Variations Of δ18O In Precipitation Research Articles

Isotopic evidence for the moisture origin and influencing factors at Urumqi Glacier No.1 in upstream Urumqi River Basin, eastern Tianshan Mountains

The stable isotope has been extensively applied as an effective tracer especially in precipitation. In glacierized area of arid northwest China, temperature is widely considered to be a major factor affecting isotopes in precipitation, while the influences of precipitation amount, relative humidity and other meteorological parameters are still not clear. Based on analyses on stable isotope values of water samples and NCEP/NCAR (National Centers of Environmental Prediction/National Center for Atmospheric Research, USA) re-analysis data, the moisture source and characteristics of isotopes in the precipitation, meltwater and river water isotopes at Urumqi Glacier No.1 of the upstream Urumqi River Basin, eastern Tianshan Mountains from spring to autumn during four years (from 2008 to 2011) was studied. Results indicated that meltwater are the main source of water for the upper Urumqi River. Seasonal variation of δ18O in precipitation demonstrated that δ18O was more enriched in summer and depleted in spring and autumn. Temperature was positively correlated with isotopes, while precipitation amount and relative humidity was negatively correlated with isotopes. The water vapor was affected by westerly air mass and regional water vapor cycle. Meanwhile, back trajectory clustering analyses showed that the moisture mainly from Europe and central Asia. The moisture was more likely to be locally sourced with the ratio was 46.8%~52.1%.

Characteristics of Hydrogen and Oxygen Isotopes in Different Water Bodies in Hilly and Gully Regions of the Loess Plateau

Studying the isotope characteristics and water body transformation relationships among different water bodies in the hilly and gully region of the Loess Plateau can provide a theoretical foundation for evaluating regional climate, ecology, and water resources. In this study, daily and monthly averaged δD and δ18O in precipitation, river water, and shallow groundwater were measured in 2017 in the Jiuyuangou watershed, which has a good ecological condition. The compositional relationship between the stable hydrogen and oxygen isotopes in difference water bodies was explored, the influence factors and spatio-temporal variation of δ18O in precipitation and river water were analyzed, and the stable isotope conversion ratios between different water bodies in the study area were calculated using the two-terminal mixed model. The main conclusions of the study are as follows:the d-excess of river water showed an increasing trend with elevation during the observation period; the δ18O of river water was enriched with increasing distance from the river source and decreased with increasing altitude; temperature, wind speed, and relative humidity had significant effects on the hydrogen and oxygen isotopes of precipitation; precipitation and shallow groundwater replenish the river during the non-flood period, the proportions of which were 46% and 54%, respectively; and during the flood season, the shallow groundwater is replenished by river water and precipitation, the proportions were 60% and 40%, respectively. These results indicate that there is a good conversion relationship between "precipitation-river-shallow groundwater" in the study area. The implementation of soil and water conservation measures has had some influence on the conversion of different water bodies in small watersheds. The results provide a basis for the establishment of water cycle models for hilly and gully regions of the Loess Plateau.

Variation of δ18O in precipitation and its response to upstream atmospheric convection and rainout: A case study of Changsha station, south-central China

The results of analyses of the stable isotopes of oxygen in precipitation (δ18Op) are presented for every rainfall event from January 2010 to December 2017 in Changsha, south-central China. Our aims were to elucidate the variations of δ18Op on different timescales and to identify the main meteorological drivers of variations in the oxygen isotopic composition of precipitation. Results showed that there were no statistically significant and consistent negative correlations between δ18Op and local precipitation amount on either daily or monthly timescale; however, changes in δ18Op in Changsha responded sensitively to the variation of precipitation in the key upstream area along air mass trajectories. Year-to-year, the strongest negative lagged correlations (r’) between δ18Op and the preceding average precipitation amount varied from −0.79 to −0.63 (all significant at the 0.001 level) in the warm half-year (from April to September) of 2010–2017. However, in the cold half-year (from October to the following March), corresponding r’ values varied from −0.79 to −0.38 that were all significant at the 0.001 level, except for the year 2012. These findings suggest that the amplitude of the isotopic signal was closely linked with the position and intensity of upstream rainout activity. This was supported by strong relationships between precipitation-weighted mean δ18O (δ18Ow) and average precipitation amount in the key upstream area at the monthly scale; correlation coefficients were − 0.76 and − 0.57 between 2010 and 2017 in the warm half-year and cold half-year, respectively (both significant at the 0.001 level). Results advance our understanding of the temporal variation of the stable oxygen isotopic composition of precipitation, and demonstrate that local isotopic proxy records may be influenced by upstream rainout processes.

