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...lacier melt at higher altitudes (e.g. Meier et al., 2013). A second grouping was clustered by higher latitudes (> 30◦ latitude) and δ18O minima in the winter months during the lowest air temperature (=-=Dansgaard, 1964-=-). The last group comprised GNIR stations within a 30◦ N/S latitude band. Those were filtered based on the phase difference between the two low-latitude zones (N–S), which was about six months, accord...

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...pic values) and GNIR samples (not averaged or discharge weighted) showed that precipitation and river samples all lie along one global meteoric water line that is well established for water isotopes (=-=Craig, 1961-=-). Although there was no coherent correlation, the seasonal amplitude of δ18O in global rivers did not increase with latitude, as in the average observed for precipitation (Fig. 4). This was related t...

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...topes in Precipitation (GNIP), a joint initiative of the International Atomic Energy Agency (IAEA), the World Meteorological Organization (WMO), and collaborating institutions as well as individuals (=-=Rozanski et al., 1993-=-; Aggarwal et al., 2010; IAEA/WMO, 2015). In order to fill isotopic data gaps between the well-known continental precipitation inputs to terrestrial landscapes and the aggregated and altered riverine ...

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... (e.g. Oki and Kanae, 2006). Recent estimates suggest that there are more than 58 000 dams sited on the world’s rivers (ICOLD, 2015), with very few rivers left in a state of natural discharge regime (=-=Dynesius and Nilsson, 1994-=-). Riverine water quality degradation may be manifested by increasing downstream water pollution (chemicals that impact human consumption or recreational use), nutrient loadings, sedimentation, altere...

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...local variations of the isotopic composition of river water were closely coupled to the well-understood regional and continental isotopic variance in precipitation (Rozanski et al., 1982, 1993, 1996; =-=Araguás-Araguás et al., 1998-=-; Bowen and Wilkinson, 2001; Feng et al., 2009). The first aim, however, was to isotopically distinguish snow- and glacier-runoff-dominated systems from direct precipitation- and runoff-dominated syst...

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...rch cling, mixing) and catchment water balance, as well as tracers of river recharge sources (direct precipitation, runoff, soil water, groundwater, lakes, snow and ice) (e.g. McDonnell et al., 1990; =-=Kendall and McDonnell, 1998-=-; Lambs, 2000; Gibson et al., 2005; Liu et al., 2008; Jasechko et al., 2013). Hydrological processes occurring between rainfall input and river discharge modify the stable isotopic composition of rive...

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...aging during soil infiltration, runoff, damming (Ogrinc et al., 2008; Koeniger et al., 2009) and seasonally differential fractional inputs of water from surface and groundwater sources (Sklash, 1990; =-=Buttle, 1994-=-; Lambs, 2004); and heavy isotope (2H, 18O) enrichment due to the effects of watershed evapotranspiration or instream evaporation (Simpson and Herczeg, 1991; Gremillion and Wanielista, 2000; Telmer an...

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...ation and riverine research cling, mixing) and catchment water balance, as well as tracers of river recharge sources (direct precipitation, runoff, soil water, groundwater, lakes, snow and ice) (e.g. =-=McDonnell et al., 1990-=-; Kendall and McDonnell, 1998; Lambs, 2000; Gibson et al., 2005; Liu et al., 2008; Jasechko et al., 2013). Hydrological processes occurring between rainfall input and river discharge modify the stable...

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.... It was assumed that seasonal and local variations of the isotopic composition of river water were closely coupled to the well-understood regional and continental isotopic variance in precipitation (=-=Rozanski et al., 1982-=-, 1993, 1996; Araguás-Araguás et al., 1998; Bowen and Wilkinson, 2001; Feng et al., 2009). The first aim, however, was to isotopically distinguish snow- and glacier-runoff-dominated systems from direc...

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...eg, 1991; Gremillion and Wanielista, 2000; Ferguson et al., 2007; Bowen et al., 2011). Alpine or high-latitude rivers may be ephemeral, dominated mostly by isotopically depleted snowmelt events (e.g. =-=Friedman et al., 1992-=-; Meier et al., 2013). Seasonal isotopic variations in rivers, nevertheless, can mirror annual variations in precipitation (e.g. Dalai et al., 2002; Lambs et al., 2005), but these variations are usual...

