, negotiation, and revision of model-based explanations about hydrologic phenomena. However, past research has shown that students, particularly early learners, often struggle to understand hydrologic phenomena and that scientific modeling remains underemphasized in elementary science learning environments. More research is, therefore, needed to understand and promote early learners' engagement in domain-specific modeling practices. To address this need, we are engaged in design-based research to foster and investigate 3rd-grade students' model-based explanations for the water cycle. Here, we report on the development of a set of empirically based learning performances that integrate core discipline-specific concepts and the practice of scientific modeling. This framework (i) grounds the iterative adaptation and enhancement of a commonly used curricular unit focused on water and (ii) lays the foundation for ongoing development of an associated learning progression that spans K-12 grades. Second, we report on findings from research investigating 3rd-grade students' model-based explanations within the context of the water cycle. Results illustrate epistemic features of mechanism-based causal claims elementary students generate and highlight both target concepts and modeling practices emphasized in students' model-based explanations for hydrologic cycling.