Numerical experiments on the impacts of surface evaporation and fractionation factors on stable isotopes in precipitation

The isotope enabled atmospheric water balance model is applied to examine the spatial and temporal variations of δ18O in precipitation, amount effect and meteoric water lines (MWL) under four scenarios with different fractionation nature and surface evaporation inputs. The experiments are conducted under the same weather forcing in the framework of the water balance and stable water isotope balance. Globally, the spatial patterns of mean δ18O and global MWLs simulated by four simulation tests are in reasonably good agreement with the Global Network of Isotopes in Precipitation observations. The results indicate that the assumptions of equilibrium fractionation for simulating spatial distribution in mean annual δ18O and the global MWL, and kinetic fractionation in simulating δ18O seasonality are acceptable. In Changsha, four simulation tests all reproduce the observed seasonal variations of δ18O in precipitation. Compared with equilibrium fractionation, the depleted degree of stable isotopes in precipitation is enhanced under kinetic fractionation, in company with a decrease of isotopic seasonality and inter-event variability. The alteration of stable isotopes in precipitation caused by the seasonal variation of stable isotopes in vapour evaporated from the surface is opposite between cold and warm seasons. Four simulations all produce the amount effect commonly observed in monsoon areas. Under kinetic fractionation, the slope of simulated amount effect is closer to the observed one than other scenarios. The MWL for warm and humid climate in monsoon areas are well simulated too. The slopes and intercepts of the simulated MWLs decrease under kinetic fractionation.

Simulation of stable water isotopic composition in the atmosphere using an isotopic Atmospheric Water Balance Model

ABSTRACTIn this article, we present a simplified isotopic Atmospheric Water Balance Model (iAWBM), to simulate spatial distribution and temporal variations of δ18O in atmospheric vapour and in precipitation, and the correlations of δ18O in precipitation with precipitation amount and with temperature. This method overcomes deficiencies of the Global Network of Isotopes in Precipitation (GNIP) due to its limitation in space and time resolution and those of the isotope‐enabled General Circulation Models (iGCMs) due to its complexity. The simulated results reproduce well the latitude effect, continent effect and seasonality of δ18O in global precipitation. The simulation suggests that the precipitation amount effect occurs mainly in the low‐mid latitude oceans and monsoon areas, and the simulated temperature effect occurs mainly in the mid‐high latitude continents. However, the simulation results also indicate some temperature effect in some low‐latitude areas where the precipitation amount effect is strong. The modelled precipitation isotopic composition is compared to 3‐year observations in Changsha, China (2010–2012). The simulations reproduce the observed seasonal variations of δ18O in precipitation. The basic characteristics that stable isotopes in precipitation are depleted during the rainy season and enriched during the dry season, and the observed amount effect is well simulated under daily time scale. The simulated weighted average δ18O (−6.58‰) in precipitation during the prevailing winter monsoon season (October–March) is almost the same as the observed δ18O in the same period (−6.56‰), but the simulated δ18O (−9.58‰) during the prevailing summer monsoon season (April–September) is lower than the observed δ18O (−7.66‰). The simulation result also shows temporal variations of vapour isotopic composition in the atmosphere, and the proportional contributions from horizontal vapour exchange, precipitation and evaporation.

Climatic significance of δ18O records from precipitation on the western Tibetan Plateau

Analysis of daily precipitation samples for stable oxygen isotopes (δ18O) collected at the Shiquanhe and Gerze (Gaize, Gertse) stations in the Ngari (Ali) region on the western Tibetan Plateau indicates that air temperature affects the δ18O variations in precipitation at these stations. In summer, Shiquanhe and Gerze show strongly similar trends in precipitation δ18O, especially in simultaneous precipitation events. Moreover, both stations experienced low δ18O values in precipitation during the active monsoon period, resulting from the southwest monsoon (the summer phase of the Indian monsoon). However, during the break monsoon period (during the summer rainy season, when the monsoon circulation is disrupted), δ18O values in summer precipitation remain relatively high and local moisture recycling generally controls the moisture sources. Air temperature correlations with δ18O strengthen during the non-monsoon period (January–June, and October–December) due to continental air masses and the westerlies. In addition, evaporation also influences the δ18O variations in precipitation. The observed temporal and spatial variations of δ18O in precipitation on the western Tibetan Plateau and adjacent regions show that the late May and early June-the late August and early September time frame provides an important period for the transportation of moisture from various sources on the Tibetan Plateau, and that the region of the West Kunlun-Tanggula Ranges acts as a significant climatic divide on the Plateau, perhaps for all of western China.

