Isotopic composition of precipitation in a topographically steep,seasonally snow‐dominated watershed and implications of variations from the global meteoric water line

The local meteoric water line (LMWL), the functional relationship between locally measured values of δ¹⁸O and δ²H in precipitation, represents the isotopic composition of water entering hydrologic systems. The degree to which the LMWL departs from the global meteoric water line (GMWL), moreover, can reveal important information about meteoric sources of water (e.g. oceanic or terrestrial) and atmospheric conditions during transport. Here we characterize the isotopic composition of precipitation within an experimental watershed in the Western US that is subject to large topographic and seasonal gradients in precipitation. Interpreting the hydrometeorologic and spatial controls on precipitation, we constructed a seasonally weighted LMWL for southwestern Idaho that is expressed by the equation δ²H = 7.40 × δ¹⁸O ? 2.17. A seasonally weighted LMWL that is based on weighting isotopic concentrations by climatic precipitation volumes is novel, and we argue better represents the significant seasonality of precipitation in the region. The developed LMWL is considerably influenced by the semiarid climate experienced in southwest Idaho, yielding a slope and y‐intercept lower than the GMWL (δ²H = 8 × δ¹⁸O + 10). Moderate to strong correlations exist between the isotopic composition of precipitation from individual events and surface meteorologic variables, specifically surface air temperature, relative humidity, and precipitation amount. A strong negative correlation exists between the annual average isotopic composition of precipitation and elevation at individual collection sites, with a lapse rate of ?0.22‰/100 m. Copyright © 2016 John Wiley & Sons, Ltd.     

Spatiotemporal variation of stable isotopic composition in precipitation: Post‐condensational effects in a humid area

In the present study, a 2‐year dataset on δ¹⁸O and δ²H in precipitation is used to investigate hydrometeorologic controls on the isotopic compositions in a temperate maritime climate. Data was collected in Denmark along a transect of Six sampling stations across a landscape with a small topographic gradient and predominant westerly winds. Data showed the local meteoric water line for this region is expressed by the equation δ²H = 7.4δ¹⁸O + 5.4‰. A significant trend correlating enriched isotopic values to humidities around 70% during dry season and more depleted isotopic values to humidities around 90% during wet season was derived from the dataset. Temperature was found to only influence the isotopic composition in a secondary way, whereas no significant relationship was obtained for precipitation amount and evapotranspiration. It is suggested that subcloud post‐condensation exchange strongly influences the isotopic composition at the study site. A simple model of evaporation on falling rain was applied with the aim to reproduce observational data and show the potential influence of changing humidity conditions on precipitation compositions. The rather simple model approach did not fully explain the observational data, but it highlights the drastic isotopic changes from a falling raindrop that potentially can occur due to its release into a dryer atmosphere. This study shows that regional conditions and especially humidity can alter the isotopic composition in precipitation substantially even in regions without major topographic and hydrometeorologic gradients.     

Tracer-based estimation of temporal variation of water sources: an insight from supra- and subglacial environments

ABSTRACT

The temporal variations in electrical conductivity and the stable isotopes of water, δD and δ¹⁸O, were examined at Chhota Shigri Glacier, India, to understand water sources and flow paths to discharge. Discharge is highly influenced by supraglacially derived meltwater during peak ablation, and subglacial meltwaters are more prominent at the end of the melt season. The slope of the best fit linear regression line for δD versus δ¹⁸O, for both supraglacial and runoff water, is lower than that for precipitation (snow and rain) and surface ice, indicating strong isotopic fractionation associated with the melting processes. The slope of the local meteoric water line (LMWL) is close to that of the global meteoric water line (GMWL), reflecting that the moisture source is predominantly oceanic. The d-excess variation in rainwater confirms that the southwest monsoon is the main contributor during summer while the remainder including winter is mostly influenced by westerlies.     

