The development of integrated resource plans (IRPs) has significantly increased in recent years. As a result, analytical tools, such as systems models developed in commercial simulation tools, have been developed to integrate the evaluation of different systems into one framework. Water resources systems models, which conceptually simulate the interaction between surface water and groundwater systems, for many years have presented the challenge to conceptualize detailed numerical models into simple relationships that can be used as transform functions in a systems model. As part of the Rancho California Water District (RCWD) IRP, an existing groundwater model was used to understand the basic behavior of the basin under different hydrologic and artificial recharge conditions. The basin was conceptualized as a series of storage units and rates in a systems dynamics model and the groundwater model output was used to develop multiple linear regression equations to define the storage units and rates. The multi-linear regression approach resulted in a systems model that closely matched the groundwater numerical model, with a level of accuracy more than adequate for the development of the IRP. The validity of the multi-linear regression approach for this specific project presents a valuable development in the use of commercial simulation tools, potentially applicable in areas beyond groundwater-surface water interaction. Given the fact that computing power is still not sufficient to evaluate integrated alternatives with detailed numerical models, the use of the systems model as a single analytical tool for the integrated alternatives results in significant savings by allowing the evaluation of multiple alternatives in a fraction of the time that it would take to run a groundwater model to achieve the same overall results. Includes 9 references, tables, figures.