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... fractionation of snowmelt (Taylor et al., 2002). All of these processes may result in markedly different average isotopic values in river discharge compared to precipitation, both in space and time (=-=Dutton et al., 2005-=-; Rock and Mayer, 2007). Generally, a review of the literature reveals that longitudinal δ18O and δ2H variations in a river strongly depend on the catchment elevation, since headwaters at high altitud...

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...oderate compared to precipitation as a result of catchment buffering and the fact that the predominant source of riverine baseflow often stems from relatively isotopically stable groundwater sources (=-=Darling and Bath, 1988-=-; Maloszewski et al., 1992; Kendall and Coplen, 2001; Dutton et al., 2005). Only a few systematic long time series (> 5 years) of monthly isotope sampling of rivers have ever been published. Those few...

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...tant long-term contributors to the river runoff in these catchments. The importance of glacier meltwater in those river systems was also affirmed by non-isotopic studies (e.g. Immerzeel et al., 2010; =-=Huss, 2011-=-). Especially in ungauged catchments, but also in addition to quantitative studies, this method may be applied to evaluate glacier or permafrost contributions, or to observe winter / summer runoff rat...

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...ipitation as a result of catchment buffering and the fact that the predominant source of riverine baseflow often stems from relatively isotopically stable groundwater sources (Darling and Bath, 1988; =-=Maloszewski et al., 1992-=-; Kendall and Coplen, 2001; Dutton et al., 2005). Only a few systematic long time series (> 5 years) of monthly isotope sampling of rivers have ever been published. Those few which have been presented...

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... and δ2H variations in a river strongly depend on the catchment elevation, since headwaters at high altitudes are generally depleted in 18O and 2H compared to lower elevation downstream regions (e.g. =-=Longinelli and Edmond, 1983-=-; Ramesh and Sarin, 1992; Pawellek et al., 2002; Winston and Criss, 2003; Rock and Mayer, 2007), except where high altitude tributaries merge into low elevation main stems (Yang et al., 1996; Yi et al...

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...r strongly depend on the catchment elevation, since headwaters at high altitudes are generally depleted in 18O and 2H compared to lower elevation downstream regions (e.g. Longinelli and Edmond, 1983; =-=Ramesh and Sarin, 1992-=-; Pawellek et al., 2002; Winston and Criss, 2003; Rock and Mayer, 2007), except where high altitude tributaries merge into low elevation main stems (Yang et al., 1996; Yi et al., 2010). The cumulative...

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... isotopic averaging during soil infiltration, runoff, damming (Ogrinc et al., 2008; Koeniger et al., 2009) and seasonally differential fractional inputs of water from surface and groundwater sources (=-=Sklash, 1990-=-; Buttle, 1994; Lambs, 2004); and heavy isotope (2H, 18O) enrichment due to the effects of watershed evapotranspiration or instream evaporation (Simpson and Herczeg, 1991; Gremillion and Wanielista, 2...

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...oil infiltration, runoff, damming (Ogrinc et al., 2008; Koeniger et al., 2009) and seasonally differential fractional inputs of water from surface and groundwater sources (Sklash, 1990; Buttle, 1994; =-=Lambs, 2004-=-); and heavy isotope (2H, 18O) enrichment due to the effects of watershed evapotranspiration or instream evaporation (Simpson and Herczeg, 1991; Gremillion and Wanielista, 2000; Telmer and Veizer, 200...

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...sus downstream isotopic patterns as they accumulate discharge and integrate various hydrological processes from contributing subcatchments (Simpson and Herczeg, 1991; Gremillion and Wanielista, 2000; =-=Ferguson et al., 2007-=-; Bowen et al., 2011). Alpine or high-latitude rivers may be ephemeral, dominated mostly by isotopically depleted snowmelt events (e.g. Friedman et al., 1992; Meier et al., 2013). Seasonal isotopic va...

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...wmelt (Taylor et al., 2002). All of these processes may result in markedly different average isotopic values in river discharge compared to precipitation, both in space and time (Dutton et al., 2005; =-=Rock and Mayer, 2007-=-). Generally, a review of the literature reveals that longitudinal δ18O and δ2H variations in a river strongly depend on the catchment elevation, since headwaters at high altitudes are generally deple...