Local meteoric water lines describe extratropical precipitation

Stable water isotopes δ¹⁸O and δ²H are used to investigate precipitation trends and storm dynamics to advance knowledge of precipitation patterns in a warming world. Herein, δ¹⁸O and δ²H were used to determine the relationship between extratropical cyclonic precipitation and local meteoric water lines (LMWLs) in the eastern Ohio Valley and the eastern United States. Precipitation volume weighted and unweighted central Ohio LMWLs, created with samples collected during 2012–2018, showed that temperature had the greatest effect on precipitation isotopic composition. HYSPLIT back trajectory modelling showed that precipitation was primarily derived from a mid-continental moisture source. Remnants of major hurricanes were collected as extratropical precipitation during the 2012–2018 sampling period in central Ohio. Extratropical precipitation samples were not significantly different from the samples that created the central Ohio LMWL. Six additional LMWLs were derived from United States Geological Survey (USGS) Atmospheric Integrated Research Monitoring Network (AIRMoN) samples collected in Pennsylvania, Delaware, Tennessee, Vermont, New Hampshire, and Oxford, Ohio. Meteoric water lines describing published samples from Superstorm Sandy, plotted with these AIRMoN LMWLs, showed isotopic composition of Superstorm Sandy precipitation was commonly more depleted than the average isotopic composition at the mid-latitude locations. Meteoric water lines describing the Superstorm Sandy precipitation were not significantly different in slope from LMWLs generated within 300 km of the USGS AIRMoN site. This finding, which was observed across the eastern Ohio Valley and eastern United States, demonstrated a consistent precipitation δ²H–δ¹⁸O relationship for extratropical cyclonic and non-cyclonic events. This work also facilitates the analysis of storm development based on the relationship between extratropical event signature and the LMWL. Analysis of extratropical precipitation in relation to LMWLs along storm tracks allows for stronger development of precipitation models and understanding of which climatic and atmospheric factors determine the isotopic composition of precipitation.     

Groundwater Origin in Qanats,Chemo-Isotopic,and Hydrogeological Evidence

A systematic study of the chemo-isotopic characteristics and origin of the groundwater was carried out at six major qanats in the hyper-arid Gonabad area, eastern Iran. These qanats as a sustainable groundwater extraction technology have a long history, supporting human life for more than a thousand years in this region. The Gonabad qanats are characterized by outlet electrical conductivity (EC) values of 750 to 3900 µS/cm and HCO₃-Na-Mg and Cl-Na water types. The Gonabad meteoric water line (G_nMWL) was drawn at the local scale as δ²H = 6.32×δ¹⁸O + 8.35 (with R² = 0.90). It has a lower slope and intercept than the global meteoric water line due to different water vapor sources and isotope kinetic fractionation effects during precipitation in this arid region. The altitude effects on isotopic content of precipitation data were derived as δ¹⁸O = (−0.0031 × H_(m.a.s.l))−1.3). The δ²H and δ¹⁸O isotopes signatures demonstrate a meteoric origin of the groundwater of these qanats. The shift of the qanat's water samples from the local meteoric water line (LMWL) in a dry period with higher temperatures is most probably due to evaporation during the infiltration process and water movement in qanat gallery. Based on the isotopic results and mass balance calculations, the qanats are locally recharged from an area between 2000 to 2400 m.a.s.l of nearby carbonate formations and coarse alluvial sediments. The dissolution of evaporate interlayers in Neogene deposits deteriorates the groundwater quality, especially in Baidokht qanat.     

Stable isotopes of atmospheric water vapour and precipitation in the northeast Qinghai‐Tibetan Plateau

