Dynamic N transport and N2O emission during rainfall events in the coastal river

The coastal zone exhibited a high population density with highly impacted by anthropogenic activities, such as river impoundment to prevent saline intrusion, which resulted in weak hydrological conditions. Rainfall events can result in dramatic changes in hydrological and nutrient transportation conditions, especially in rivers with weak hydrological conditions. However, how the nitrogen transport and N2O emissions or biogeochemistry responds to the different types of rainfall events in the weak hydrodynamics rivers is poorly understood. In this study, the hydrological, nitrogenous characteristic, as well as N2O dynamics, were studied by high-frequency water sampling during two distinct rainfall events, high-intensity with short duration (E1) and low-intensity with long duration (E2). The results displayed that the hydrologic condition in E1 with a wider range of d-excess values (from -9.50 to 32.1 ‰), were more dynamic than those observed in E2. The N2O concentrations (0.01-3.33 μmol/L) were higher during E1 compared to E2 (0.03-1.11 μmol/L), which indicated that high-intensity rainfall has a greater potential for N2O emission. On the contrary, the concentrations of nitrogen (e.g., TN and NO3--N) were lower during E1 compared to E2. Additionally, hysteresis was observed in both water and nitrogen components, resulting in a prolonged recovery time for pre-rainfall levels during the long-duration event. Moreover, the results showed that the higher average N2O flux (78.3 μmol/m2/h) in the rainfall event period was much larger than that in the non-rainfall period (1.63 μmol/m2/h). The frequency dam regulation resulted in the water level fluctuation, which could enhance wet-dry alternation and simulated N2O emissions. This study highlighted the characteristic of N dynamic and hydrological responses to diverse rainfall events occurrences in the coastal river. Rainfall could increase the N2O emission, especially during high-intensity rainfall events, which cannot be ignored in the context of annual N2O release.

Stable isotope compositions and vapor sources of precipitation in the Yellow River Delta, China

To uncover the vapor source, formation mechanism, and the influence of meteorological factors on precipitation in the saline land of the Yellow River Delta, I employed stable isotopes of precipitation, especially for δ17O and 17O-excess, along with the Hybrid Single Particle Lagrangian Integrated Trajectory Model (HYSPLIT), to analyze the isotopic variation characteristics of precipitation and water vapor sources at different temporal scales and precipitation intensities [(<5, 5-10, 10-25, 25-50, >50 mm·d-1)] from May to October in Dongying, located in the Yellow River Delta. There were wide ranges of isotopes in the daily precipitation data between May and October, with smaller ranges and enriched average values during the dry season. The slope between δ'18O and δ'17O was the minimum of 0.5211 when precipitation intensity was below 5 mm·d-1, indicating the potential influence of evaporation from the moisture source site on precipitation. The maximum value was 0.5268 when precipitation intensity was between 10 mm·d-1 and 25 mm·d-1. For precipitation intensities below 50 mm·d-1 with four different intensities, δ2H, δ18O and δ17O decreased with the increase of precipitation. During the dry season, 17O-excess exhibited a positive relationship with temperature, suggesting the influence of continental circulating water vapor on precipitation. Conversely, in the wet season, 17O-excess displayed a negative relationship with relative humidity (RH), indicating less influence of evaporation. Analysis of air mass back trajectories using the HYSPLIT model indicated that precipitation during the dry season was primarily influenced by the continental monsoon, while precipitation during the wet season was affected by both oceanic and continental monsoons. In conclusion, precipitation in the Yellow River Delta is influenced by the evaporation of various water vapor sources, local meteorological factors, and atmospheric water vapor sources, resulting in different isotopic signatures across different scales. The fin-dings would provide a scientific basis for the allocation of scarce water resources in the Yellow River Delta.

