Water Isotopes In Precipitation Research Articles

Control of the temperature signal in Antarctic proxies by snowfall dynamics

Abstract. Antarctica, the coldest and driest continent, is home to the largest ice sheet, whose mass is predominantly recharged by snowfall. A common feature of polar regions is the warming associated with snowfall, as moist oceanic air and cloud cover increase the surface temperature. Consequently, snow that accumulates on the ice sheet is deposited under unusually warm conditions. Here we use a polar-oriented regional atmospheric model to study the statistical difference between average and snowfall-weighted temperatures. During snowfall, the warm anomaly scales with snowfall amount, with the strongest sensitivity occurring at low-accumulation sites. Heavier snowfall in winter helps to decrease the annual snowfall-weighted temperature, but this effect is overwritten by the event-scale warming associated with precipitating atmospheric systems, which particularly contrast with the extremely cold conditions that occur in winter. Consequently, the seasonal range of snowfall-weighted temperature is reduced by 20 %. On the other hand, the annual snowfall-weighted temperature shows 80 % more interannual variability than the annual temperature due to the irregularity of snowfall occurrence and its associated temperature anomaly. Disturbances of the apparent annual temperature cycle and interannual variability have important consequences for the interpretation of water isotopes in precipitation, which are deposited with snowfall and commonly used for paleotemperature reconstructions from ice cores.

Response of Water Isotopes in Precipitation to a Collapse of the West Antarctic Ice Sheet in High-Resolution Simulations with the Weather Research and Forecasting Model

Abstract The West Antarctic Ice Sheet (WAIS) may have collapsed during the last interglacial period, between 132 000 and 116 000 years ago. The changes in topography resulting from WAIS collapse would be accompanied by significant changes in Antarctic surface climate, atmospheric circulation, and ocean conditions. Evidence of these changes may be recorded in water-isotope ratios in precipitation archived in the ice. We conduct high-resolution simulations with an isotope-enabled version of the Weather Research and Forecasting Model over Antarctica, with boundary conditions provided by climate model simulations with both present-day and lowered WAIS topography. The results show that while there is significant spatial variability, WAIS collapse would cause detectable isotopic changes at several locations where ice-core records have been obtained or could be obtained in the future. The most robust signals include elevated δ18O at SkyTrain Ice Rise in West Antarctica and elevated deuterium excess and δ18O at Hercules Dome in East Antarctica. A combination of records from multiple sites would provide constraints on the timing, rate, and magnitude of past WAIS collapse.

Sources of water vapor and their effects on water isotopes in precipitation in the Indian monsoon region: a model-based assessment

Climate records of ratios of stable water isotopes of oxygen (δ18O) are used to reconstruct the past Indian monsoon precipitation. Identifying the sources of water vapor is important in understanding the role of monsoonal circulation in the δ18O values, to aid in monsoon reconstructions. Here, using an isotope-enabled Earth system model, we estimate the contributions of oceanic and terrestrial water vapor sources to two major precipitation seasons in India—the Southwest monsoon and the Northeast monsoon, and their effects on the δ18O in precipitation (δ18Op). We find that the two monsoon seasons have different dominant sources of water vapor because of the reversal in atmospheric circulation. While Indian Ocean regions, Arabian Sea, and recycling are the major sources of the Southwest monsoon precipitation, North Pacific Ocean and recycling are two crucial sources of Northeast monsoon precipitation. The δ18Op of the Southwest monsoon precipitation is determined by contributions from the Indian Ocean sources and recycling. Despite reduced precipitation, more negative δ18Op values are simulated in the Northeast monsoon season due to larger negative δ18Op contributions from the North Pacific. Our results imply that changes in atmospheric circulation and water vapor sources in past climates can influence climate reconstructions using δ18O.