Stable water isotopes (δ¹⁸O and δ²H) are an important source signature for understanding the hydrological cycle and altered climate regimes. However, the mechanisms underlying atmospheric water vapour isotopes in the northeast Qinghai‐Tibetan Plateau of central Asia remain poorly understood. This study initially investigated water vapour isotopic composition and its controls during the premonsoon and monsoon seasons. Isotopic compositions of water vapour and precipitation exhibited high variability across seasons, with the most negative average δ¹⁸O values of precipitation and the most positive δ¹⁸O values of water vapour found during the premonsoon periods. Temperature effect was significant during the premonsoon period but not the monsoon period. Both a higher slope and intercept of the local meteoric water line were found during the monsoon period as compared with in the premonsoon period, suggesting that raindrops have been experienced a greater kinetic fractionation process such as reevaporation below the cloud during the premonsoon periods. The δ²H and δ¹⁸O signatures in atmospheric water vapour tended to be depleted with the occurrence of precipitation events especially during the monsoon period and probably as a result of rainout processes. The monthly average contribution of evaporation from the lake to local precipitation was 35.2%. High d‐excess values of water vapour were influenced by the high proportion of local moisture mixing, as indicated by the gradually increasing relative humidity along westerly and Asian monsoon trajectories. The daily observation (observed ε) showed deviations from the equilibrium fractionation factors (calculated ε), implying that raindrops experienced substantial evaporative enrichment during their descent. The average fraction of raindrops reevaporation was estimated to be 16.4± 12.9%. These findings provide useful insights for understanding the interaction between water vapour and precipitation, moisture sources, and help in reconstructing the paleoclimate in the alpine regions.     

Flow dynamics of groundwater and soil water in the former heap Gessenhalde at the uranium mining area of Ronneburg: a stable isotope approach

Stable isotopes, ²H_water, ¹⁸O_water as well as ¹⁸O_sulphate and ³⁴S_sulphate, were used to study the flow system of shallow groundwater and soil water at the base area of a former leaching heap at the uranium mining area of Ronneburg, Germany. The flow paths and water‐retention times were estimated by comparison of δ²H and δ¹⁸O values in groundwater and soil water to the δ²H and δ¹⁸O signature of precipitation, giving distinctive inputs of summer or winter precipitation. The points of measuring the groundwater were divided into three categories with different flow conditions: rapid flow, stagnant conditions and a transition zone by hierarchical cluster analysis of δ²H and δ¹⁸O values of groundwater. The transit time of groundwater in the rapid flow area is less than 6 months, whereas water in the stagnant zone is stored for at least 1 year. In soil water, a clear response to different input signals is detectable only in the 30‐cm horizon (retention time is about 6 months), whereas at deeper levels a mixing with older water is taking place. The isotopic composition of the dissolved sulphate was used to identify oxidation of sulphides as the source of sulphate. Copyright © 2010 John Wiley & Sons, Ltd.     

Seasonal variations of deuterium and oxygen‐18 isotopes and their response to moisture source for precipitation events in the subtropical monsoon region

Deuterium and oxygen‐18 are common environmental tracers in water used to investigate hydrological processes such as evaporation and groundwater recharge, and to trace moisture source. In this study, we collected event precipitation from 01 January 2010 to 28 February 2011 at a site in Changsha, Yangtze River Basin to estimate the influence of moisture source and atmospheric conditions on stable isotope compositions. The local meteoric water line, established as δD = (8.45 ± 0.13) δ¹⁸O + (17.7 ± 0.9) (r² = 0.97, n = 189), had a higher slope and intercept than global meteoric water line. Temperature–δ¹⁸O exhibited complex correlations, with positive correlations during Nov.–Apr. superior to during Jun.–Sep., which was attributed to distinctive moisture sources, but vague the overall period; amount effect examined throughout the year. Linear regressions between δ¹⁸O and δD value in different precipitation event size classes revealed progressively decreasing slope and intercept values with decreasing precipitation amount and increasing vapour pressure deficit, indicating that small rainfall events (0–5 mm) were subject to secondary evaporation effects during rainwater descent. In contrast, snowfall and heavy precipitation events exhibited high slope and intercepts for the regression equation between δ¹⁸O and δD. High concentrations of heavy isotopes were associated with precipitation events sourced from remote westerly air masses, degenerated tropical marine air masses from the Bay of Bengal (BoB), and inland moisture in the pre‐monsoon period, as determined from backward trajectories assessed in the HYSPLIT model. Meanwhile, low concentrations of heavy isotopes were found to correspond with remote maritime moisture from BoB, the South China Sea, and the west Pacific at three different air pressures in summer monsoon and post‐monsoon using HYSPLIT and records of typhoon paths. These findings suggest that stable isotope compositions in precipitation events are closely associated with the meteorological conditions and respond sensitively to moisture source in subtropical monsoon climates. Copyright © 2013 John Wiley & Sons, Ltd.     

