Stable Isotopic Composition Of Precipitation Research Articles

Monitoring of stable isotope composition of precipitation reveals thunderstorm dynamics

ABSTRACT The summer of 2019 is particularly well known for the famous heatwaves that swept across the European continent, with its associated drought and record-breaking air temperatures. This was followed by powerful thunderstorms, characterised by hail and heavy rain that damaged the crops on a regional scale. Here, we investigated one of the largest storm cells, lasting more than 6 h, which struck southwestern Romania. High-temporal resolution sampling of storm precipitation was performed for stable isotope measurements, rainfall and air temperature, to follow the storm dynamics. Hydrogen and oxygen isotope measurements show an abrupt decreasing temporal trend followed by superimposed V-shaped patterns interpreted as reflecting moisture replenishment by successive rain bands. To model the stable isotope values of precipitation in relation to the general trend of decreasing air temperatures, we applied a numerical Rayleigh condensation model for a non-constant α isotopic fractionation factor between liquid water and water vapour. The storm is powered by four consecutive moisture fronts, each following a Rayleigh distribution. About 40 % of the water vapour condenses during the sampled storm due to adiabatic expansion and cooling, which lowers saturation. Condensation ceases when cooling and absolute humidity can no longer sustain the dew point, stopping the rain. The timing of the event, occurring late at night and early in the morning, its duration of over 6 h as well as its synoptic scale may indicate a mesoscale convective complex.

A decade (2008–2017) of water stable isotope composition of precipitation at Concordia Station, East Antarctica

Abstract. A 10-year record of oxygen and hydrogen isotopic composition of precipitation is presented here: from 2008 to 2017, 1483 daily precipitation samples were collected year-round on a raised platform at Concordia Station, East Antarctica. Weather data were retrieved from the Italian Antarctic Meteo-Climatological Observatory automatic weather station (AWS), while ERA5 was used to estimate total precipitation. The δ–temperature relationships were moderately high for daily data (r2=0.63 and 0.64 for δ18O and δ2H, respectively) and stronger using monthly data (r2=0.82 for both δ18O and δ2H), with a slope of about 0.5 ‰ °C−1 for δ18O/TAWS (3.5 ‰ °C−1 for δ2H/TAWS), which remains consistent also using annual averages. The isotopic composition of precipitation is the input signal of the snow–ice system, and this dataset will be useful to improve the interpretation of paleoclimate records and promote a better understanding of the post-depositional processes affecting the isotopic signal in ice cores. This dataset represents a benchmark for the evaluation of isotope-enabled general circulation models. Here, the ECHAM6-wiso output was compared to experimental data, showing moderately good relationships for δ18O and δ2H but not for d-excess, nonetheless marking a substantial improvement from the previous release of the model.

Hydrochemical and isotopic characteristics on the southern and northern slopes of the Himalayas: spatio-temporal controls and source apportionment

Water-soluble ions, inorganic nitrogen, and stable isotopes in precipitation were assessed from the southern (Koshi Tappu and Khandbari) and northern slopes (Lhasa and SET) of the Himalayas to understand the sources, chemistry of regional precipitation, and climatic processes. Water soluble ions showed distinct seasonal variation, with higher concentrations in the non-monsoon. The concentration of ionic species was highest in Koshi Tappu, followed by Lhasa, SET, and Khandbari. The sources were from the terrigenous (Ca2+, HCO3−), marine (Na+ and Cl−), anthropogenic (SO42−, NO3−, and NH4+), terrigenous and marine (Mg2+), and biomass-burning (K+). The southern slope, relative to the northern, was more prone to anthropogenic emissions with higher deposition. Among all sites, inorganic nitrogen deposition at Koshi Tappu was higher than the threshold value (10 kg ha−1 y−1). The isotopic composition during the study period was higher in non-monsoon, started declining from June, and depleted in July and August compared to other months, i.e., the monsoon mature phase, along the south-to-north transect. The diminished value of stable isotopes in precipitation with increasing altitude underlines the evidence of the orographic effect in isotopic composition. Our study delineated that the higher/lower d-excess value increased with altitude on the southern/northern slope of the Himalayas. The backward trajectory analysis and the National Centers for Environmental Prediction's Final (NCEP FNL) datasets identified that most of the trajectories arrived from warm and humid low-latitude regions during monsoon and westerlies in non-monsoon. Thus, the chemical characteristics and stable isotopic composition of precipitation differed on the southern and northern slopes of the Himalayas by orographic effect and various sources. This study provides new insights into the atmospheric environment and climatic control of stable isotopes in the Himalayan Tibetan Plateau and facilitates monitoring of transboundary air pollution.