Impact of Indian summer monsoon in westerly dominated water resources of western Himalayas

We used stable water isotopes of oxygen and hydrogen to identify and estimate the seasonal contribution of precipitation to the regional hydrology of Sindh and Rambiara catchments of western Himalayas. The different source waters exhibit significant spatio-temporal variations that correspond to the change in seasonal meteorology, precipitation form and moisture sources. The two-component hydrograph separation based on d-excess suggests that the western disturbances (WD) contribute dominantly (76 ± 4 %) to the regional hydrology, compared to Indian summer monsoon (ISM) rainfall (24 ± 4 %). A comparison of d-excess values of WD and ISM indicates the groundwater consists of 90 ± 3 % WD sources and 10 ± 2 % ISM sources, signifying distinct seasonal variations in groundwater recharge sources. The sine wave model results showed that the annual mean residence time (MRT) of groundwater for the Sindh catchment (5.8 ± 0.6 months) is greater than the Rambiara groundwater (3.6 ± 0.5 months). The lower isotope values observed in the river water than in the precipitation suggest its origin from the snowmelt. This study provides valuable insights into the hydrological processes operating in the high altitude Himalayan catchments to facilitate the improved understanding of runoff generation mechanisms and water resource management in future climate change scenarios.

[Stable Isotopes of Precipitation in the Eastern Tarim River Basin and Water Vapor Sources]

The in-situ monitoring of stable hydrogen and oxygen isotopes (δ¹⁸O and δ²H) in precipitation is helpful to understand the processes of water phase change, mixing, and transportation. Due to the arid climate in the eastern Tarim River basin, water resources are the key factor affecting the regional sustainable development. However, the understanding of stable hydrogen and oxygen isotopes in precipitation is still limited for this region. Based on the isotope data of 103 precipitation samples collected at four stations in the eastern Tarim River basin from June 2019 to September 2020, the spatial and temporal characteristics of stable hydrogen and oxygen isotopes in precipitation were analyzed, and the connections between stable isotopes and moisture sources were discussed. The findings provide a reference for the application of environmental isotope tracers in arid areas. The results show that, ① the four sampling stations generally presented an increasing trend in precipitation stable isotopic values from north to south, and the isotopic values were higher in summer and lower in winter. The d-excess value in the study region was smaller than the national average, and the slope of the local meteoric water line exhibited a significant arid characteristic. ② There was a positive correlation between precipitation stable isotopes and air temperature in the study region, whereas a weak negative relationship was seen between d-excess and air temperature. The negative correlation occurred between δ¹⁸O and relative humidity, and the d-excess value exhibited a positive correlation with relative humidity. ③ The backward trajectory showed that the eastern Tarim River basin is mainly controlled by the westerlies paths. The proportion of air mass that comes from the middle and short distance is relatively large. The concentration weighted trajectory method showed that the farther from the precipitation sampling site, the more stable the d-excess value was. The closer to the sampling site, the more frequently the concentration changed, indicating that the d-excess value was greatly affected by the local water vapor recycling.

[Analysis of Hydrogen and Oxygen Stable Isotope Characteristics and Vapor Sources of Precipitation in the Guanzhong Plain]

To improve the understanding of hydrogen and oxygen stable isotope characteristics and vapor sources in the Guanzhong Plain, we collected 98 precipitation samples and corresponding meteorological data between 2015 and 2018 in Yangling, Shanxi Province, which is located in the central area of the Guanzhong Plain. The composition characteristics of the local hydrogen and oxygen stable isotopes of precipitation (δ²H, δ¹⁸O, and δ¹⁷O) and their environmental controls were analyzed, and the local meteoric water line (LMWL) and the meteoric water line of the triple oxygen isotopes were established. Three indicators (δ¹⁸O, d-excess, and ¹⁷O-excess) were used to explore the possible vapor sources of local precipitation and to quantify the contributions of ocean-source and inland-source water vapor to the precipitation. The results showed that there were obvious seasonal changes in the hydrogen and oxygen stable isotopes of precipitation in the Yangling area:water isotopes were depleted in the wet season (May to October) and enriched in the dry season (November to April of the next year). Both the slope (7.7) and intercept (9.1) of the LMWL were lower than those of the global meteoric water line (GMWL), indicating that the annual precipitation in the research area experienced variable degrees of secondary evaporation under cloud cover. The slope of the meteoric water line of the triple oxygen isotopes is 0.528, which is between that of seawater equilibrium fractionation (0.529) and water vapor diffusion into dry air (0.518), consistent with the fact that the Guanzhong area is located on the migration path of marine air mass to inland arid regions. Comprehensive analysis of δ¹⁸O, d-excess, and ¹⁷O-excess confirmed that the precipitation in the study area is jointly contributed to by the warm and humid air mass from the southeast monsoon and the dry and cold air mass from the westerly wind. Of these, approximately 55%-79% of the precipitation water vapor comes from the ocean, mainly in June to August, and about 21%-45% of the water vapor comes from inland and local evaporation, mainly from October to April. The water vapor sources of precipitation in May and September are complex and may intermittently originate from ocean and inland water vapor.