A Pliocene Precipitation Isotope Proxy‐Model Comparison Assessing the Hydrological Fingerprints of Sea Surface Temperature Gradients

The Pliocene offers insights into future climate, with near-modern atmospheric pCO2 and global mean surface temperature estimated to be 3-4°C above pre-industrial. However, the hydrological response differs between future global warming and early Pliocene climate model simulations. This discrepancy results from the use of reduced meridional and zonal sea surface temperature (SST) gradients, based on foraminiferal Mg/Ca and Alkenone proxy evidence, to force the early Pliocene simulation. Subsequent, SST reconstructions based on the organic proxy TEX86, have found warmer temperatures in the warm pool, bringing the magnitude of the gradient reductions into dispute. We design an independent test of Pliocene SST scenarios and their hydrological cycle "fingerprints." We use an isotope-enabled General Circulation Model, iCAM5, to model the distribution of water isotopes in precipitation in response to four climatological SST and sea-ice fields representing modern, abrupt 4×CO2, late Pliocene and early Pliocene climates. We conduct a proxy-model comparison with all the available precipitation isotope proxy data, and we identify target regions that carry precipitation isotopic fingerprints of SST gradients as priorities for additional proxy reconstructions. We identify two regions with distinct precipitation isotope (D/H) fingerprints resulting from reduced SST gradients: the Maritime Continent (D-enriched due to reduced convective rainfall) and the Sahel (wetter, more deep convection, D-depleted). The proxy-model comparison using available plant wax reconstructions, mostly from Africa, is promising but inconclusive. Additional proxy reconstructions are needed in both target regions and in much of the world for significant tests of SST scenarios and dynamical linkages to the hydrological cycle.

Water stable isotopes reveal the hydrological response of Costa Rican glacial lakes to climate variability

Lakes are generally considered a key information source for understanding paleoclimate. Here, we use stable water isotopes (δ2H and δ18O) in a high-elevation tropical glacial lake to inform about modern climate variability and local hydrological conditions. We report a data set of water isotopes in precipitation and lake water collected between June 2015 and November 2020 in the Chirripó National Park, Costa Rica. Using a linear resistance model, the isotopic information coupled with lake water temperature and hydrometeorological data allowed the calculation of seasonal evaporation to inflow ratios (E/I) for Lake Ditkevi. Isotopic variations (δ18O and d-excess) reflect seasonal changes in the evaporative conditions, with E/I ratios ranging from 2.7 ± 1.3% to 10.8 ± 5.4%. Significant correlations between lake d-excess, lake water temperature, and sea surface temperature anomalies in El Niño 3.4 region revealed the sensitive hydrological response of this high elevation tropical glacial lake to the cyclic deviations in the ocean-atmosphere domain of warm/cold El Niño Southern Oscillation episodes. Our findings contribute to i) a better understanding of the climatic controls on high-elevation precipitation and surface water systems in the tropics and ii) inform paleoclimate reconstructions in Central America.

Below‐cloud evaporation effect on stable water isotopes in precipitation at the eastern margin of Qinghai‐Tibet Plateau

AbstractQuantitatively assessing the evaporation‐caused stable water isotope fractionation in precipitation below the cloud base is of great significance to understand the modern hydrological circulation especially in a plateau climate. Based on the hourly meteorological data at eight meteorological stations of the Gannan Plateau at the eastern margin of Qinghai‐Tibet Plateau in 2020, we examined the spatial and temporal variations of below‐cloud evaporation effect on precipitation isotopes, and then discussed the factors influencing the below‐cloud evaporation across the Gannan Plateau. The contemporary observation of precipitation isotopes at one site in the Gannan Plateau is also applied to understand the local meteoric water line associated with the below‐cloud evaporation. From 9:00 to 21:00 Beijing Time, the impact of below‐cloud evaporation is greater than that during the rest periods. Within the study region, the below‐cloud evaporation in the central area is usually stronger than that of the surrounding area. The diurnal and monthly variability of relative humidity is highly consistent with that of the remaining raindrop mass fraction after evaporation as well as the isotope variation in raindrop below the cloud base. When the relative humidity is greater than 95%, there is a linear relationship between the remaining raindrop mass fraction and the below‐cloud deuterium‐excess variation of approximate 1.1‰/%. Global warming will increase the below‐cloud evaporation effect across the Gannan Plateau. The findings are useful to understand the below‐cloud evaporation and isotope fractionation under a plateau climate.

