Problems of water system integration occur when a model's boundaries are too narrow to capture interactions and feedbacks across the terrestrial water cycle. We propose that integrated water systems models are required to overcome them and are necessary to understand emergent system behaviour, to expand model boundaries, to evaluate interventions, and to ensure simulations reflect stakeholder goals. We present the Water Systems Integrated Modelling framework (WSIMOD) software as one such approach and describe its theoretical basis, covering the node and arc nature of simulations, the integration framework that enables communication between model elements, and the model orchestration to customise interactions. We highlight data requirements for creating such a model and the potential for future development and refinement. WSIMOD offers a flexible and powerful approach to represent water systems, and we hope it will encourage further research and application into using model integration towards achieving sustainable and resilient water management.