Characterization of precipitation δ 18O variation in Nagqu, central Tibetan Plateau and its climatic controls

The stable isotopic composition of precipitation in different regions reflects climatic factors such as temperature, precipitation, moisture sources, and transport process. However, the isotopic variation in the region is usually much complicated due to the combined influences of these factors. A good understanding of climatic controls on the isotopic composition of precipitation can contribute to the study on isotopic tracer for climate and hydrology. To investigate the isotopic variation of precipitation and its climatic controls in the middle of the Tibetan Plateau, a monitoring station for stable isotope in precipitation has been established in Nagqu region, central Tibetan Plateau. We obtained 79 daily samples at Nagqu Meteorological Station in 2000. The observed δ18O in precipitation showed a distinctly seasonal pattern with higher values in spring and winter and lower values in summer, despite of individually low values in winter due to extremely low temperature. To further understand this pattern, we evaluated the influence of temperature, precipitation, moisture sources, and moisture transport process on precipitation δ18O. A multiple linear regression model represents quantitatively the dependence of precipitation δ18O on precipitation and temperature: δ18Oppt = −0.30P − 0.11T − 14.8 (R2 = 0.13, n = 79, P = 0.005), which indicates δ18O values in precipitation are more dependent on precipitation amount than on temperature. In contrast, when the temperature is low enough (<2°C), δ18O values in precipitation are mainly dependent on temperature: δ18Oppt = 0.53T − 10.2 (R2 = 0.44, n = 19, P = 0.002). The variation of δ18O in precipitation is also closely related to moisture origins and transport trajectories. A model is set up to trace the trajectories for air masses arriving in the observed region, and the results demonstrated that humid marine air masses from the Indian Ocean generally have significantly lower δ18O values than dry continental air masses from the north or local re-evaporation. During monsoon precipitation, the distance and depth of moisture transport as well as convective precipitation all lead to the large variability of δ18O in precipitation.

Influence of moisture transport on stable isotope in precipitation in Yarlungzangbo River basin

The relationship between the variation of δ 18O in precipitation in Yarlungzangbo River basin and the moisture flux was analyzed with NCEP/NCAR reanalysis grid data and δ 18O in precipitation at four stations (Lazi, Nugesha, Yangcun and Nuxia) of the region investigated. In terms of spatial variations, there is obviously a positive correlation between them for the entire basin. With the decrease in moisture flux from the downstream to the upstream area, δ 18O in precipitation became gradually decreased. However, in terms of temporal variations, higher δ 18O in precipitation during spring is linked to small moisture flux while low δ 18O in precipitation during summer is linked to large moisture flux. A model involving meteorological data from NCEP/NCAR was subsequently set up which successfully traced the moisture transport trajectories at Yangcun station. Based on the traced moisture transport trajectories and the δ 18O in precipitation at Yangcun station, the relationship between δ18O in precipitation in Yarlungzangbo River basin and the moisture transport history was discussed. We found that the humid marine air mass from the Indian Ocean in general has significantly lower δ 18O values than the continental air mass from the north or local re-evaporation. The fluctuation of δ 18O in precipitation during the monsoon season is very pronounced; the lower values are usually related to farther distance and multilayer moisture transport, as well as moisture crossing the Himalaya Mountains.