Atmospheric River Brings Warmth and Rainfall to the Northern Antarctic Peninsula During the Mid‐Austral Winter of 2023

AbstractContrasting the extensive research on summer atmospheric rivers (ARs) in the Antarctic Peninsula (AP), winter AR impacts are less understood. This study examines a unique warming event from 1 to 3 July 2023, using in situ winter observations and ERA5 reanalysis. On 2 July, Frei station experienced an extreme warm event with a temperature of 2.7°C and a significant rise in the freezing level, coinciding with winter rainfall. A pressure dipole pattern over the AP, with contrasting circulations over Bellingshausen and Weddell Seas, facilitated an AR, carrying warm, humid air initially from South America/Atlantic and then the southeast Pacific. This shift resulted in anomalous water stable isotope composition in precipitation. Trends suggest a strengthening winter pressure dipole, associated with increased AR frequency and higher temperatures in northern AP. These findings highlight the importance of winter observations in exploring AR impacts, bridging knowledge gaps about winter AR behaviors.

The Effect of Convective/Advective Precipitation Partitions on the Precipitation Isotopes in the Monsoon Regions of China: A Case Study of Changsha

Abstract Convective/advective precipitation partitions refer to the divisions of precipitation that are either convective or advective in nature, relative to the total precipitation amount. These distinct partitions can have a significant influence on the stable isotope composition of precipitation. This study analyzed and compared the effect of precipitation partitions on δ18O in precipitation (δ18Op) by using daily precipitation stable isotope data from Changsha station and monthly precipitation stable isotope data from the Global Network of Isotopes in Precipitation (GNIP), under different time scales, time intervals (i.e., annual, warm season, and cold season), and precipitation intensities. The results showed that the correlation between the convective precipitation fraction (CPF) and total precipitation amount was influenced by the intensity of convection in different time intervals. On both the daily and monthly scales, the CPF decreased as the precipitation amount increased in the warm season, while it increased with increasing precipitation amount in the cold season. Regardless of the season, daily δ18Op at Changsha station consistently increased with an increase in daily CPF. On a daily scale, the effect of convective activity on δ18Op was stronger than that of the “precipitation amount effect” in the cold season, as compared to the situation in the warm season. As a result, the regression line slope between δ18Op and CPF increased with increasing precipitation intensity in the warm season, meaning that as the CPF increased, the δ18Op increased at a faster rate under higher precipitation intensity. Similarly, the slope increased with increasing precipitation intensity in the cold season. This suggests that precipitation intensity and convection intensity can affect the relationship between δ18Op and CPF. Our findings shed light on how different precipitation partitions affect stable isotope composition of precipitation, thus enhancing our understanding of the variability of precipitation stable isotopes in the monsoon regions of China. Significance Statement This study aims to better elucidate the influence of different precipitation partitions on precipitation stable isotopes. In the eastern monsoon region of China, stable isotopes in precipitation showed a robust positive relationship with convective precipitation faction. On a daily scale, the convective activity enhanced the influences of the “precipitation amount effect” on precipitation stable isotopes in the warm season and reduced such influences in the cold season. These results improve our understanding of stable isotopic variability of precipitation in the eastern monsoon region, China.

Role of stable isotopes in revealing moisture sources and rainfall variability in India

