An online dynamic decision support system (DDSS) was developed, to support the farm water management in surface irrigation. The online DDSS was based on the formulation and integration of three components: a dynamic-relational data base, an administrator model, and a graphical user interface. The DDSS allows routines of actualization, edition and addition of online data, providing information in real time. The online DDSS was applied in an orange orchard (Citrus sinensis L. Osbeck) cv. Valencia Late under furrow irrigation. The results pointed out that the time irrigation cutoff was the main significant management factor, to decrease the hazard of leaching, superficial runoff and percolation. Applying the results obtained with the DDSS, furrow irrigation efficiencies could be improved up to values equals to 95.89% for application efficiency and 94.61% for total distribution efficiency. As a conclusion, the DDSS demonstrated to be a useful tool to assist the decision making process, providing proper information for the management of the available water resource at farm level.