Water sources in semiarid northeast Asia as revealed by field observations and isotope transport model

Origin of water vapor, which falls as precipitation over arid/semiarid area in northeast Asia, is investigated by modeling and observational approaches. The regional climate model evaluates the spatiotemporal variations of precipitation and water vapor budget, which are used to drive the single‐layer isotope circulation model. Intraseasonal variations of δ18O in precipitation are well simulated during June‐July‐August of 2003. The δ18O in water vapor experiences rapid decreases according to the passage of synoptic‐scale disturbances. The rapid decreases of δ18O are attributed to two processes. (1) The δ18O in air mass locally decreases over the western mountains associated with the mountain precipitation; the light vapor is advected to eastern Mongolia by the prevailing westerly wind. (2) Convective systems pass by in the vicinity of the observation site, which persistently keep the lower δ18O in the air mass. Origin of the water vapor during June‐July‐August in 2003 is evaluated by the colored moisture analysis in which the tracers are assigned depending on the region where the water vapor finally evaporated. Seasonal mean result indicates that the regions contributing to precipitation in Mongolia are not low‐latitude regions but central Asia and western Siberia located to the northwest of Mongolia. Observed multilevel isotopic composition supports the model estimation. The moisture transport along the southwesterly wind of the Asian summer monsoon has difficulty reaching Mongolia as a monthly/seasonal mean perspective. However, eastern Mongolia and northeast China are situated on the border area between westerly wind moisture transport (by midlatitude synoptic cyclones) and southerly wind moisture transport (by Asian summer monsoon).

Temporal and spatial variations of δ18O in precipitation of the Yarlung Zangbo River Basin

This paper reveals the temporal and spatial variations of stable isotope in precipitation of the Yarlung Zangbo River Basin based on the variations of δ18O in precipitation at four stations (Lhaze, Nugesha, Yangcun and Nuxia) in 2005. The results show that δ18O of precipitation has distinct seasonal changes in the Yarlung Zangbo River Basin. The higher value of δ18O occurs in spring prior to monsoon precipitation, and the lower value occurs during monsoon precipitation. From the spatial variations, with the altitude-effect and rainout process during moisture transport along the Yarlung Zangbo River Valley, 18O of precipitation is gradually depleted. Thus, δ18O of precipitation decreases gradually from the downstream to the upstream, and the lapse rate of δ18O in precipitation is approximately 0.34‰/100m and 0.7‰/100km for the two reasons. During monsoon precipitation, spatial variation of δ18O in precipitation is dominated by the amount effect in the large scale synoptic condition.

Variations of δ18O in precipitation along vapor transport paths

Three sampling cross sections along the south path starting from the Tropics through the vapor passage in the Yunnan-Guizhou Plateau to the middle-low reaches of the Yangtze River, the north path from West China, via North China, to Japan under the westerlies, and the plateau path from South Asia over the Himalayas to the northern Tibetan Plateau, are set up, based on the IAEA (International Atomic Energy Agency)/WMO global survey network and sampling sites on the Tibetan Plateau. The variations, and the relationship with precipitation and temperature, of the δ18O in precipitation along the three cross sections are analyzed and compared. Along the south path, the seasonal differences of mean δ18O in precipitation are small at the stations located in the Tropics, but increase markedly from Bangkok towards the north, with the δ18O in the rainy season smaller than in the dry season. The δ18O values in precipitation fluctuate on the whole, which shows that there are different vapor sources. Along the north path, the seasonal differences of the mean δ18O in precipitation for the stations in the west of Zhengzhou are all greater than in the east of Zhengzhou. During the cold half of the year, the mean δ18O in precipitation reaches its minimum at Urumqi with the lowest temperature due to the wide, cold high pressure over Mongolia, then increases gradually with longitude, and remains at roughly the same level at the stations eastward from Zhengzhou. During the warm half of the year, the δ18O values in precipitation are lower in the east than in the west, markedly influenced by the summer monsoon over East Asia. Along the plateau path, the mean δ18O values in precipitation in the rainy season are correspondingly high in the southern parts of the Indian subcontinent, and then decrease gradually with latitude. A sharp depletion of the stable isotopic compositions in precipitation takes place due to the very strong rainout of the stable isotopic compositions in vapor in the process of lifting over the southern slope of the Himalayas. The low level of the δ18O in precipitation is from Nyalam to the Tanggula Mountains during the rainy season, but δ18O increases persistently with increasing latitude from the Tanggula Mountains to the northern Tibetan Plateau because of the replenishment of vapor with relatively heavy stable isotopic compositions originating from the inner plateau. During the dry season, the mean δ18O values in precipitation basically decrease along the path from the south to the north. Generally, the mean δ18O in precipitation during the rainy season is lower than in the dry season for the regions controlled by the monsoons over South Asia or the plateau, and opposite for the regions without a monsoon or with a weak monsoon.

