The first part of a model for the management of the water resource is presented. The model is set for a city and it includes the water extraction form the environment, the cleaning process and the distribution of the potable water, the use of the water, the collection and the treatment of the sewage and finally the restitution of the water to the environment. The water fluxes are analyzed and the energy is incorporated by the use of functions and adjustable parameters. A set of equations for modeling a detailed overview is obtained. The model will be validated by using a complete set of real data.