Precipitation is a crucial component of the water cycle and is essential for the livelihood of people and ecosystems; therefore, understanding precipitation parameters is vital. Stable isotopes in precipitation can provide important information on precipitation sources, atmospheric circulation patterns, and hydrological processes. In this study, stable isotopes in precipitation for four cities in India were analyzed, namely New Delhi, Hyderabad, Shillong, and Calicut, using data from the International Atomic Energy Agency (IAEA) Global Network for Isotopes in Precipitation (GNIP). The GNIP data were supplemented with in-situ measurements. Results showed the correlations between climate-related factors such as surface air temperature and precipitation levels with the stable isotope composition of precipitation. The relationships critically explore interannual variations in the isotope data over the last three decades. The Local Meteoric Water Line (LMWL) for New Delhi and Hyderabad had intercepts less than 10‰, implying a higher evaporation effect over precipitation, consistent with arid and semi-arid regions with increased altitude. The weighted average value of d-excess for southern and Himalayan points were 10.7 and 12.7, respectively, and the average value of δ¹⁸O were − 3.65 and − 5.84, and δ²H were − 16.8 and − 35.8. The d-excess value was significantly lower in the Northern part (New Delhi), with an average weighted value of 6.6. The key values include the isotopic composition of rainfall in different regions of India, the LMWL for different stations, the d-excess value, and the consistency of meteoric water lines with regional and global values. The results of this study provide valuable information on the variability of stable isotopes in precipitation in India. The study's outcomes can be compared with the isotopic composition of surface water and groundwater. This discovery offers more understanding of the isotopic differences that occur on a smaller scale during organized convection and the factors that affect them. As a result, it enhances our ability to decipher the paleoclimate data in arid, semi-arid, and subtropical monsoon regions.

The relations between summer droughts/floods and oxygen isotope composition of precipitation in the Dongting Lake basin

AbstractThe stable oxygen isotope composition of precipitation (δ18Op) in southern China is considered as a valuable proxy of climatic conditions. However, their interpretations have been controversial. In this study, based on the observed and simulated data (isoGSM2) on oxygen isotope composition of precipitation, the linkage between summer precipitation (P) and δ18Op in the Dongting Lake basin and their possible influencing factors were investigated. The results indicate that the interannual variation of summer δ18Op is consistent with that of annual δ18Op. They both show a significantly negative correlation with the summer P, suggesting that the stable isotope composition in precipitation may be considered as a proxy of summer precipitation in the Dongting Lake basin. Statistically, the amount effect and circulation effect are significant in the isotope composition of precipitation in the basin. Based on either the observed data in Changsha or the simulated data for the basin, the local amount effect appears more important than large‐scale circulation for δ18Op during extreme summers. These results can potentially improve the reconstruction of paleoclimate in the East Asian monsoon region. Further study is needed to determine the contribution of local and large‐scale factors to the oxygen isotope composition of precipitation and to quantify the integral rainout along the moisture transporting paths.

Comparisons of Precipitation Isotopic Effects on Daily, Monthly and Annual Time Scales—A Case Study in the Subtropical Monsoon Region of Eastern China

The study on precipitation isotope variation can potentially improve the understanding of weather processes, regional water cycle and paleoclimate reconstruction in the subtropical monsoon region. Based on the measured stable isotope composition in precipitation (δ18Op) and daily precipitation from January 2010 to December 2021 in Changsha of the subtropical monsoon region of eastern China, the δ18Op variations, amount effect and local meteoric water line (LMWL) were analyzed and compared on daily, monthly and annual time scales, as well as under different precipitation intensities. The results showed that, on the daily time scale, δ18Op was significantly and negatively correlated with precipitation in the study area. Influenced by subcloud evaporation, small precipitation events (≤5 mm/d) could change the rainout level of precipitation isotopes. There were significant differences in the slope and intercept of the LMWL on different time scales, in different seasons and under different precipitation intensities. On the daily and monthly time scales, the slope and intercept of the LMWL in the cold half of the year were significantly smaller and larger than those in the warm half of the year, respectively, and the slope and intercept of the LMWL increased significantly with precipitation intensity, and then remained largely stable. On the annual time scale, the slope and intercept of the LMWL in the cold half of the year were smaller than those in the warm half of the year. The possible reasons for the differences in the LMWL on different time scales are the combined effects of seasonal differences in precipitation intensity and water vapor sources.

