Growing interest in global environmental issues has led to the need for global and regional assessment of water resources. A global water assessment model called "WaterGAP 2" is described, which consists of two main components-a Global Water Use model and a Global Hydrology model. These components are used to compute water use and availability on the river basin level. The Global Water Use model consists of (a) domestic and industry sectors which take into account the effect of structural and technological changes on water use, and (b) an agriculture sector which accounts especially for the effect of climate on irrigation water requirements. The Global Hydrology model calculates surface runoff and groundwater recharge based on the computation of daily water balances of the soil and canopy. A water balance is also performed for surface waters, and river flow is routed via a global flow routing scheme. The Global Hydrology model provides a testable method for taking into account the effects of climate and land cover on runoff. The components of the model have been calibrated and tested against data on water use and runoff from river basins throughout the world. Although its performance can and needs to be improved, the WaterGAP 2 model already provides a consistent method to fill in many of the existing gaps in water resources data in many parts of the world. It also provides a coherent approach for generating scenarios of changes in water resources. Hence, it is especially useful as a tool for globally comparing the water situation in river basins.

Most studies on the impact of climate change on regional water resources focus on longterm average flows or mean water availability, and they rarely take the effects of altered human water use into account. When analyzing extreme events such as floods and droughts, the assessments are typically confined to smaller areas and case studies. At the same time it is acknowledged that climate change may severely alter the risk of hydrological extremes over large regional scales, and that human water use will put additional pressure on future water resources. In an attempt to bridge these various aspects, this paper presents a first-time continental, integrated analysis of possible impacts of global change (here defined as climate and water use change) on future flood and drought frequencies for the selected study area of Europe. The global integrated water model WaterGAP is evaluated regarding its capability to simulate high and low-flow regimes and is then applied to calculate relative changes in flood and drought frequencies. The results indicate large 'critical regions' for which significant changes in flood or drought risks are expected under the proposed global change scenarios. The regions most prone to a rise in flood frequencies are northern to northeastern Europe, while southern and southeastern Europe show significant increases in drought frequencies. In the critical regions, events with an intensity of today's 100-year floods and droughts may recur every 10-50 years by the 2070s. Though interim and preliminary, and despite the inherent uncertainties in the presented approach, the results underpin the importance of developing mitigation and adaptation strategies for global change impacts on a continental scale.

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New global models provide the opportunity to generate quantitative information about the world water situation. Here the WaterGAP 2 model is used to compute globally comprehensive estimates about water availability, water withdrawals, and other indicators on the river-basin scale. In applying the model to the current global water situation, it was found that about 24% of world river basin area has a withdrawal to availability ratio greater than 0.4, which some experts consider to be a rough indication of "severe water stress"; the impacts of this stress are expected to be stronger in developing countries than in industrialized ones. Under a "businessas-usual" scenario of continuing demographic, economic and technological trends up to 2025, water withdrawals are expected to stabilize or decrease in 41% of world river basin areas because of the saturation of water needs and improvement in water-use efficiency. Withdrawals grow elsewhere because population and economic growth will lead to rising demand for water, and this outweighs the assumed improvements in water-use efficiency. An uncertainty analysis showed that the uncertainty of these estimates is likely to have a strong geographic variability.Key words global water resources; hydrological model; integrated assessment; scenario analysis; water scarcity; water stress; water availability; water use; water withdrawals Estimations globales actuelles et futures, en conditions de continuité, de la disponibilité de l'eau et des prélèvements Résumé Les nouveaux modèles globaux donnent l'opportunité de générer de l'information quantitative au sujet de la situation hydrologique mondiale. Nous utilisons le modèle WaterGAP 2 pour calculer des estimations à vocation globale de la disponibilité en eau, des prélèvements d'eau et d'autres indicateurs, au niveau des bassins versants. L'application du modèle à la situation hydrologique globale actuelle montre que 24% environ de la surface des bassins versants du monde présentent un rapport entre prélèvement et disponibilité supérieur à 0.4, ce que certains experts considèrent comme une indication grossière d'un "stress hydrique sévère"; les impacts de ce stress étant estimés plus forts dans les pays en voie de développement que dans les pays industrialisés. Selon un scénario de continuité dans les tendances démo-graphiques, économiques et technologiques jusqu'en 2025, les prélèvements d'eau se stabilisent ou diminuent sur 41% de la surface des bassins versants à cause de la saturation des besoins en eau et de l'amélioration de l'efficience de l'utilisation de l'eau. Ailleurs, les prélèvements croissent parce que la croissance démographique et économique augmente les besoins en eau, ce qui surpasse les améliorations supposées dans l'efficience de l'utilisation de l'eau. Une analyse d'incertitude montre que l'incertitude liée à ces estimations présente une forte variabilité géographique.Mots clefs ressources en eau globales; modèle hydrologique; évaluation intégrée; analyse de scénario; manque d'eau; stress hydrique; di...

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