Controls on stable isotopic characteristics of water vapour over Thailand

This study examined the weekly water vapour isotopic composition (δ¹⁸O_v) in Thailand. The water vapour was cryogenically collected from eight sites across the country. Two observational samples were collected over one 24-h period each week (a daytime and a night-time sample), from September 2013 to September 2014. The primary aim was to investigate the environmental factors influencing water vapour isotopes. The results revealed differences in water vapour isotopic values between day and night samples. Three periods of depleted δ¹⁸O_v were associated with large-scale convective systems in September, December, and May. The statistical relationship between the climate variables and water vapour isotopes indicated that the amount of precipitation and relative humidity were the primary controls on both diurnal and seasonal isotopic variability. The temperature did not affect the δ¹⁸O_v, mainly because the atmospheric processes are a function of vertical convection rather than temperature in tropical regions. The water vapour deuterium excess (d-excess) showed greater variability in 2013 than in 2014. The d-excess variation reflected the differences in convection occurring in the day and night. In addition, the vapour phase data were combined with the local meteoric water line to identify the local water vapour line and the interaction between the isotopic composition of water vapour and liquid water. The water vapour isotopic patterns paralleled the precipitation isotopes on rainy days because of equilibrium isotopic exchange. Water vapour and precipitation were isotopically similar under low humidity but showed greater differences from each other under wetter conditions. The study results provide insight into water vapour isotopic characteristics in tropical regions and constrain the role of large-scale atmospheric processes relative to isotopic variability of water vapour in Thailand and nearby countries.     

Using stable isotopes to understand seasonal and interannual dynamics in moisture sources and atmospheric circulation in precipitation

Stable isotopes in precipitation are useful tracers to strengthen understanding of climate change and hydrological processes. In this study, the moisture sources of 190 precipitation events in Beijing were analysed using the Hybrid Single‐particle Lagrangian Integrated Trajectory model, based on which we studied the relation between variations in precipitation δ¹⁸O and dynamics in moisture sources and atmospheric circulation in seasonal and interannual timescales. Categorization of 7 groups of moisture sources was performed, among which oceanic moisture sources presented lower δ¹⁸O in precipitation than continental moisture sources. The results show that seasonal variations of precipitation δ¹⁸O were caused by changes of moisture sources. In summer, moisture from proximal oceans dominated vapour transport to Beijing due to increasing monsoon strength and resulted in a relatively small variation of precipitation δ¹⁸O. At the interannual timescale, the variations of δ¹⁸O in summer precipitation were related to dynamics in oceanic moistures, showing depleted values when the contribution of oceanic moistures, especially the proportion of long‐distance oceanic moisture, was high. Further analysis indicated that changes of oceanic moisture sources were controlled by the strength of summer monsoons. These findings address the complexity of moisture sources in midlatitude monsoon areas and suggest that isotopic signals in precipitation have the potential to deduce changes in moisture sources and atmospheric circulation and can therefore serve for palaeoclimate reconstruction.     

Spatial and temporal patterns of stable water isotopes along the Yangtze River during two drought years

Changes in the level of the Yangtze River caused by anthropogenic water regulation have major effects on the hydrological processes and water cycle in surrounding lakes and rivers. In this study, we obtained isotopic evidence of changes in the water cycle of Yangtze River during the two drought years of 2006 and 2013. Isotopic evidence demonstrated that the δ¹⁸O and δD levels in Yangtze River exhibited high spatial heterogeneity from the upper to lower reaches, which were controlled by atmospheric precipitation, tributary/lake water mixing, damming regulation, and water temperature. Both the slope and intercept of Yangtze River evaporative line (δD = 7.88 δ¹⁸O + 7.96) were slightly higher than those of local meteoric water line of Yangtze River catchment (δD = 7.41 δ¹⁸O + 6.01). Most of the river isotopic values were located below the local meteoric water line, thereby implying that the Yangtze River water experienced a certain degree of evaporative enrichment on isotopic compositions of river water. The high fluctuations in the isotopic composition (e.g., deuterium excess [d‐excess]) in the middle to lower reaches during the initial stage of operation for the Three Gorges Dams (2003–2006) were due to heterogeneous isotopic signatures from the upstream water. In contrast to the normal stage (after 2010) characterized by the maximum water level and largest water storage, a relatively small variability in the deuterium excess was found along the middle to lower reaches because of the homogenization of reservoir water with a longer residence time and complete mixing. The effects of water from lakes and tributaries on the isotopic compositions in mainstream water were highlighted because of the high contributions of lakes water (e.g., Dongting Lake and Poyang Lake) efflux to the Yangtze River mainstream, which ranged from 21% to 85% during 2006 and 2013. These findings suggest that the retention and regulation of the Three Gorges Dams has greatly buffered the isotopic variability of the water cycle in the Yangtze catchment, thereby improving our understanding of the complex lake–river interactions along the middle to lower reaches in the future.     