δ18O and δ2H characteristics of moisture sources and their role in surface water recharge in the north-east of Iran

The north-east of Iran is a semi-arid region and faces a water shortage crisis. Therefore, monitoring water resources using accurate methods such as stable isotopes technique is vitally important. In this study, precipitation events were sampled in 10 stations in the Mashhad basin and the Bojnourd region in 2008, 2009, 2011, and 2015, additional surface and groundwater. These samples were analysed at the Ottawa University for both oxygen and hydrogen isotopes. In addition, the moisture sources were determined using the backward trajectories of the HYSPLIT model. The backward trajectories showed that both high- and low-latitude water bodies provide moisture for the north-east of Iran. However, the role of high-latitude water bodies including the Caspian, the Black, and the Mediterranean Seas is stronger. On the other hand, the stable isotopes showed large variations and the developed meteoric water lines deviated in both slope and intercept from the global meteoric water line. This showed that the precipitation events of the north-east of Iran were provided by various air masses and moisture sources. Finally, plotting the isotope values of the surface water resources on high- and low-latitude meteoric water lines demonstrated that these water resources were dominantly recharged by precipitation events originating from high-latitude water bodies.

Spatial distribution and controlling factors of surface water stable isotope values (δ18O and δ2H) across Kazakhstan, Central Asia

Climate change is expected to alter hydrological and biogeochemical processes in Central Asia (CA), and surface water stable isotope values (δ¹⁸O and δ²H) can be used to examine these changes. Spatially extensive stable isotope data, however, are sparse, which constrains the understanding of hydrological processes in transboundary rivers across Kazakhstan. Therefore, we conducted a survey of surface water stable isotopes across the region. River and lake water isotope values exhibit spatial variability that was closely associated with isotope values of precipitation, physiographic factors, landscape characteristics, and local hydrological processes, e.g., evaporation and mixing of waters from different sources. River water was characterized by lower δ¹⁸O and δ²H values and higher d-excess relative to lake water, suggesting evaporative enrichment of lake water. Analysis of δ¹⁸O versus δ²H for rivers and lakes yielded distinct regressions, (river [RWL], δ²H = 6.08δ¹⁸O - 16.7, r² = 0.837, p < 0.001) and (lake [LWL], δ²H = 6.23δ¹⁸O - 22.1, r² = 0.924, p < 0.001). The slope and intercept of the RWL and LWL were slightly lower than the local meteoric water line [LMWL] (δ²H = 6.96 δ¹⁸O - 1.0, r² = 0.942, p < 0.001). River water δ¹⁸O showed a significant negative correlation with elevation and longitude, but not with latitude. The spatial distributions of δ¹⁸O and d-excess values showed a remarkable gradient from west to east across Kazakhstan that was associated with moisture moving from the Mediterranean region to Kazakhstan. We also found generally higher δ¹⁸O values and lower d-excess values in low-elevation areas because of high evaporation rate in Kazakhstan. These baseline data will be useful for documenting the effects of climate change on the hydrological cycle in Central Asia.

[Analysis of Stable Hydrogen and Oxygen Isotope Characteristics and Vapor Sources of Event-based Precipitation in Chengdu]

