The conjunctive use of surface and subsurface water is used commonly for offering stable supplies in water resources management. Conventionally, the process of supply distribution model development can be classified as simulation method, describing system behaviors and operating rules directly, and optimization method, describing system behaviors by constraint equation and imitating operating rules by objective function. Owing to the allocation is spatial dependency and operating rules is temporal dependency, many researches prefer to construct conjunctive use models by "optimization method" then by "simulation method". However, subject to the selected optimal methodology, such as linear programming or dynamic programming, modelers need to hypothesize or simplify the system. Hence, this study provides a powerful object oriented simulation modeling, system dynamics, alternative to optimization method for representing complex systems and analyzing their dynamics behavior. In system dynamics, the relation between structure and behavior is based on the concept of stock-flow diagrams. The process of model development, combining program flowchart with spatial system configuration, provokes modeler can build model easily. In this study, we also consider system expansion such as increasing new supply sources, and capacity expansion such as water treatment plant. We attempt to demonstrate this approach is a suitable methodology for constructing the complex water resources modeling.