The significant worldwide increase in observed river runoff has been tentatively attributed to the stomatal ''antitranspirant'' response of plants to rising atmospheric CO 2 [Gedney N, Cox PM, Betts RA, Boucher O, Huntingford C, Stott PA (2006) Nature 439: 835-838]. However, CO 2 also is a plant fertilizer. When allowing for the increase in foliage area that results from increasing atmospheric CO 2 levels in a global vegetation model, we find a decrease in global runoff from 1901 to 1999. This finding highlights the importance of vegetation structure feedback on the water balance of the land surface. Therefore, the elevated atmospheric CO 2 concentration does not explain the estimated increase in global runoff over the last century. In contrast, we find that changes in mean climate, as well as its variability, do contribute to the global runoff increase. Using historic land-use data, we show that landuse change plays an additional important role in controlling regional runoff values, particularly in the tropics. Land-use change has been strongest in tropical regions, and its contribution is substantially larger than that of climate change. On average, land-use change has increased global runoff by 0.08 mm/year 2 and accounts for Ϸ50% of the reconstructed global runoff trend over the last century. Therefore, we emphasize the importance of land-cover change in forecasting future freshwater availability and climate.atmospheric CO2 ͉ water cycle ͉ climate change ͉ land cover change C limate change and human intervention are expected to strongly alter the global hydrological cycle in the coming decades (1-5). Previous reconstruction of global runoff data suggests that global river runoff increased significantly during the 20th century (2). However, it is difficult to estimate whether this trend in runoff is caused by natural or anthropogenic factors, because the characteristics and dynamic properties of the hydrological cycle depend on many interrelated links among climate, atmosphere, soil, and vegetation dynamics. Long-term changes in runoff depend on the balance of precipitation and evapotranspiration. The latter term is not only driven by climatic factors, such as temperature, wind speed, surface humidity, and solar radiation, but also is modulated by physiological (e.g., stomatal) and structural [e.g., leaf area index (LAI)] components of vegetation. It is well known that stomata respond to elevated atmospheric CO 2 concentrations by partial closure (6). Accordingly, a recent modeling analysis suggested that the rising atmospheric CO 2 concentration is the main driver of the observed increase in continental runoff (1). Nevertheless, the results of the Gedney et al.(1) study should be viewed with caution because only the direct effect of atmospheric CO 2 concentrations on stomatal conductance was considered. Structural changes in vegetation in response to increased productivity under higher atmospheric CO 2 levels, particularly changes in LAI (7), were not taken into account in their study.Land use is another key...

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Ongoing global climatic change initiated by the anthropogenic release of carbon dioxide is a matter of intense debate. We focus both on the impact of these climatic changes on the global hydrological cycle and on the amplitude of the increase of global and continental runoff over the last century, in relation to measured temperature increases. In this contribution, we propose an original statistical wavelet-based method for the reconstruction of the monthly discharges of worldwide largest rivers. This method provides a data-based approximation of the evolution of the annual continental and global runoffs over the last century. A consistent correlation is highlighted between global annual temperature and runoff, suggesting a 4% global runoff increase by 1 °C global temperature rise. However, this global trend should be qualified at the regional scale where both increasing and decreasing trends are identified. North America runoffs appear to be the most sensitive to the recent climatic changes. Finally, this contribution provides the first experimental data-based evidence demonstrating the link between the global warming and the intensification of the global hydrological cycle. This corresponds to more intense evaporation over oceans coupled to continental precipitation increase or continental evaporation decrease. This process finally leads to an increase of the global continental runoff.

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International audienceA numerical model of chemical weathering in soil horizons and underlying bedrock (WITCH) has been coupled to a numerical model of water and carbon cycles in forest ecosystems (ASPECTS) to simulate the concentration of major species within the soil horizons and the stream of the Strengbach granitic watershed, located in the Vosges Mountains (France). For the first time, simulations of solute con- centrations in soil layers and in the catchment river have been performed on a seasonal basis. The model is able to reproduce the con- centrations of most major species within the soil horizons, as well as catching the first-order seasonal fluctuations of aqueous calcium, magnesium and silica concentrations. However, the WITCH model underestimates concentrations of Mg2+ and silica at the spring of the catchment stream, and significantly underestimates Ca2+ concentration. The deficit in calculated calcium can be compensated for by dis- solution of trace apatite disseminated in the bedrock. However, the resulting increased Ca2+ release yields important smectite precipi- tation in the deepest model layer (in contact with the bedrock) and subsequent removal of large amount of silica and magnesium from solution. In contrast, the model accurately accounts for the concentrations of major species (Ca, Mg and silica) measured in the catchment stream when precipitation of clay minerals is not allowed. The model underestimation of Mg2+ and H4SiO4 concentrations when precipitation of well crystallized smectites is allowed strongly suggests that precipitation of well crystallized clay minerals is over- estimated and that more soluble poorly crystallized and amorphous materials may be forming. In agreement with observations on other watersheds draining granitic rocks, this study indicates that highly soluble trace calcic phases control the aqueous calcium budget in the Strengbach watershed

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