Isotopic composition (δ18O and δD) of precipitation and groundwater in a semi‐arid,mountainous area (Guadiana Menor basin,Southeast Spain)

We characterize the precipitation and groundwater in a mountainous (peaks slightly above 3000 m a.s.l.), semi‐arid river basin in SE Spain in terms of the isotopes ¹⁸O and ²H. This basin, with an extension of about 7000 km², is an ideal site for such a study because fronts from the Atlantic and the Mediterranean converge here. Much of the land is farmed and irrigated both by groundwater and runoff water collected in reservoirs. A total of approximately 100 water samples from precipitation and 300 from groundwater have been analysed. To sample precipitation we set up a network of 39 stations at different altitudes (800–1700 m a.s.l.), with which we were able to collect the rain and snowfall from 29 separate events between July 2005 and April 2007 and take monthly samples during the periods of maximum recharge of the aquifers. To characterize the groundwater we set up a control network of 43 points (23 springs and 20 wells) to sample every 3 months the main aquifers and both the thermal and non‐thermal groundwater. We also sampled two shallow‐water sites (a reservoir and a river). The isotope composition of the precipitation forms a local meteoric water line (LMWL) characterized by the equation δD = 7·72δ¹⁸O + 9·90, with mean values for δ¹⁸O and δD of − 10·28‰ and − 69·33‰, respectively, and 12·9‰ for the d‐excess value. To correlate the isotope composition of the rainfall water with groundwater we calculated the weighted local meteoric water line (WLMWL), characterized by the equation δD = 7·40δ¹⁸O + 7·24, which takes into account the quantity of water precipitated during each event. These values of (dδD/dδ¹⁸O)< 8 and d‐excess (δD–8δ¹⁸O)< 10 in each curve bear witness to the ‘amount effect’, an effect which is more manifest between May and September, when the ground temperature is higher. Other effects noted in the basin were those of altitude and the continental influence. The isotopic compositions of the groundwater are represented by the equation δD = 4·79δ¹⁸O − 18·64. The groundwater is richer in heavy isotopes than the rainfall, with mean values of − 8·48‰ for δ¹⁸O and − 59·27‰ for δD. The isotope enrichment processes detected include a higher rate of evaporation from detrital aquifers than from carbonate ones, the effects of recharging aquifers from irrigation return flow and/or from reservoirs' leakage and enrichment in δ¹⁸O from thermal water. Copyright © 2010 John Wiley & Sons, Ltd.     

Factors controlling spatial and seasonal distributions of precipitation δ18O in China

Utilising datasets from the Global Network of Isotopes in Precipitation of the International Atomic Energy Agency, and previous isotopic studies, we investigated δ¹⁸O spatial and temporal patterns in Chinese precipitation. Significantly positive relationships existed between precipitation δ¹⁸O and air temperature above the north of 35°N and in high altitude regions above 32°N. Significantly negative relationships between precipitation δ¹⁸O and the precipitation amount existed below south of 35°N. These temperature and precipitation effects became stronger with increasing altitude except in high altitude regions between 32°N and 35°N. The NCEP/NCAR reanalysis data from 1980 to 2004 showed that variations in spatial and seasonal wind fields at 700 hpa and total precipitable water from the ground to the top of the atmosphere were correlated with the monthly spatial distribution of precipitation δ¹⁸O. Basing on this relationship, we established quantitative correlations between the mean monthly precipitation δ¹⁸O and both latitude and temperature in different seasons. We found that spatial variations in precipitation δ¹⁸O could be described well using the Bowen–Wilkinson model and second‐order equations developed during the present study only in winter (from December to February). During the rest of the year, patterns were too complex to predict using simple models. The results suggest that it is difficult to demonstrate variations of precipitation δ¹⁸O throughout the year and for all regions of China using a single model. Moreover, the new models for the relationships among precipitation, latitude, and temperature were better able to depict the variations in precipitation δ¹⁸O than the Bowen–Wilkinson model. Copyright © 2011 John Wiley & Sons, Ltd.     

