Global-scale information on natural river flows and anthropogenic river flow alterations is required to identify areas where aqueous ecosystems are expected to be strongly degraded. Such information can support the identification of environmental flow guidelines and a sustainable water management that balances the water demands of humans and ecosystems. This study presents the first global assessment of the anthropogenic alteration of river flow regimes, in particular of flow variability, by water withdrawals and dams/reservoirs. Six ecologically relevant flow indicators were quantified using an improved version of the global water model WaterGAP. WaterGAP simulated, with a spatial resolution of 0.5 degree, river discharge as affected by human water withdrawals and dams around the year 2000, as well as naturalized discharge without this type of human interference. Compared to naturalized conditions, long-term average global discharge into oceans and internal sinks has decreased by 2.7% due to water withdrawals, and by 0.8% due to dams. Mainly due to irrigation, long-term average river discharge and statistical low flow Q 90 (monthly river discharge that is exceeded in 9 out of 10 months) have decreased by more than 10% on one sixth and one quarter of the global land area (excluding Antarctica and Greenland), respectively. Q 90 has increased significantly on only 5% of the land area, downstream of reservoirs. Due to both water withdrawals and reservoirs, seasonal flow amplitude has decreased significantly on one sixth of the land area, while interannual variability has increased on one quarter of the land area mainly due to irrigation. It has decreased on only 8% of the land area, in areas downstream of reservoirs where consumptive water use is low. The impact of reservoirs is likely underestimated by our study as small reservoirs are not taken into account. Areas most affected by anthropogenic river Correspondence to: P. Döll

Abstract. When assessing global water resources with hydrological models, it is essential to know about methodological uncertainties. The values of simulated water balance components may vary due to different spatial and temporal aggregations, reference periods, and applied climate forcings, as well as due to the consideration of human water use, or the lack thereof. We analyzed these variations over the period 1901–2010 by forcing the global hydrological model WaterGAP 2.2 (ISIMIP2a) with five state-of-the-art climate data sets, including a homogenized version of the concatenated WFD/WFDEI data set. Absolute values and temporal variations of global water balance components are strongly affected by the uncertainty in the climate forcing, and no temporal trends of the global water balance components are detected for the four homogeneous climate forcings considered (except for human water abstractions). The calibration of WaterGAP against observed long-term average river discharge Q significantly reduces the impact of climate forcing uncertainty on estimated Q and renewable water resources. For the homogeneous forcings, Q of the calibrated and non-calibrated regions of the globe varies by 1.6 and 18.5 %, respectively, for 1971–2000. On the continental scale, most differences for long-term average precipitation P and Q estimates occur in Africa and, due to snow undercatch of rain gauges, also in the data-rich continents Europe and North America. Variations of Q at the grid-cell scale are large, except in a few grid cells upstream and downstream of calibration stations, with an average variation of 37 and 74 % among the four homogeneous forcings in calibrated and non-calibrated regions, respectively. Considering only the forcings GSWP3 and WFDEI_hom, i.e., excluding the forcing without undercatch correction (PGFv2.1) and the one with a much lower shortwave downward radiation SWD than the others (WFD), Q variations are reduced to 16 and 31 % in calibrated and non-calibrated regions, respectively. These simulation results support the need for extended Q measurements and data sharing for better constraining global water balance assessments. Over the 20th century, the human footprint on natural water resources has become larger. For 11–18% of the global land area, the change of Q between 1941–1970 and 1971–2000 was driven more strongly by change of human water use including dam construction than by change in precipitation, while this was true for only 9–13 % of the land area from 1911–1940 to 1941–1970.

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Chemical tracers in seawater, as well as physical parameters such as temperature and salinity, have been measured to better characterize the dynamics of water convection and its spatiotemporal changes in the Sea of Japan (also called the Japan Sea), a semi-closed, hyperoxic marginal sea (maximum depth: ∼3,800 m) in the northwestern corner of the Pacific Ocean. Repeated conductivity, temperature, and depth (CTD) observations and measurements of dissolved oxygen, for more than 30 years, have confirmed that the bottom layer of the Japan Sea, with a thickness of ∼1 km below the boundary at a depth of ∼2,500 m, is characterized by vertical homogeneity with fluctuations of potential temperature and dissolved oxygen of <0.001• C and <0.5 µmol kg −1 , respectively. The timescale of the abyssal circulation in the Japan Sea has been estimated to be 100-300 years, using 14 C and other chemical tracers. Stable isotope analyses for dissolved He, O 2 and CH 4 have given us information on their unique geochemical cycles in the Japan Sea. Profiles of the short-lived radioisotope 222 Rn just above the sea bottom have brought new insights into the short-term lateral water movement with a timescale of several days in the Japan Sea bottom water. It is of special concern that the gradual deoxygenation and warming of the bottom water over the last 30 years have resulted in an ∼10% decrease in dissolved oxygen and ∼0.04• C increase in potential temperature, suggesting a change of the deep convection system in the Japan Sea. The temporal changes in the vertical profiles of tritium from 1984 to 1998 have suggested a shift of the abyssal circulation pattern from a "total (overall) convection mode" to a "shallow (partial) convection mode". It is likely that the global warming since the last century has hindered the formation of dense surface seawater and its ability to sink down to the bottom, isolating the bottom layer from the deep convection loop that is indispensable as the source of cold and oxygen-rich water. However, the decreasing trend of bottom dissolved oxygen between 1977 and 2010 was not monotonous; rather, it was interrupted by an occasional break in the winter of 2000-2001, when severely cold weather may have resulted in especially dense surface water to sink down to the bottom layer for its ventilation.