Defining a Precipitation Stable Isotope Framework in the Wider Carpathian Region

The eastern part of Europe is very poorly represented in the Global Network for Isotopes in Precipitation (GNIP) database, mainly because the monitoring of the stable isotopes in precipitation started only recently compared with other regions. In this respect, the main objective of this article is to fill the gap in the GNIP database over the eastern part of Europe and show the temporal variability and potential drivers of an extended network of δ18O values in precipitation collected from 27 locations in Romania and the Republic of Moldova. We also present the first high-resolution map of the spatio-temporal distribution of δ18O values in precipitation in Romania and the Republic of Moldova, according to an observational dataset. According to our results, the stations from western and northern Romania tend to have LMWLS with higher values than those from southwestern Romania. The monthly variation of the δ18O and δ2H showed a clearly interannual variation, with distinct seasonal differences, following the seasonal temperatures. The analysis of the spatial distribution of stable isotopes in precipitation water was made on the basis of both observational data and modeled data. This allowed us to study the origin of the air moisture and the interaction with regional and local patterns and to analyze the link between the spatial δ18O variations and the large-scale circulation patterns on a seasonal scale.

Variability in observed stable water isotopes in snowpack across a mountainous watershed in Colorado

AbstractIsotopic information from 81 snowpits was collected over a 5‐year period in a large, Colorado watershed. Data spans gradients in elevation, aspect, vegetation, and seasonal climate. They are combined with overlapping campaigns for water isotopes in precipitation and snowmelt, and a land‐surface model for detailed estimates of snowfall and climate at sample locations. Snowfall isotopic inputs, describe the majority of δ18O snowpack variability. Aspect is a secondary control, with slightly more enriched conditions on east and north facing slopes. This is attributed to preservation of seasonally enriched snowfall and vapour loss in the early winter. Sublimation, expressed by decreases in snowpack d‐excess in comparison to snowfall contributions, increases at low elevation and when seasonal temperature and solar radiation are high. At peak snow accumulation, post‐depositional fractionation appears to occur in the top 25 ± 14% of the snowpack due to melt‐freeze redistribution of lighter isotopes deeper into the snowpack and vapour loss to the atmosphere during intermittent periods of low relative humidity and high windspeed. Relative depth of fractionation increases when winter daytime temperatures are high and winter precipitation is low. Once isothermal, snowpack isotopic homogenization and enrichment was observed with initial snowmelt isotopically depleted in comparison to snowpack and enriching over time. The rate of δ18O increase (d‐excess decrease) in snowmelt was 0.02‰ per day per 100‐m elevation loss. Isotopic data suggests elevation dictates snowpack and snowmelt evolution by controlling early snow persistence (or absence), isotopic lapse rates in precipitation and the ratio of energy to snow availability. Hydrologic tracer studies using stable water isotopes in basins of large topographic relief will require adjustment for these elevational controls to properly constrain stream water sourcing from snowmelt.

Orbitally driven evolution of Asian monsoon and stable water isotope ratios during the Holocene: Isotope-enabled climate model simulations and proxy data comparisons

The variation of stable water isotopes (δ18O, δD) in climate archives is an important proxy to understand the evolution of South Asian monsoon (SA) precipitation over the Holocene. In this study, using an isotope-enabled climate-model, we examine the responses of water isotopes in precipitation over the SA region to orbital changes in the early to late Holocene (8 ka to 0 ka). Precipitation is enhanced during 8 ka to 4 ka in response to increased summertime insolation, and correspondingly, larger magnitudes of negative water isotope ratios in precipitation (δ18Oprecip) are simulated. The model-simulated wettest period and the corresponding period of maximum depletion of δ18Oprecip in the SA region is the 8 ka period. We find that strengthened circulation, increased convection, and precipitation led to the δ18Oprecip depletion in the SA region. In the tropical Indian monsoon region, where convective precipitation is dominant, the δ18Oprecip values are inversely correlated with the local convection and the amount of precipitation. The δ18Oprecip values in the East Asian Summer Monsoon (EASM) region are not well-correlated with local precipitation, likely due to the enhanced convection and depletion of vapor in upstream areas and mixed precipitation types in the region. The modeled δ18Oprecip values are evaluated against δ18O inferred from proxy archives. Proxy data and model-simulated isotope ratios in precipitation agree on the larger magnitude of negative isotopic values during the 4 ka, 6 ka, and 8 ka periods relative to the present, although model-simulated enrichment in the 2 ka period is inconsistent with some proxy-records.

