In the future, most of the population will settle in large cities and metropolitan areas, in basins where pressure over water resources is already very large. In this situation, the operators of water and sanitation services must make a continuous effort of planning, to reach effective application of the reduced natural and economic resources. However, the urban water systems are composed of many different elements that interact with each other, exhibiting feedback loops and emerging behaviors, so that no simple models could be used. In short, there are complex systems. Under these conditions, decision makers require appropriate methods and tools, such that allow them to examine several alternatives and their impacts in time. The systems approach and adaptive dynamic modeling is one of the most active and promising research fields in water resources management. In this paper the use of dynamic modeling applied to supply water to cities is proposed, a general model is shown and applied to the city of Puebla and its metropolitan area. A trend and a balance scenarios are analyzed, the second composed by a group of conservation, rainwater harvesting and reuse actions over a 15 years horizon, demonstrating the possibility of leading the system from a deficit into a surplus of water, and exemplifying the benefits of dynamic modeling in urban water systems.