Based on analysis of hydrogen and oxygen isotopes in 113 rainfall samples collected from September 2016 to October 2017 in Chengdu, which is a typical representative of humid areas affected by multiple moisture sources, the compositional characteristics of hydrogen and oxygen isotopes (²H, ¹⁸O, and ¹⁷O) and the water vapor sources of precipitation were analyzed. It was found that δD, δ¹⁸O, δ¹⁷O, d-excess, and ¹⁷O-excess in atmospheric event-based precipitation have significant seasonal variation. In the dry season they are high and in the wet season are low, reflecting the different moisture sources during two seasons (dry and wet). The slope and intercept of the Local Meteoric Water Line were small, indicating that the precipitation originated from sources with various stable isotope ratios and that raindrops were subject to secondary evaporation during their landing process. The Local Meteoric Water Line slope for the triple oxygen isotopes (δ'¹⁷O=0.5289δ'¹⁸O+0.0075) ranged between the slopes for seawater vapor and dry air, and the value of ¹⁷O-excess was far larger than that of seawater. This indicates that the Chengdu area lies in the path of marine air masses moving toward inland regions. The atmospheric precipitation mainly came from these marine air masses and the isotope had undergone serious enrichment in the process of reaching the area. The d values were close to the global average, and the extremely low value of d-excess in the dry season may be affected by artificial rainfall operations. In addition to the relative humidity of the water vapor source, ¹⁷O-excess is also affected by the upstream air mass convection; moreover, the ¹⁷O-excess of the precipitation was not affected by the meteorological factors over the whole study period, so the ¹⁷O-excess could be considered tracers of evaporative conditions at the vapor source in Chengdu. The precipitation ¹⁷O-excess in different seasons provides additional information to better understand the precipitation formation processes in Chengdu.

Pleistocene age paleo-groundwater inferred from water-stable isotope values in the central part of the Baltic Artesian Basin

A new data set of δ(2)H and δ(18)O in the groundwater from the central part of the Baltic Artesian Basin is presented. The hydrogeological section is subdivided into stagnation, slow exchange and active exchange zones. Na-Ca-Cl brine found at the deepest part - the stagnation zone - is characterized by δ(18)O values above -5 ‰ and δ(2)H values approaching -40 ‰ with respect to Vienna Standard Mean Ocean Water. The slow exchange zone where waters of mostly intermediate salinity reside is characterized by δ(18)O values around -11.7 ‰ and δ(2)H values around -85.3 ‰. Mean δ(18)O and δ(2)H values of the fresh groundwater in the active water exchange zone are -11.1 and -79.9 ‰, respectively. Characteristically, the groundwater in the active and slow exchange zone is isotopically more depleted compared with the precipitation values observed, and the depletion increases with depth down to the level where strongly enriched brines are encountered.

Studies on stable isotopic composition of daily rainfall from Kozhikode, Kerala, India

The stable isotopic compositions of all major daily rain fall samples (n = 113) collected from Kozhikode station in Kerala, India, for the year 2010 representing the pre-monsoon, southwest and northeast monsoon seasons are examined. The isotopic variations δ(18)O, δ(2)H and d-excess in daily rainfall ranged from δ(18)O: -4.4 to 2 ‰, δ(2)H: -25.3 to 13.8 ‰, and d-excess: -2.4 to 15.3 ‰; δ(18)O: -9.7 to -0.6 ‰, δ(2)H: -61.7 to 5.3 ‰, and d-excess 5.8 to 17.4 ‰; δ(18)O -11.3 to -1.4 ‰, δ(2)H: -75.3 to 0.9 ‰, and d-excess: 8.8 to 21.3 ‰ during the pre-, southwest and northeast monsoon periods, respectively. Thus, daily rainfall events during two monsoon periods had a distinct range of isotopic variations. The daily rain events within the two monsoon seasons also exhibited periodic variations. The isotopic composition of rain events during pre-monsoon and a few low-intensity events during the southwest monsoon period had imprints of secondary evaporation. This study analysing the stable isotopic characteristics of individual rain events in southern India, which is influenced by dual monsoon rainfall, will aid in a better understanding of its mechanism.

Interannual variation of water isotopologues at Vostok indicates a contribution from stratospheric water vapor

Combined measurements of water isotopologues of a snow pit at Vostok over the past 60 y reveal a unique signature that cannot be explained only by climatic features as usually done. Comparisons of the data using a general circulation model and a simpler isotopic distillation model reveal a stratospheric signature in the (17)O-excess record at Vostok. Our data and theoretical considerations indicate that mass-independent fractionation imprints the isotopic signature of stratospheric water vapor, which may allow for a distinction between stratospheric and tropospheric influences at remote East Antarctic sites.