Deuterium Excess in Precipitation Reveals Water Vapor Source in the Monsoon Margin Sites in Northwest China

The deuterium excess (d) in precipitation, determined by the stable hydrogen and oxygen isotopes (δ2H and δ18O), is a widely applied parameter in tracing the water vapor source. Based on the multiple-year observations of stable water isotopes in precipitation at four stations in the Lanzhou city, Northwest China, we analyzed the variations in deuterium excess in precipitation at the Asian monsoon margin region. The mean value of deuterium excess at the study region is 11.0‰ in the dry season and 8.0‰ in the wet season. The d value in precipitation negatively correlates with air temperature and vapor pressure. The low d value during the wet season reflects the monsoon moisture transported from long distances. During the dry season, the continental air masses correspond to the higher d value in precipitation. The moisture regimes based on reanalysis data are generally consistent with the findings using a stable isotopic approach, and the monsoon moisture is highlighted in summer precipitation at these monsoon margin sites.

Catchment storage and residence time in a periodically irrigated watershed

AbstractUnderstanding anthropogenic impacts on water storage and water flow pathways in catchments is an ongoing challenge in hydrology. Here, we study the dynamics of subsurface storage and residence time of water in a catchment in Berkeley, California, that is within a regional park but contains diverse land use within its perimeter, including a periodically irrigated golf course. Our study combines several isotopic tracers with water budget data to examine sources of water in a stream draining the site. Irrigation water, applied to a small area of the watershed, is a minor component of the water budget. However, geochemical tracers reveal that irrigation water is a significant fraction of stream flow downstream of the golf course during baseflow and during precipitation events. Isotopic tracers indicate that the watershed has a preference to release young water for stream flow generation, resulting in contrasting tritium ages for stream water and groundwater of 1.3 ± 0.5 year and 8.2 ± 1.7 year, respectively. We determined that the older water is a very small component (0.7%) of the stream water in the tail of an assumed exponential distribution. We used the seasonal variation of stable water isotopes in precipitation and stream water over two water years to explain the damping of the isotopic signature of stream water, which yields information about the catchment's response to the input signal. The methods described here may be applicable to other urban or suburban headwater catchments in areas with a component of non‐natural recharge from, for example, leaky infrastructure, storm water routing or dry season irrigation.

New water fractions and transit time distributions at Plynlimon, Wales, estimated from stable water isotopes in precipitation and streamflow

Abstract. Long-term, high-frequency time series of passive tracers in precipitation and streamflow are essential for quantifying catchment transport and storage processes, but few such data sets are publicly available. Here we describe, present, and make available to the public two extensive data sets of stable water isotopes in streamflow and precipitation at the Plynlimon experimental catchments in central Wales. Stable isotope data are available at 7-hourly intervals for 17 months, and at weekly intervals for 4.25 years. Precipitation isotope values were highly variable in both data sets, and the high temporal resolution of the 7-hourly streamwater samples revealed rich isotopic dynamics that were not captured by the weekly sampling. We used ensemble hydrograph separation to calculate new water fractions and transit time distributions from both data sets. Transit time distributions estimated by ensemble hydrograph separation were broadly consistent with those estimated by spectral fitting methods, suggesting that they can reliably quantify the contributions of recent precipitation to streamflow. We found that on average, roughly 3 % of streamwater was made up of precipitation that fell within the previous 7 h, and 13 %–15 % of streamwater was made up of precipitation that fell within the previous week. The contributions of recent precipitation to streamflow were highest during large events, as illustrated by comparing new water fractions for different discharges and precipitation rates. This dependence of new water fractions on water fluxes was also reflected in their seasonal variations, with lower new water fractions and more damped catchment transit time distributions in spring and summer compared to fall and winter. We also compared new water fractions obtained from stable water isotopes against those obtained from concentrations of chloride, a solute frequently used as a passive tracer of catchment transport processes. After filtering the chloride data for dry deposition effects, we found broadly similar new water fractions using chloride and stable water isotopes, indicating that these different tracers may yield similar inferences about catchment storage and transport, if potentially confounding factors are eliminated. These stable isotope time series comprise some of the longest and most detailed publicly available catchment isotope data sets. They complement extensive solute data sets that are already publicly available for Plynlimon, enabling a wide range of future analyses of catchment behavior.

