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.

A global water model is used to analyse the impacts of climate change and socio-economic driving forces (derived from the A2 and B2 scenarios of IPCC) on future global water stress. This work extends previous global water research by analysing not only the impact of climate change and population, but also the effects of income, electricity production, water-use efficiency and other driving forces, on water stress. Depending on the scenario and climate model, water stress increases (between current conditions and the 2050s) over 62.0-75.8% of total river basin area and decreases over 19.7-29.0% of this area. The remaining areas have small changes. The principal cause of decreasing water stress (where it occurs) is the greater availability of water due to increased annual precipitation related to climate change. The principal cause of increasing water stress is growing water withdrawals, and the most important factor for this increase is the growth of domestic water use stimulated by income growth. (Population growth was a much less important factor and irrigated area was assumed to remain constant.) To address the uncertainty of water stress estimates, three different indicators of water stress were computed and compared. The overlap area of their computation of "severe stress" in the 2050s was large (approximately 23 10 6 km 2 or 56-73 % of the total "severe stress" area). This indicates a moderate level of agreement and robustness in estimates of future water stress. At the same time the indicators disagreed in many other areas, suggesting that work is still needed to elaborate general indicators and concepts of water stress.Key words global water resources; global change; water scenarios; water availability; water stress; climate change and water Changements futurs à long terme dans les ressources en eau globales forcés par les changements climatiques et socio-économiquesRésumé Un modèle hydrologique global est utilisé pour analyser les impacts du changement climatique et des forçages socio-économiques (déduits des scénarios A2 et B2 de l'IPCC) sur le stress hydrique global futur. Ce travail étend les recherches hydrologiques globales précédentes en analysant non seulement l'impact du changement climatique et de la population, mais aussi les effets des intrants, de la production électrique, de l'efficience des usages de l'eau et d'autres forçages sur le stress hydrique. Selon le scénario et le modèle climatique, le stress hydrique augmente (entre les conditions actuelles et les années 2050) sur 62.0-75.8% de l'aire totale des bassins hydrographiques et diminue sur 19.7-29.0% de cette aire. L'aire restante présente de faibles changements. La cause principale de la décroissance du stress hydrique (là où elle survient) est la plus grande disponibilité de l'eau due à l'augmentation de la précipitation annuelle en relation avec le changement climatique. La cause principale de l'augmentation du stress hydrique est l'augmentation des prélèvements d'eau, dont le facteur le plus important est la croissance des...

Major rivers worldwide have experienced dramatic changes in flow, reducing their natural ability to adjust to and absorb disturbances. Given expected changes in global climate and water needs, this may create serious problems, including loss of native biodiversity and risks to ecosystems and humans from increased flooding or water shortages. Here, we project river discharge under different climate and water withdrawal scenarios and combine this with data on the impact of dams on large river basins to create global maps illustrating potential changes in discharge and water stress for dam‐impacted and free‐flowing basins. The projections indicate that every populated basin in the world will experience changes in river discharge and many will experience water stress. The magnitude of these impacts is used to identify basins likely and almost certain to require proactive or reactive management intervention. Our analysis indicates that the area in need of management action to mitigate the impacts of climate change is much greater for basins impacted by dams than for basins with free‐flowing rivers. Nearly one billion people live in areas likely to require action and approximately 365 million people live in basins almost certain to require action. Proactive management efforts will minimize risks to ecosystems and people and may be less costly than reactive efforts taken only once problems have arisen.

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.

The planned expansion of biofuel plantations in Brazil could potentially cause both direct and indirect land-use changes (e.g., biofuel plantations replace rangelands, which replace forests). In this study, we use a spatially explicit model to project land-use changes caused by that expansion in 2020, assuming that ethanol (biodiesel) production increases by 35 (4) x 10 9 liter in the 2003-2020 period. Our simulations show that direct land-use changes will have a small impact on carbon emissions because most biofuel plantations would replace rangeland areas. However, indirect land-use changes, especially those pushing the rangeland frontier into the Amazonian forests, could offset the carbon savings from biofuels. Sugarcane ethanol and soybean biodiesel each contribute to nearly half of the projected indirect deforestation of 121,970 km 2 by 2020, creating a carbon debt that would take about 250 years to be repaid using these biofuels instead of fossil fuels. We also tested different crops that could serve as feedstock to fulfill Brazil's biodiesel demand and found that oil palm would cause the least land-use changes and associated carbon debt. The modeled livestock density increases by 0.09 head per hectare. But a higher increase of 0.13 head per hectare in the average livestock density throughout the country could avoid the indirect land-use changes caused by biofuels (even with soybean as the biodiesel feedstock), while still fulfilling all food and bioenergy demands. We suggest that a closer collaboration or strengthened institutional link between the biofuel and cattle-ranching sectors in the coming years is crucial for effective carbon savings from biofuels in Brazil.B razil's government and biofuel industry are planning a large increase in the production of biofuels in the next 10 years. This increase is driven by internal and external market demand (ethanol), as well as by government-enforced blending (biodiesel) (1-3). Although Brazilian sugarcane ethanol is often considered to have one of the best production systems with respect to carbon savings (4-8), there are concerns about the land-use changes (LUC) that would be incurred by an expansion of biofuel croplands (6, 7). Soybean plantations, from which most of the Brazilian biodiesel is produced (1, 3), already occupy 35% of the country's cultivated land (9). It is known that biofuels can replace vast areas of farmland and native habitats, driving up food prices and resulting in little reduction of or even increasing greenhouse gas (GHG) emissions (6,7,(10)(11)(12)(13)(14)(15).Previous studies focused on the direct land-use changes (DLUC) and the "carbon debt" caused by the replacement of native habitats by biofuel crops in Brazil (7,8,10,11). Others pointed to the probable indirect land-use changes (ILUC) in Brazil caused by future expansion of biofuel croplands in the United States (14-16). Overall, these studies show that potential LUC must be taken into account to assess the efficacy of a given biofuel. However, these studies were neither spatially...