Predicting spatial distribution of stable isotopes in precipitation by classical geostatistical- and machine learning methods

Stable isotopes of precipitation are important natural tracers in hydrology, ecology, and forensics. The spatially explicit predictions of oxygen and hydrogen isotopes in precipitation are obtained through different interpolation techniques. In the present study we aim to examine the performance of various interpolation techniques when predicting the spatial distribution of precipitation stable isotopes. The efficiency of combined geostatistical tools (i.e. regression kriging; RK) and various machine learning methods (including regression enhanced random forest methods: MRRF, RERF) are compared in interpolating the spatial variability of precipitation stable oxygen isotope values from two different sampling networks in Europe. To assess the performance of the models, mean squared error (MSE), nonparametric Kling Gupta efficiency (KGE), absolute differences and relative mean absolute error metrics were employed. It was found that the combination of the different regression techniques with Random Forest can produce estimations with comparable accuracy in terms of descending order of overall average MSE, MRRF: 2.61, RK: 2.77, RERF: 2.99, RF: 3.08. The best performing combined random forest model variant (MRRF) outperformed regression kriging in terms of a hybrid error metric (KGE) by 7.5%. Sequential random rarefying the station networks showed that machine-learning methods are more capable of maintaining high prediction accuracy even with fewer input data. This can be a great advantage when a suitable method is needed to predict the stable isotope composition of precipitation for large spatial domains where the spatial density of data stations shows large differences.

Mixed Signals From the Stable Isotope Composition of Precipitation and Plant Waxes in the Northern Tropical Andes

AbstractWe evaluate the efficacy of the stable isotope composition of precipitation and plant waxes as proxies for paleoaltimetry and paleohydrology in the northern tropical Andes. We report monthly hydrogen (δ2Hp) and oxygen (δ18Op) isotope values of precipitation for an annual cycle, and hydrogen isotope values of plant waxes (δ2Hwax) obtained from modern soils along the eastern and western flanks of the Eastern Cordillera of Colombia. δ2Hp, δ18Op, as well as the unweighted mean δ2Hwax values of n‐C29, n‐C31, and n‐C33 n‐alkanes in the eastern flank show a dependence on elevation (R2 = 0.90, 0.82, and 0.65, respectively). In stark contrast, the stable isotope compositions of neither precipitation nor plant waxes from the western flank correlate with elevation (R2 < 0.23), on top of a negligible (p‐value >0.05) correlation between δ2Hwax and δ2Hp. In general, δ2Hwax values along the eastern flank of the Eastern Cordillera seem to follow the trend of a simple Rayleigh distillation process that is consistent with studies elsewhere on the eastern side of the Andes in South America. Neither δ2Hp nor δ18Op, and therefore δ2Hwax, offer reliable estimates of past elevations in the western flank, due perhaps to water vapor source mixing, evaporation overprint, contrasting plant communities, and/or differences in evapotranspiration. Thus, δ2Hwax is only reliable for paleohydrology and paleoaltimetry reconstructions on the eastern flank of the Andes, whereas interpretations based on δ2Hp and/or δ18Op west of the highest point of the Eastern Cordillera need to consider mixing of moisture sources in addition to precipitation amount.

Determining key upstream convection and rainout zones affecting δ18O in water vapor and precipitation based on 10-year continuous observations in the East Asian Monsoon region

An increasing number of studies have recognized the crucial impact of upstream convective activities and rainout processes on the stable isotopic composition of precipitation (δ18Op) and water vapor (δ18Ov) at mid and low latitudes. However, it is difficult to precisely identify the upstream convection and rainout zones using the common method based on the spatial distribution of time-lagged temporal correlation. Using a 10-year continuous high-resolution δ18Ov and δ18Op dataset at Nanjing (southeast China), the longest record of this kind, we develop an improved time-lagged correlation method for Key Upstream Convection Zones Identification (KUCZI). Utilizing this method, we find that summer δ18Ov and δ18Op at Nanjing are primarily controlled by the convective activities and rainout processes along the moisture transport pathway from the Maritime Continent (MC), via the Indo-China Peninsula and South China Sea (ICP_SCS), to Southeast China (SEC), particularly over SEC. Contrary to the conclusion of many existing studies, there is a possibility that the Indian Ocean is not a major convection zone affecting δ18Op and δ18Ov at Nanjing. Our results indicate that we may need to reconsider the role of the Indian Ocean on the paleoclimate interpretation of stalagmite δ18O records in the East Asian Monsoon (EAM) region.

