Water resources management usually relies on mathematical and computer-based models to explore the likely impacts of management decisions. These models are often integrated, meaning that they combine elements from a range of different disciplines, including social, economic and ecological components. The core of the integrated model is the hydrological component, which simulates water availability given climatic conditions and water demands. In many Australian river basins, surface water and groundwater resources are highly interconnected and they must be modelled, and managed, as a single resource. Despite the undeniable importance of representing the interactions between connected surface water – groundwater (SW-GW) systems, there is a lack of consensus regarding a suitable approach, particularly for large study areas with sparse observational data. This PhD thesis presents the development of an integrated SW-GW model suitable for use in large-scale modelling applications where data and computational resources may be limited. Two alternative models are presented, both apply simple, ‘mass-balance’ approaches for representing surface water, but differing levels of complexity for the groundwater domain. We find that, while there is no single “best” solution, simple representations of groundwater processes can significantly improve the simulation of baseflows in groundwater dependent streams.