In this paper, a decision-support tool named HYDROPTIM has been developed that allows the flow management in a large scale water systems including reservoirs, open channels for water transport, water treatment plants, pressurized water pipe networks, tanks, flow/pressure control elements and a telemetry/telecontrol system. Predictive optimal control is used to generate flow control strategies from the sources to the consumer areas to meet future demands with appropriate pressure levels, optimizing operational goals such as network safety volumes and flow control stability. The tool allows as well to build the network model graphically and then to automatically generate the model equations used by the predictive optimal controller. Additionally, the tool can work off-line (in simulation) and on-line (in real-time mode). The case study of Guadiana River is presented to exemplify the control results obtained using the tool.