Delineation of water flow paths in a tropical Andean headwater catchment with deep soils and permeable bedrock

AbstractTraditional hydrometric data combined with environmental tracers such as water stable isotopes contributes to improve the understanding of catchment hydrology. Nevertheless, the application of isotopic tracers in headwater catchments of the tropical Andes with deep soils and permeable parent material influenced by recent volcanism remains limited. In this study, the stable isotopic composition of precipitation, soil water, wetlands, and streamflow was studied to provide insights into the hydrology of a small tropical Andean catchment with deep and permeable volcanic soils, ash layers, and fractured bedrock resulting from Holocene volcanic activity. Although local precipitation forms under isotopic equilibrium conditions, the stable isotopic composition of precipitation is influenced by atmospheric moisture recycling processes. The spatial and temporal variability of isotopic signals and the analysis of inverse transit time proxies (ITTPs) of surface (streamflow) and subsurface (soil and wetlands) waters indicate that vertical flow paths through the deep volcanic ash soils are dominant across the catchment. The strongly damped isotopic composition of these waters points to high soil and wetlands water storage, increasing the transit time or age of stream water in the hydrological system. These findings indicate that water mobilizing through subsurface flow paths–that is, volcanic soils, ash layers, and cracks in the fractured bedrock resulting from Holocene volcanism–is the main contributor to streamflow generation. Comparison with previously published work from Andean catchments and other volcanic areas shows the diversity of hydrological conditions that can be found as a result of pedological and lithological differences shaped by volcanic activity. Therefore, site‐specific strategies may be needed to improve water resources management.

Stable isotope composition in precipitation and groundwater of Shwan Sub-Basin, Kirkuk governorate, northeast of Iraq

Abstract Stable isotope composition of δ2H and δ18O was investigated in the water resources of the Shwan sub-Basin northeast of Iraq. The study objects conceived the possible factors that affect the stable isotopes’ composition in precipitation additionally to achieve information concerning recharge processes and estimate the groundwater recharge sources. In this study, four precipitation samples were collected at the study area for the 2020–2021 hydrological year. Thirty-two groundwater samples and one surface water sample from Lesser Zab River (LZR) were collected during the same period for two sampling seasons. The results of observed meteorological data show a very small amount of precipitation for the sampling year. This year is considered a dry hydrological year with total annual precipitation of 100.62 mm compared with the previous 40 hydrological years with total annual precipitation of 325.43 mm. The isotopic composition in precipitation was highly varied as it primarily depends on environmental conditions. The depleted values are recognized with the increasing precipitation amount, whereas the enriched values were the most affected by evaporation. Back trajectory analysis revealed that stable isotopes in precipitation are primarily influenced by air masses and moisture sources. The sources of the trajectory that came from the Mediterranean Sea, Arabian Gulf and the Red Sea would lead to variations in the values of precipitation stable isotope. Stable isotope values in groundwater showed that the samples for both periods are located between the East Mediterranean water line (EMWL) and global meteoric water line (GMWL) close to the local meteoric water line (LMWL) indicating that the groundwater recharge is mainly through precipitation. Groundwater recharges by an indirect recharge mechanism from the LZR, based on stable isotope similarity between depleted stable isotopes in groundwater and river water. The estimated groundwater recharge based on weighted oxygen isotopes is about 9.2% of annual rainfall infers that the recharge during the sampling year was very low. The low recharge value experiences dry weather conditions from low precipitation amounts besides increasing evaporation during the current study.

Hydrometeorological factors controlling the stable isotopic composition of precipitation in the highlands of south Ecuador

Abstract Knowledge about precipitation generation remains limited in the tropical Andes due to the lack of water stable isotope (WSI) data. Therefore, we investigated the key factors controlling the isotopic composition of precipitation in the Páramo highlands of southern Ecuador using event-based (high frequency) WSI data collected between November 2017 and October 2018. Our results show that air masses reach the study site preferentially from the eastern flank of the Andes through the Amazon basin (73.2%), the Orinoco plains (11.2%), and the Mato Grosso Massif (2.7%), whereas only a small proportion stems from the Pacific Ocean (12.9%). A combination of local and regional factors influences the δ18O isotopic composition of precipitation. Regional atmospheric features (Atlantic moisture, evapotranspiration over the Amazon Forest, continental rain-out, and altitudinal lapse rates) are what largely control the meteoric δ18O composition. Local precipitation, temperature, and the fraction of precipitation corresponding to moderate to heavy rainfalls are also key features influencing isotopic ratios, highlighting the importance of localized convective precipitation at the study site. Contrary to δ18O, d-excess values showed little temporal variation and could not be statistically linked to regional or local hydrometeorological features. The latter reveals that large amounts of recycled moisture from the Amazon basin contributes to local precipitation regardless of season and predominant trajectories from the east. Our findings will help to improve the isotope-based climatic models and enhance paleoclimate reconstructions in the southern Ecuador highlands.