Seasonality of stable isotope composition of atmospheric water input at the southern slopes of Mt. Kilimanjaro,Tanzania

To understand the moisture regime at the southern slopes of Mt. Kilimanjaro, we analysed the isotopic variability of oxygen (δ¹⁸O) and hydrogen (δD) of rainfall, throughfall, and fog from a total of 2,140 samples collected weekly over 2 years at 9 study sites along an elevation transect ranging from 950 to 3,880 m above sea level. Precipitation in the Kilimanjaro tropical rainforests consists of a combination of rainfall, throughfall, and fog. We defined local meteoric water lines for all 3 precipitation types individually and the overall precipitation, δD_prec = 7.45 (±0.05) × δ¹⁸O_prec + 13.61 (±0.20), n  = 2,140, R ² = .91, p  < .001. We investigated the precipitation‐type‐specific stable isotope composition and analysed the effects of amount, altitude, and temperature. Aggregated annual mean values revealed isotope composition of rainfall as most depleted and fog water as most enriched in heavy isotopes at the highest elevation research site. We found an altitude effect of δ¹⁸O_rain = ?0.11‰ × 100 m^?1, which varied according to precipitation type and season. The relatively weak isotope or altitude gradient may reveal 2 different moisture sources in the research area: (a) local moisture recycling and (b) regional moisture sources. Generally, the seasonality of δ¹⁸O_rain values follows the bimodal rainfall distribution under the influences of south‐ and north‐easterly trade winds. These seasonal patterns of isotopic composition were linked to different regional moisture sources by analysing Hybrid Single Particle Lagrangian Integrated Trajectory backward trajectories. Seasonality of d excess values revealed evidence of enhanced moisture recycling after the onset of the rainy seasons. This comprehensive dataset is essential for further research using stable isotopes as a hydrological tracer of sources of precipitation that contribute to water resources of the Kilimanjaro region.     

Estimating the criterion for determining water vapour sources of summer precipitation on the northern Tibetan Plateau

According to the precipitation and δ¹⁸O data obtained during the GEWEX Asian Monsoon Experiment–Tibet Intensive Observation Period, based on the knowledge that δ¹⁸O is lower in precipitation formed from ocean air mass vapour than that from local evaporation vapour, the water vapour sources can be identified from the δ¹⁸O values in precipitation. We attempt to give the identification criterion of δ¹⁸O values in precipitation. The threshold values chosen to distinguish between ocean and local sources are δ¹⁸O < ?20‰ and δ¹⁸O > ?13‰ respectively. According to this criterion, the proportion of local evaporation‐formed precipitation and ocean air‐mass‐formed precipitation in total precipitation was estimated. The average value of precipitation at three sites (NODA, Amdo and AQB) is 249·76 mm. Among this, precipitation formed directly by the ocean air mass vapour is 80·08 mm at most. Precipitation formed by water vapour evaporated from local places is 117·05 mm at least. That is to say that precipitation formed directly by the ocean air mass vapour accounts for 32·06% of the total precipitation at most. Precipitation formed by water vapour evaporated from local places accounts for 46·86% of the total precipitation at least. At least 21·8% of the total precipitation came from water vapour that was evaporated on the way and transported by the monsoon circulation. Copyright © 2005 John Wiley & Sons, Ltd.     

