WeSenseIt will develop a citizen-based observatory of water, which will allow citizens and communities to become active stakeholders in information capturing, evaluation and communication.We propose:(i) data collection: (a) a first “hard” layer consisting of low-cost, static and portable devices that sense and transfer water information when automatically monitored or when initiated by citizens from their mobile devices; (b) a second “soft” layer consisting of techniques to harness citizens’ Collective Intelligence, i.e. the information, experience and knowledge embodied within individuals and communities, both in terms of enabling direct messages to the authorities (with mobile-phone pictures, messages, etc.) and in terms of crowd-sourcing (e.g. by mining social networks like Twitter and Facebook, as well as bulletin boards, RSS feeds, etc.).(ii) the development of descriptive and predictive models and decision-making tools integrating sensor and citizen-based data; the data suppliers (physical sensors or people) are seen as nodes of an integrated heterogeneous data collection network which undergoes progressive multi-objective optimization and tuning.(iii) two-way feedback and exchange of environmental knowledge/experience between citizens and authorities for decision-making and governance within an e-collaboration framework ,enabling improved transparency, knowledge management, accountability and responsiveness, as well as facilitating participation in water management.We will test, experiment and demonstrate the citizen observatory of water in three different case studies in water management with civil protection agencies in UK, NL and Italy. The topic is the entire hydrologic cycle with a major focus on variables responsible for floods and drought occurrences.The project results have the potential to fundamentally change the traditional concept of environmental monitoring and forecasting, as well as models of governance.