Control of seasonal water vapor isotope variations at Lhasa, southern Tibetan Plateau

Water isotopes in precipitation provide new insight on the controlling factors for moisture source variations on the Tibetan Plateau. However, understanding the intra-annual water isotope cycle is difficult for a region lack of winter precipitation. Here, we reported water vapor δ18O and δ2H time series over two and half years at Lhasa on the southern Tibetan Plateau. We compare our values with concurrent daily precipitation isotopes to assess the seasonal controls of vapor isotopes and possible correlation to precipitation isotopes. Results show an intra-seasonal dependence of water vapor δ18O on large scale meteorological regime. During the winter months, vapor water δ18O shows positive dependence on regional-scale temperature, confirming a Rayleigh-distillation controlling moisture transport. During the summer months, lower vapor δ18O is weekly in association with the heavy precipitation over tropical Indian Ocean and on the Tibetan Plateau, and this relation is further enhanced by the evaporation of precipitation at the sampling site. We also found a weak seasonal variation of vapor d-excess, likely resulted from climate seasonality over the large moisture sources region. Moreover, the isotope dataset contains a high-frequency of fluctuations for both δ18O and d-excess lasting several days during the summer monsoon season. These high frequency fluctuations are in phase with precipitation events, confirming significant local evaporation effects on the short-term water cycle, which further complicated the control of near surface water vapor isotopes. These findings improve our regional scale understanding of hydrological cycle on the southern Tibetan Plateau, and will potentially improve our understanding of isotope variations in proxy archives in the Tibetan Plateau.

Sensitivity of young water fractions to hydro-climatic forcing and landscape properties across 22 Swiss catchments

Abstract. The young water fraction Fyw, defined as the proportion of catchment outflow younger than approximately 2–3 months, can be estimated directly from the amplitudes of seasonal cycles of stable water isotopes in precipitation and streamflow. Thus, Fyw may be a useful metric in catchment inter-comparison studies that investigate landscape and hydro-climatic controls on streamflow generation. Here, we explore how Fyw varies with catchment characteristics and climatic forcing, using an extensive isotope data set from 22 small- to medium-sized (0.7–351 km2) Swiss catchments. We find that flow-weighting the tracer concentrations in streamwater resulted in roughly 26 % larger young water fractions compared to the corresponding unweighted values, reflecting the fact that young water fractions tend to be larger when catchments are wet and discharge is correspondingly higher. However, flow-weighted and unweighted young water fractions are strongly correlated with each other among the catchments. They also correlate with terrain, soil, and land-use indices, as well as with mean precipitation and measures of hydrologic response. Within individual catchments, young water fractions increase with discharge, indicating an increase in the proportional contribution of faster flow paths at higher flows. We present a new method to quantify the discharge sensitivity of Fyw, which we estimate as the linear slope of the relationship between the young water fraction and flow. Among the 22 catchments, discharge sensitivities of Fyw are highly variable and only weakly correlated with Fyw itself, implying that these two measures reflect catchment behaviour differently. Based on strong correlations between the discharge sensitivity of Fyw and several catchment characteristics, we suggest that low discharge sensitivities imply greater persistence in the proportions of fast and slow runoff flow paths as catchment wetness changes. In contrast, high discharge sensitivities imply the activation of different dominant flow paths during precipitation events, such as when subsurface water tables rise into more permeable layers and/or the river network expands further into the landscape.