Orographic distillation and spatio‐temporal variations of stable isotopes in precipitation in the North Atlantic

Little is known about the spatial and temporal variability of the stable isotopic composition of precipitation in the North Atlantic and its relationship to the North Atlantic Oscillation (NAO) and anthropogenic climate change. The islands of the Azores archipelago are uniquely positioned in the middle of the North Atlantic Ocean to address this knowledge gap. A survey of spatial and temporal variability of the stable isotope composition of precipitation in Azores is discussed using newly presented analyses along with Global Network of Isotopes in Precipitation data. The collected precipitation samples yield a new local meteoric water line (δ²H = 7.1 * δ¹⁸O + 8.46) for the Azores region and the North Atlantic Ocean. The annual isotopic mean of precipitation shows a small range for the unweighted and precipitation mass‐weighted δ¹⁸O‐H₂O values. Results show an inverse relation between the monthly δ¹⁸O‐H₂O and the amount of precipitation, which increases in elevation and into the interior of the island. Higher amounts of precipitation (from convective storm systems) do not correspond to the most depleted values of stable isotopes in precipitation. Precipitation shows an orographic effect with depleted δ¹⁸O‐H₂O values related to the Rayleigh effect. Monthly δ¹⁸O‐H₂O values for individual precipitation sampling stations show little relationship to air temperature. Results show a local source of moisture during the summer with the characteristics of the first vapour condensate. The stable isotope composition of precipitation is strongly correlated to the NAO index, and δ¹⁸O‐H₂O values show a statistically significantly trend towards enrichment since 1962 coincident with the increased air temperature and relative humidity due to climate change. Results are in line with observations of increasing sea surface temperature and relative humidity.     

Analyzing Effects of Shrub Canopy on Throughfall and Phreatic Water Using Water Isotopes,Western China

Alpine shrub Quercus aquifolioides was selected to study the effects of shrub canopy on throughfall and phreatic water by analyzing the isotopic time series of precipitation, canopy throughfall and phreatic water and examining correlations among these series in Wolong Nature Reserve, Western China. Based on analysis of precipitation data in 2003, the local meteoric water line during the rainy season was δD = 8.28 × δ¹⁸O + 8.93, and the primary precipitation moisture in this region originated from the Pacific Ocean in the summer. Stable isotope analysis showed that the main supply of throughfall and phreatic water was from precipitation, and the shrub canopy has an important effect on the processes of rainwater transmuted into throughfall and phreatic water. Moreover, the differences of δD and δ¹⁸O values between rainwater and throughfall were relevant to rainfall. Due to interception of the shrub canopy, there had a response hysteresis of phreatic water to the various rainfall events, which was mostly 2 days, except that this hysteresis was ≤1 day when rainfall was >15 mm/day.     

Water isotopic variability in Mallorca: a path to understanding past changes in hydroclimate

This paper reports the first results on δ¹⁸O and δ²H analysis of precipitations, cave drip waters, and groundwaters from sites in Mallorca (Balearic Islands, western Mediterranean), a key region for paleoclimate studies. Understanding the isotopic variability and the sources of moisture in modern climate systems is required to develop speleothem isotope‐based climate reconstructions. The stable isotopic composition of precipitation was analysed in samples collected between March 2012 and March 2013. The values are in the range reported by GNIP Palma station. Based on these results, the local meteoric water line (LMWL) δ²H = 7.9 (±0.3) δ¹⁸O + 10.8 (±2.5) was derived, with slightly lower slope than Global Meteoric Water Line. The results help tracking two main sources of air masses affecting the study sites: rain events with the highest δ¹⁸O values (> ?5‰) originate over the Mediterranean Sea, whereas the more depleted samples (< ?8‰) are sourced in the North Atlantic region. The back trajectory analysis and deuterium excess values, ranging from 0.4 to 18.4‰, further support our findings. To assess the isotopic variation across the island, water samples from eight caves were collected. The δ¹⁸O values range between ?6.9 and ?1.6‰. With one exception (Artà), the isotopic composition of waters in caves located along the coast (Drac, Vallgornera, Cala Varques, Tancada, and Son Sant Martí) indicates Mediterranean‐sourced moisture masses. By contrast, the drip water δ¹⁸O values for inland caves (Campanet, ses Rates Pinyades) or developed under a thick (>50 m) limestone cap (Artà) exhibit more negative values. A well‐homogenized aquifer supplied by rainwaters of both origins is clearly indicated by groundwater δ¹⁸O values, which show to be within 2.4‰ of the unweighted arithmetic mean of ?7.4‰. Although limited, the isotopic data presented here constitute the baseline for future studies using speleothem δ¹⁸O records for western Mediterranean paleoclimate reconstructions. Copyright © 2016 John Wiley & Sons, Ltd.     