Temporal variations and evaporation control effect of the stable isotope composition of precipitation in the subtropical monsoon climate region, Southwest China

Stable hydrogen and oxygen isotopes in precipitation, which are sensitive to climate change and the water cycle, are widely used as natural tracers to investigate hydrological processes and paleoclimate reconstruction. High resolution event-based precipitation sampling was conducted in Guilin, a subtropical monsoon climate region in Southwest China, from 2010 to 2016 to identify the temporal variability of stable isotopes of precipitation and its dominant meteorological factors. The results showed that the stable isotopes of precipitation exhibited significant seasonal variability, with lower values in the wet season and higher values in the dry season, reflecting the changes in the character of precipitation in response to seasonal variability of the moisture sources and atmospheric dynamics. δ18O values were significantly negatively correlated with temperature and potential evapotranspiration (ET0) on the daily and monthly timescales; however, there was no significant correlation between monthly δ18O and the amount of rainfall. Multivariate regression analysis showed that when temperature, precipitation amount and ET0 were considered, 39% of the variability of the monthly oxygen isotope composition of precipitation was explained. Monthly mean temperature had a higher covariation with precipitation δ18O, indicating that the variations of precipitation isotope were more dependent on temperature, but not substantially dependent on the amount effect of rainfall. Howerver, the main influence of meteorological factors on the isotopic variations of precipitation varied seasonally. Temperature strongly influenced the isotopic composition of precipitation in summer, while the amount effect was prominent due to the enhanced convective activity characterized by more depleted δ18O values. Although evapotranspiration is a minor contributor of precipitation isotope variability, the impact of evapotranspiration on the enrichment of precipitation isotopes in autumn is a substantial control and should be considered, which makes a major contribution to compensate for the depletion in heavy isotopes of rainfall. The non-linear relationship between d-excess and ET0 was attributed to the variable contribution of large-scale convective moisture and local evaporation, as well as water vapor transfer processes. The response of precipitation stable isotopes to convective events was characterized by distinct stage patterns, associated with the water vapor sources and physical processes during rainout. The significant depletion of precipitation δ18O values during convective events was not due to the ‘amount effect’, but due to large-scale convection activity. These findings provide further insight into intraevent-scale isotopic variations associated with organized convection and their key influencing factors, which ultimately improves our interpretation of the paleoclimate records in subtropical monsoon regions.

Influence of below-cloud secondary evaporation on stable isotope composition in precipitation and its relationship with meteorological factors in Shiyang River Basin, Northwest China

In arid regions, the process of precipitation from the cloud base to the ground is often affected by the below-cloud secondary evaporation, which changes the ratios of hydrogen and oxygen stable isotopes in falling raindrops. Based on stable isotopes of 670 precipitation samples and meteorological records at 11 stations in the Shiyang River Basin, and combined with the improved Stewart model, this study discussed the impact of different meteorological factors on below-cloud secondary evaporation. The result showed that the Local Meteoric Water Line (LMWL) of the Shiyang River Basin is δD = 7.70δ18O + 8.90 (r2 = 0.98, p < 0.01, n = 670). The secondary evaporation is stronger in spring and summer and weaker in autumn and winter, and it is stronger in the mid-downstream than that in the upstream. When temperature ranges from 10 °C to 20 °C, precipitation is less than 10 mm, and relative humidity is lower than 30%, the phenomenon of isotope dynamic fractionation caused by the secondary evaporation is more obvious. As temperature increases, relative humidity, precipitation amount, or raindrop diameter decreases, the linear relationship between the evaporation remaining ratios of raindrops (f) and the difference of deuterium excess from cloud base to ground (Δd) weakens gradually. Δd has a stronger response to the changes in relative humidity than that in temperature and precipitation intensity, and the impact of raindrop diameters more than 1.2 mm on Δd is very limited.

Spatiotemporal dynamics of water sources in a mountain river basin inferred through δ2H and δ18O of water.

