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G. P. Weedon, S. Gomes, P. Viterbo, W. J. Shuttleworth, E. Blyth, H. Österle, J. C. Adam, N. Bellouin, O. Boucher, and M. Best

of the twentieth century and that resolves the full diurnal cycle. An analysis of changes in the external drivers of evaporation that is relevant to both researchers and water-resource engineers is also made. The European Union WATCH project ( ) seeks to assess the terrestrial water cycle in the context of global change in the twentieth and twenty-first centuries. A major component of the study is use of land surface models (LSMs) and general hydrological models (GHMs) to

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Ingjerd Haddeland, Douglas B. Clark, Wietse Franssen, Fulco Ludwig, Frank Voß, Nigel W. Arnell, Nathalie Bertrand, Martin Best, Sonja Folwell, Dieter Gerten, Sandra Gomes, Simon N. Gosling, Stefan Hagemann, Naota Hanasaki, Richard Harding, Jens Heinke, Pavel Kabat, Sujan Koirala, Taikan Oki, Jan Polcher, Tobias Stacke, Pedro Viterbo, Graham P. Weedon, and Pat Yeh

1. Introduction The global water balance has been the subject of modeling studies for decades, both from a climate perspective where the main interest is the influence of the water balance on surface heat fluxes and from a hydrological perspective focusing on water availability and use. However, there are still many uncertainties in our understanding of the current water cycle, and to date the results of land surface models (LSMs) and global hydrology models (GHMs) have not been compared in a

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D. Gerten, J. Heinke, H. Hoff, H. Biemans, M. Fader, and K. Waha

simulation from 1901 to 2099), while results are presented as 30-yr country averages (blending results for rain-fed and irrigated land) for the present (1971–2000) and for a future time slice (2070–99 or “2080s”), respectively. 2. Model and methods In the following we will briefly present the model used for our calculations ( section 2a ); describe how the blue, green, and joint green-blue water resources and availabilities per capita were computed at grid cell level and how they were then scaled to the

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Stefan Hagemann, Cui Chen, Jan O. Haerter, Jens Heinke, Dieter Gerten, and Claudio Piani

in situations where the hydrological climate change signal is unclear ( Mudelsee et al. 2003 ; Milly et al. 2002 ). Global climate models (GCMs) are used to investigate possible trends in the past and future global climate. To quantify details of projected changes in the hydrological cycle and their potential impacts on water resources, commonly used global hydrology models (GHMs) or land surface hydrology models (LSHMs) are forced with GCM output. These hydrological simulations largely depend

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Christel Prudhomme, Simon Parry, Jamie Hannaford, Douglas B. Clark, Stefan Hagemann, and Frank Voss

environmental consequences. Climate change influences on the hydrological cycle are often studied using large-scale gridded models, which enable elements of the hydrological cycle (e.g., rainfall, evapotranspiration, and runoff) and potential perturbations to the cycle as a result of climate change to be studied on regional (e.g., across Europe) to global scales. Such studies have predominantly analyzed climatic variables such as temperature and precipitation (e.g., Burke et al. 2006 ) or land surface

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Philippe Lucas-Picher, Jens H. Christensen, Fahad Saeed, Pankaj Kumar, Shakeel Asharaf, Bodo Ahrens, Andrew J. Wiltshire, Daniela Jacob, and Stefan Hagemann

description of the regional forcings, such as the mountain orography, land cover, and the land–sea contrasts. Many studies have been carried out to verify the ability of RCMs to simulate the Indian monsoon. Initial studies performed short simulations using GCMs as lateral boundary conditions (LBCs), focusing on a few months or years to verify the validity of the approach ( Bhaskaran et al. 1996 ; Jacob and Podzun 1997 ; Ji and Vernekar 1997 ; Bhaskaran et al. 1998 ; Vernekar and Ji 1999 ). Then

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Richard Harding, Martin Best, Eleanor Blyth, Stefan Hagemann, Pavel Kabat, Lena M. Tallaksen, Tanya Warnaars, David Wiberg, Graham P. Weedon, Henny van Lanen, Fulco Ludwig, and Ingjerd Haddeland

for ecosystem and human services ( Bates et al. 2008 ; Strzepek and Boehlert 2010 ; Ward et al. 2010 ; Vörösmarty et al. 2010 ). As a result, in many regions water (particularly groundwater) is being exploited in an unsustainable way, leading to long-term declines in groundwater levels ( McGuire 2009 ; Rodell et al. 2009 ). Strong interactions between the climate, hydrology, and land use occur ( Claussen 2004 ; Falloon and Betts 2010 ). The snow–climate feedback is well known and described (e

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Lukas Gudmundsson, Lena M. Tallaksen, Kerstin Stahl, Douglas B. Clark, Egon Dumont, Stefan Hagemann, Nathalie Bertrand, Dieter Gerten, Jens Heinke, Naota Hanasaki, Frank Voss, and Sujan Koirala

discharge from large, continental-scale river basins. Generally these studies show that the uncertainty in river discharge introduced by the use of different atmospheric forcing models ( Nohara et al. 2006 ; Hagemann and Jacob 2007 ) and different land surface schemes ( Materia et al. 2010 ) can be reduced by ensemble techniques. Several studies have compared soil moisture simulations from the GSWP to monthly observations from a global observation network (e.g., Gao and Dirmeyer 2006 ; Guo and

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Pete Falloon, Richard Betts, Andrew Wiltshire, Rutger Dankers, Camilla Mathison, Doug McNeall, Paul Bates, and Mark Trigg

general circulation models (GCMs) or river-routing schemes driven by runoff produced by GCMs. The standard form of the previous version of the Hadley Centre GCM, the third climate configuration of the Met Office Unified Model (HadCM3; Gordon et al. 2000 ), used a very simple river-routing scheme in which surface and subsurface runoff were instantaneously advected from the land surface through outflow points to the ocean. A basinwide accounting scheme such as this is reasonably effective at annual

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Kerstin Stahl, Lena M. Tallaksen, Lukas Gudmundsson, and Jens H. Christensen

and Cultural Organization (UNESCO) Flow Regimes from International Experimental and Network Data (FRIEND) European Water Archive (EWA) and the U.S. Hydro-Climatic Data Network of stream gauges, however, have contributed to a better understanding of large-scale processes at the land surface, including changes across a range of scales (e.g., Krakauer and Fung 2008 ; Stahl et al. 2010 ). In the United States, streamflow data from small basins have been used to calibrate and evaluate land surface

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