Comparisons on seasonal and annual variations of δ18O in precipitation

The spatial and temporal variations of stable oxygen isotope in precipitation on different time scales are analyzed according to the data from the IAEA/WMO stations with long survey series in the Northern Hemisphere. Temperature effect is mainly distributed in mid-high latitudes on seasonal scale except for Bamako and Addisababa stations. The δ18O/temperature slope displays the positive correlation against altitude for most of the statistical stations. Amount effect appears primarily in the region south of 30°N and coastal areas. The δ18O/precipitation slope is indirectly proportional to precipitation amount. For some of the sampling stations at mid-high latitudes where their seasonal distribution of precipitation is contrary to that of temperature, coupled with temperature effect, the amount effect appears synchronistically. Either the temperature effect or the amount effect on seasonal scale, there are positive correlations to a certain extent between the annual weighted mean δ18O and the annual mean temperature for almost all the stations. The correlation between composite δ18O and temperature on spatial scale is much more marked, compared with that of individual station. There is a good agreement between 10-year moving average temperature curves I and II, with the values of the former all markedly smaller than corresponding ones of the latter, calculated by the monthly mean series group I and the annual mean series group II, respectively. However, two calculated dδ18O/dT curves display the distinct difference: the variation amplitude of slope series II is larger than that of slope series I. Both curves had similar ascending trend from the 1960s to the 1970s, and then, their variations display the anti-phase. Moreover, the analyses show that there is negative correlation between slope series FI and temperature series II. However, the status is different for slope series I and temperature series I. Both series have contrary trend from the 1960s to the 1970s, whereas the same trend since the 1980s.

Stable Isotope Variations in Monsoon Precipitation on the Tibetan Plateau.

Precipitation samples were collected in a large-scale region (more than 2,000 km from the north to the south of the Tibetan Plateau) and a middle scale region (with an area about 9,000 km2) in the Tibetan Plateau to study the impact of monsoon precipitation on stable isotopes. In the large-scale region, samples of precipitation and river water were collected at several stations from the north to the south along the Qinghai-Tibet Highway on the plateau. Results show that the Indian monsoon not only influences the spatial variation of stable isotope in precipitation, but also the relationship between δ18O and δD. Deuterium excees (d) in areas where precipitation is directly controlled by monsoon precipitation is lower, while in the northern Tibetan Plateau, d is in higher value due to the influence of continental air mass and strong local hydrological recycling. A detailed study on oxygen stable isotope in precipitation was carried out on the middle scale in the Nagqu Basin area. The temporal variation of δ18O in daily precipitation at 6 sites shows quite similar trends, indicating that δ18O in precipitation in the middle Tibetan Plateau is mainly controlled by large-scale synoptic condition, rather than the local meteorological factors. The spatial variation of δ18O in precipitation in the basin agrees with the Tibet-scale spatial variation trend and local hydrological recycling. Temporal variations of δ18O in precipitation also show a close relation to the movements of monsoon precipitation. The onset of monsoon in the earlier summer leads to low δ18O value. There is a negative relation between δ18O and SWI (south wind index). High SWI, which represents intensive transport of moisture from south with the monsoon movement, is coincided with lower δ18O values, while low SWI, which represents moisture from inland air mass or local evaporated moisture, is associated with higher δ18O values.

Spring Flood Meltwater or Groundwater?

The environmental isotope oxygen-18 was used as a tracer to separate stream discharge into flows originating from groundwater and fresh meltwater. Two, mainly forested, watersheds (areas 6.6 and 4.0 km2) in southern Sweden were studied during snowmelt 1979. The seasonal variation of δ18O in precipitation makes the δ18O of meltwater different from that of groundwater, hence making the separation possible. The major part of the streamflow this spring originated from groundwater. Of the total water volumes that left the areas, during the two periods of snowmelt that occurred this year, the fraction of groundwater ranged between 0.7 ± 0.2 and 0.9(±) 0.2 The volume of meltwater in the streams corresponded to the volume of snowmelt and precipitation over 10-15% of the catchments. A field survey indicated that these percentages reasonably well describe the extension of the saturated areas, i.e. the effluent areas for groundwater where no infiltration can take place.