The influence of the synoptic regime on stable water isotopes in precipitation at Dome C, East Antarctica

Abstract. The correct derivation of paleotemperatures from ice cores requires exact knowledge of all processes involved before and after the deposition of snow and the subsequent formation of ice. At the Antarctic deep ice core drilling site Dome C, a unique data set of daily precipitation amount, type, and stable water isotope ratios is available that enables us to study in detail atmospheric processes that influence the stable water isotope ratio of precipitation. Meteorological data from both automatic weather station and a mesoscale atmospheric model were used to investigate how different atmospheric flow patterns determine the precipitation parameters. A classification of synoptic situations that cause precipitation at Dome C was established and, together with back-trajectory calculations, was utilized to estimate moisture source areas. With the resulting source area conditions (wind speed, sea surface temperature, and relative humidity) as input, the precipitation stable isotopic composition was modeled using the so-called Mixed Cloud Isotope Model (MCIM). The model generally underestimates the depletion of 18O in precipitation, which was not improved by using condensation temperature rather than inversion temperature. Contrary to the assumption widely used in ice core studies, a more northern moisture source does not necessarily mean stronger isotopic fractionation. This is due to the fact that snowfall events at Dome C are often associated with warm air advection due to amplification of planetary waves, which considerably increases the site temperature and thus reduces the temperature difference between source area and deposition site. In addition, no correlation was found between relative humidity at the moisture source and the deuterium excess in precipitation. The significant difference in the isotopic signal of hoarfrost and diamond dust was shown to disappear after removal of seasonality. This study confirms the results of an earlier study carried out at Dome Fuji with a shorter data set using the same methods.

Stable water isotopes and large‐scale vertical motions in the tropics

AbstractA complete understanding of the interaction between convection and the large‐scale circulation in the tropics remains an outstanding problem. Although there is evidence that the vertical structure of convective heating has great influence in the large‐scale response and that this structure also presents considerable geographical variations, more need to be done. One of the questions that are still unanswered is how the vertical structure of the convective heating, or, similarly, of vertical velocity, varies across the tropical Pacific. Here it is suggested that some light can be shed on this debate by considering stable water isotopes. Because these tend to be progressively less abundant with increasing height, precipitation associated with top heavy profiles should be expected to be more depleted than that associated with bottom heavy profiles. This claim is verified with a variety of data: first, using observations from IAEA/WMO Global Network of Isotopes in Precipitation stations; then, using a simple model based on the budget of water isotopes in precipitation; finally, using a more complex isotope‐enabled general circulation model. Evidence provided by these sources confirms that different structures of vertical velocities are associated with different isotopic abundances, with top heavy profiles giving rise to more depleted rainfall. Finally, the data from over the Pacific, although scarce, seem to suggest that precipitation in the eastern part is more enriched than in the western, thus hinting at velocity profiles over the East being more bottom heavy than over the West Pacific.

Modeling precipitation <i>δ</i><sup>18</sup>O variability in East Asia since the Last Glacial Maximum: temperature and amount effects across different timescales

Abstract. Water isotopes in precipitation have played a key role in the reconstruction of past climate on millennial timescales and longer. However, for midlatitude regions like East Asia with complex terrain, the reliability behind the basic assumptions of the temperature effect and amount effect is based on modern observational data and still remains unclear for past climate. In the present work, we reexamine the two basic effects on seasonal, interannual, and millennial timescales in a set of time slice experiments for the period 22–0 ka using an isotope-enabled atmospheric general circulation model (AGCM). Our study confirms the robustness of the temperature and amount effects on the seasonal cycle over China in the present climatic conditions, with the temperature effect dominating in northern China and the amount effect dominating in the far south of China but no distinct effect in the transition region of central China. However, our analysis shows that neither temperature nor amount effect is significantly dominant over China on millennial and interannual timescales, which is a challenge to those classic assumptions in past climate reconstruction. Our work helps shed light on the interpretation of the proxy record of δ18O from a modeling point of view.

