Problems in the sustainable exploitation of water resources are mainly due to the spatial distribution of uses and are worsened by uncontrolled withdrawals by different activities. In this context, in order to evaluate and manage the available water resources, a decision support system has been developed to support the decision-making processes. This system was implemented on a web platform, in order to manage spatial data and to analyze information on water resources. The system also integrates a WEB-GIS engine for the elaboration and regionalization of data over the river network. The web architecture also makes these tools widely accessible and easily shareable by all stakeholders. The Web-based application was tested successfully in the Tiber River Basin (Central Italy) and focuses on the building of a hydrological database together with an evaluation model for surface water resources. The results provide synthetic information on the sustainability of water allocation scenarios, with particular regard to the feasibility of allowing new dissipative water withdrawal, such as dissipative water use in agricultural production for precision irrigation. The software technology of this project relies on open source code and can also be applied in other fields for the sustainable management of environmental resources.