Stable isotopic compositions and controlling factors of precipitation and atmospheric vapour in the headwaters of the Shule River

Stable isotopic compositions of precipitation (δ¹⁸O_p, δ²H_p and d-excess_p) and atmospheric vapour (δ¹⁸O_v, δ²H_v and d-excess_v) with high spatial–temporal resolution are crucial in revealing hydrologic cycle. Based on the variation characteristics of δ¹⁸O_p/δ¹⁸O_v, δ²H_p/δ²H_v and d-excess_p/d-excess_v in the headwaters of the Shule River (HSR) on hourly and daily scales from June to September 2018, this study analysed the relationships between δ¹⁸O_p/δ²H_p and δ¹⁸O_v/δ²H_v combined with the equilibrium fractionation model, as well as δ¹⁸O_p/δ¹⁸O_v and meteorological factors. The slopes of local meteoric water line (LMWL) and the δ²H_v-δ¹⁸O_v fitting equation were similar (7.96 and 7.94) with both intercepts exceeding 10, reflecting the great contribution of recycling moisture. The values of δ¹⁸O_v/δ²H_v were lower than δ¹⁸O_p/δ²H_p but with consistent variation patterns throughout the period. The equilibrium simulation results suggested that precipitation and atmospheric vapour almost approached isotopic equilibrium state, especially during monsoon intrusion period. Affected by monsoon intrusion, the slopes and intercepts of the LMWLs and the δ²H_v-δ¹⁸O_v fitting equations were smaller than those during non-monsoon period and d-excess and δ¹⁸O were negatively correlated. Relative humidity had significant negative correlations with δ¹⁸O_p and δ¹⁸O_v in the whole period, however, the positive correlations between δ¹⁸O_p/δ¹⁸O_v and temperature were observed during non-monsoon and monsoon intrusion period, respectively. Our results demonstrated that precipitation and atmospheric vapour isotopic compositions exhibited consistency under the influence of diverse moisture sources, while more complex relationships were found between δ¹⁸O_p/δ¹⁸O_v and meteorological factors. This research provided evidence for using the isotopic compositions of atmospheric vapour to indicate moisture sources, and can improve understanding of the water cycle and eco-hydrological process from the perspective of the interaction between water and gas phases of the inland river basin in northwest China.     

Spatial and temporal variability of stable isotopes (δ18O and δ2H) in surface waters of arid,mountainous Central Asia

Characterization of spatial and temporal variability of stable isotopes (δ¹⁸O and δ²H) of surface waters is essential to interpret hydrological processes and establish modern isotope–elevation gradients across mountainous terrains. Here, we present stable isotope data for river waters across Kyrgyzstan. River water isotopes exhibit substantial spatial heterogeneity among different watersheds in Kyrgyzstan. Higher river water isotope values were found mainly in the Issyk‐Kul Lake watershed, whereas waters in the Son‐Kul Lake watershed display lower values. Results show a close δ¹⁸O–δ²H relation between river water and the local meteoric water line, implying that river water experiences little evaporative enrichment. River water from the high‐elevation regions (e.g., Naryn and Son‐Kul Lake watershed) had the most negative isotope values, implying that river water is dominated by snowmelt. Higher deuterium excess (average d = 13.9‰) in river water probably represents the isotopic signature of combined contributions from direct precipitation and glacier melt in stream discharge across Kyrgyzstan. A significant relationship between river water δ¹⁸O and elevation was observed with a vertical lapse rate of 0.13‰/100 m. These findings provide crucial information about hydrological processes across Kyrgyzstan and contribute to a better understanding of the paleoclimate/elevation reconstruction of this region.