The identification of precipitation amount effect with a water isotope-enabled threshold model in vadose zone: a case study in Ordos Plateau

Accurate estimation of precipitation amount recharged to the groundwater is essential to the water balance calculation of a groundwater system in regional scale. In the torrid zones, the proportion of heavier water isotopes in precipitation presents a negative correlation with the precipitation amount. The physical processes explicating this so-called amount effect have not been well understood. This paper focuses on the δD relationships of local precipitation and groundwater to explain the isotopic variability in Ordos Plateau, China. In order to better understand the physical process interpreting the amount effect, a numerical threshold model in the vadose zone to relate the ineffective precipitation amount and isotopic abundance of precipitation and groundwater was supposed. By comparing the modelled δD with that in groundwater, ineffective precipitation, a value of 6.12 mm was obtained in Ordos Plateau. Thus, the potential groundwater recharge from the precipitation in the region can be deduced. The methodology proposed in this study will provide useful insights into the estimation of precipitation contribution to groundwater in other similar areas in the world.

Factors controlling stable isotope composition of precipitation in arid conditions: an observation network in the Tianshan Mountains, central Asia

Approximately one-third of the Earth's arid areas are distributed across central Asia. The stable isotope composition of precipitation in this region is affected by its aridity, therefore subject to high evaporation and low precipitation amount. To investigate the factors controlling stable water isotopes in precipitation in arid central Asia, an observation network was established around the Tianshan Mountains in 2012. Based on the 1052 event-based precipitation samples collected at 23 stations during 2012–2013, the spatial distribution and seasonal variation of δD and δ18O in precipitation were investigated. The values of δD and δ18O are relatively more enriched in the rainfall dominant summer months (from April to October) and depleted in the drier winter months (from November to March) with low D-excess due to subcloud evaporation observed at many of the driest low elevation stations. The local meteoric water line (LMWL) was calculated to be δD=7.36δ18O – 0.50 (r2=0.97, p<0.01) based on the event-based samples, and δD=7.60δ18O+2.66 (r2=0.98, p<0.01) based on the monthly precipitation-weighted values. In winter, the data indicate an isotopic rain shadow effect whereby rainout leads to depletion of precipitation in the most arid region to the south of the Tianshan Mountains. The values of δ18O significantly correlate with air temperature for each station, and the best-fit equation is established as δ18O=0.78T – 16.01 (r2=0.73, p<0.01). Using daily air temperature and precipitation derived from a 0.5° (latitude)×0.5° (longitude) gridded data set, an isoscape of δ18O in precipitation was produced based on this observed temperature effect.

Analysis of long-term stable isotopic composition in German precipitation

Summary Stable water isotopes in precipitation ( 18 O and 2 H) have been frequently used as environmental tracers to understand processes and timescales in hydrology and climate research. Capturing changes of isotopic composition over time and investigating long-term processes requires long-term data set analysis. In Germany, we have one of the world’s densest national networks for long-term isotopic analysis of precipitation covering up to 36 years of time series at 28 locations. These data were used to identify the average as well as the temporal evolution of isotopic composition in German precipitation and how it is related to meteorological and geographical parameters. We found that individual Local Meteoric Water Lines (LMWL) and the long-term averages of δ 18 O and δ 2 H depend on latitude and elevation. More variable isotopic compositions and more enriched averages were found at the coast compared to more stable compositions and depleted averages in the South, South-East and at higher elevations. This continentality effect was strongly influenced by seasonal isotope-temperature dependencies. Removing the seasonality and looking at the changes over time compared to long-term averages indicate similar patterns for temperatures and 18 O at some locations. We concluded that temperature and isotopes are in equilibrium in inland air masses only. The trend in temperature evolution was consistent on the national level, and temperature increases were observed in almost all stations. In contrast, temporal patterns of 18 O revealed different patterns and increases were only observed in 20 out of 28 locations. Therefore, changes in isotopic composition in precipitation are not only influenced by large scale processes (i.e. temperature) but also by local factors which need to be further investigated.