This paper is the outcome of a community initiative to identify major unsolved scientific problems in hydrology motivated by a need for stronger harmonisation of research efforts. The procedure involved a public consultation through online media, followed by two workshops through which a large number of potential science questions were collated, prioritised, and synthesised. In spite of the diversity of the participants (230 scientists in total), the process revealed much about community priorities and the state of our science: a preference for continuity in research questions rather than radical departures or redirections from past and current work. Questions remain focused on the process-based understanding of hydrological variability and causality at all space and time scales. Increased attention to environmental change drives a new emphasis on understanding how change propagates across interfaces within the hydrological system and across disciplinary boundaries. In particular, the expansion of the human footprint raises a new set of questions related to human interactions with nature and water cycle feedbacks in the context of complex water management problems. We hope that this reflection and synthesis of the 23 unsolved problems in hydrology will help guide research efforts for some years to come. ARTICLE HISTORY

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The Mediterranean basin has been identified as one of the world's regions most vulnerable to climatic and anthropogenic changes. A methodology accounting for the basin specific conditions is developed to assess the impacts of these changes on water resources. Based on global climate projections and water-use scenarios inspired by national reports, the current water stress state is addressed first and then it is explored for the mediumterm. Currently, the southern and eastern rims are experiencing high to severe water stress. By the 2050 horizon, this stress could increase over the whole Mediterranean basin, notably because of a 30-50% decline in freshwater resources as a result of climate change. In addition, under a business-as-usual water-use scenario, total water withdrawals are projected to double on the southern and eastern rims. These worrying trends indicate the need to develop mitigation scenarios. In accord with the Mediterranean Strategy for Sustainable Development, an alternative water-use scenario based on improvements in the efficiency of water distribution networks and of irrigated agriculture is investigated. Such progress would stabilize total water withdrawals over the Mediterranean basin and even make them decrease (10-40%) in many northern catchments. Water stress could thus be tempered in some eastern catchments and remain low on the northern rim. This study highlights the importance of developing sustainable development strategies to cope with climatic and anthropogenic changes in order to explore their impacts at regional scales. It supports the need to focus on the most vulnerable areas within the Mediterranean basin. Key words regional assessment; water stress index; climate scenarios; water-use scenarios; sustainable development strategies; Mediterranean basin Etat actuel des ressources en eau en Méditerranée et tendances futures sous contraintes de changements climatiques et anthropiques Résumé Le bassin méditerranéen a été identifié comme l'une des régions au monde les plus vulnérables aux changements climatiques et anthropiques. Une méthode intégrant les spécificités du bassin a été développée afin d'évaluer les impacts de ces changements sur les ressources en eau. Elle s'appuie sur des projections climatiques globales et des scénarios d'usages de l'eau, définis à partir de rapports nationaux, pour évaluer l'état actuel du stress hydrique et son évolution à moyen terme. Actuellement, le sud et l'est de la méditerranée doivent faire face à un stress hydrique sévère, voire à des pénuries d'eau. D'ici 2050, le stress hydrique pourrait augmenter sur l'ensemble du pourtour méditerranéen, en particulier du fait d'une diminution de 30 à 50 % des ressources en eau associée aux changements climatiques. De plus, selon un scénario d'usages de l'eau tendanciel, les prélèvements totaux en eau pourraient doubler sur les rives sud et est de la Méditerranée. Ces tendances suggèrent le besoin de développer des scénarios alternatifs. En se fondant sur la Stratégie Méditerranéenne pour le Développement D...