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...of hydropower reservoirs show important impacts on the natural seasonal isotopic amplitude, indicating for example the fluctuating mixing ratios of water sources due to reservoir storage and release (=-=Rank et al., 1998-=-, 2014; Schotterer, 2010). Long-term patterns of isotopes in rivers generally correlate with that of local precipitation; however, the catchment signals may be delayed up to several years (Rank et al....

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...to important river alterations and processes, e.g. the freezing of upstream surface water, which changes the river runoff components in winter (e.g. Torne River downstream of Lake Torneträsk, Sweden; =-=Burgman et al., 1981-=-); the averaging of different water sources due to cumulative dam systems (e.g. Euphrates River, Syria; Kattan, 2012, and Waikato River, New Zealand; Mook, 1982); and the mixing of evaporated water an...

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...Longinelli and Edmond, 1983; Ramesh and Sarin, 1992; Pawellek et al., 2002; Winston and Criss, 2003; Rock and Mayer, 2007), except where high altitude tributaries merge into low elevation main stems (=-=Yang et al., 1996-=-; Yi et al., 2010). The cumulative effect of catchment-scale evapotranspiration and instream evaporative processes may additionally increase δ18O and δ2H values in the downstream direction. Rivers tha...

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...mponents in winter (e.g. Torne River downstream of Lake Torneträsk, Sweden; Burgman et al., 1981); the averaging of different water sources due to cumulative dam systems (e.g. Euphrates River, Syria; =-=Kattan, 2012-=-, and Waikato River, New Zealand; Mook, 1982); and the mixing of evaporated water and reverse seasonal flow from the outflow of regulated reservoirs having long water residence times (e.g. Zambezi Riv...

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...cost laser spectroscopy techniques for conducting water isotope analysis, the widespread adoption of stable isotope tracers is now achievable in many national river water quality monitoring programs (=-=Kendall et al., 2010-=-) as well as in aquatic ecological studies. We aim to demonstrate the benefits of routinely applying water stable isotopes as key tracers in evaluating hydrological processes in the worlds’ rivers and...

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...ematical models to derive the comparability of the isotopic composition of rivers to rainfall, but these models rely on discrete but sparsely distributed GNIP station data or were applied regionally (=-=Landwehr and Coplen, 2006-=-; Bowen et al., 2011). To overcome this GNIP coverage limitation, we used a catchment-constrained version of the regionalized cluster-based water isotope prediction (RCWIP) model based on GNIP data (T...

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... part of the IAEA Water Resources Programme. GNIR began as a pilot project in 2002–2005 and focussed on the stable isotopes and tritium content of various world river catchments (Vitvar et al., 2007; =-=Michel et al., 2014-=-). The aim of the GNIR programme is to collect and disseminate time-series and synoptic collections of riverine isotope data from the world’s rivers and to inform a range of scientific disciplines inc...

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...correlate with that of local precipitation; however, the catchment signals may be delayed up to several years (Rank et al., 2014), or differ for rivers within a geographical region (Schotterer, 2010; =-=Stumpp et al., 2015-=-). Hence, long-term riverine isotopic time series are key to providing scientific information for water managers and researchers to gain insights to study hydrological processes and better focus integ...

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...and the mixing of evaporated water and reverse seasonal flow from the outflow of regulated reservoirs having long water residence times (e.g. Zambezi River downstream of Cahora Bassa Dam, Mozambique; =-=Talma et al., 2012-=-). Despite all of the above caveats, most rivers still reflected the seasonal variation of δ18O values in precipitation that was expected based on the topography and latitude of the river basin, even ...

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...6; Bowen et al., 2011). To overcome this GNIP coverage limitation, we used a catchment-constrained version of the regionalized cluster-based water isotope prediction (RCWIP) model based on GNIP data (=-=Terzer et al., 2013-=-). This catchment constrained model modification (CCRCWIP) was used to estimate the average amount-weighted isotopic composition of rainfall in the upstream catchment of a selected GNIR station. The u...