Social processes are essential components of human-environment systems and their dynamics. However, modeling a tightly coupled socio-environmental system over a large area and across wide social and environmental diversity presents several challenges, given the complexity of the interactions and their spatial heterogeneity. The transboundary Rio Grande/RÃo Bravo (RGB) Basin is an excellent case study to address these challenges. Water scarcity and over-allocation of water are present in a highly engineered system with extensive damming and a complex structure of agreements and compacts that govern the distribution of hydrological resources among users. Since no basin-wide approaches to modeling the RGB as a socio-environmental system exist, we attempt to close this gap. Building on data collected through extensive ethnographic fieldwork, we used a structured, collaborative, and integrative approach for documenting existing knowledge on and modeling of the RGB socio-environmental system. We assess different models for conceptualizing human behavior applied in the RGB, identify a need to redefine the (spatial) boundaries of the system and produce inductively generated knowledge about the interlinkages of social processes with environmental system components in the form of a semi-quantitative conceptual model. Our research demonstrates an alternative to ad-hoc approaches to defining â€œthe socialâ€ in socio-environmental models and is a first step towards the development of a basin-wide computer simulation model of the RGB socio-environmental system.