In mountainous river basins of the Pacific Northwest, climate models predict that winter warming will result in increased precipitation falling as rain and decreased snowpack. A detailed understanding of the spatial and temporal dynamics of water sources across river networks will help illuminate climate change impacts on river flow regimes. Because the stable isotopic composition of precipitation varies geographically, variation in surface water isotope ratios indicates the volume-weighted integration of upstream source water. We measured the stable isotope ratios of surface water samples collected in the Snoqualmie River basin in western Washington over June and September 2017 and the 2018 water year. We used ordinary least squares regression and geostatistical Spatial Stream Network models to relate surface water isotope ratios to mean watershed elevation (MWE) across seasons. Geologic and discharge data was integrated with water isotopes to create a conceptual model of streamflow generation for the Snoqualmie River. We found that surface water stable isotope ratios were lowest in the spring and highest in the dry, Mediterranean summer, but related strongly to MWE throughout the year. Low isotope ratios in spring reflect the input of snowmelt into high elevation tributaries. High summer isotope ratios suggest that groundwater is sourced from low elevation areas and recharged by winter precipitation. Overall, our results suggest that baseflow in the Snoqualmie River may be resilient to predicted warming and subsequent changes to snowpack in the Pacific Northwest.

Implications for groundwater recharge from stable isotopic composition of precipitation in Hawai'i during the 2017–2018 La Niña

AbstractThe project captured a subset of the hydrological cycle for the tropical island of O'ahu, linking precipitation to groundwater recharge and aquifer storage. We determined seasonal storm events contributed more to aquifer recharge than year‐round baseline orographic trade wind rainfall. Hydrogen and oxygen isotope values from an island‐wide rain collector network with 20 locations deployed for 16 months and sampled at 3‐month intervals were used to create the first local meteoric water line for O'ahu. Isotopic measurements were influenced by the amount effect, seasonality, storm type, and La Niña, though little elevation control was noted. Certain groundwater compositions from legacy data showed a strong similarity with collected precipitation from our stations. The majority of these significant relationships were between wet season precipitation and groundwater. A high number of moderate and heavy rainfall days during the dry season, large percentage of event‐based rainfall, and wind directions outside of the typical NE trade wind direction were characteristics of the 2017–2018 wet season. This indicates that the majority of wet season precipitation is from event‐based storms rather than typical trade wind weather. The deuterium‐excess values provided the strongest evidence of a relationship between groundwater and different precipitation sources, indicating that this may be a useful metric for determining the extent of recharge from different rain events and systems.

Stable isotopic composition of precipitation in a tropical rainforest region of the Niger Delta, Nigeria

ABSTRACT The isotopic compositions of oxygen (δ 18O) and hydrogen (δ 2H) of precipitation were determined from three different locations in the western Niger Delta (Warri, Ughelli and Abraka) between 2014 and 2015. 18O and 2H in wet season precipitation were more depleted compared to the dry season. Similarly, d-excess computed for wet season precipitation is lower than that for the dry season. The δ 18O and δ 2H variations in precipitation suggest the effect of the convective system and north-easterly and south-westerly trade winds. The decrease in δ 18O and δ 2H was also observed in precipitation data of a continuous rain event of two successive days. The local meteoric water lines estimated for Warri, Ughelli and Abraka were δ 2H = 8.8 δ 18O + 9.1 ‰ (R 2 = 0.93), δ 2H = 6.9 δ 18O + 10.7 ‰ (R 2 = 0.98) and δ 2H = 7.9 δ 18O + 11.3 ‰ (R 2 = 0.87), respectively. The Niger Delta regional meteoric water line of δ 2H = 7.7 δ 18O + 10.2 ‰ (R 2 = 0.91) was derived from the monthly average from the three locations. The provided local meteoric water line for the Niger Delta from unweighted stable isotopic data represents a baseline for regional water resources studies.

Identifying hydrological conditions of the Pihe River catchment in the Chengdu Plain based on spatio-temporal distribution of 2H and 18O

Stable isotope composition in precipitation, river water, and groundwater was used to investigate the hydrological conditions in the Pihe River catchment of Southwest China. Significant seasonal isotopic variations exist in precipitation and river water, and there is 1-month delay in the river water response to precipitation. The river water is gradually enriched under the influence of evaporation. However, due to the influence of recharge from external water source, the evaporation ratios do not strictly increase with the flow distance. The seasonal isotopic values in groundwater fluctuate the least. River water supplies groundwater more than precipitation in dry season.