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The isotope-enabled general circulation models (GCM) have been widely applied to simulate the variability of stable isotopes in meteoric water at various time scales. The in-situ observations of water vapour isotopes are an important basis for assessing the performance of isotope-enabled GCMs, although they are still limited. Here we compiled the observations of near-surface water vapour isotopes on a daily scale at 17 stations in East Asia, and assessed the skill and the association between isotope error and meteorological errors on a daily scale. Generally, the spatial pattern and seasonal variability can be well simulated in the isotope-enabled GCMs. The models show better skill for warm and humid backgrounds, which also corresponds to the monsoonal regions with lower latitudes in East Asia. As spatial resolution is finer, the skill of models is better, which can be seen from the two GCMs. According to the correlation coefficient, the improvement of resolution is more obvious in summer than in winter, especially for IsoGSM. In addition, the correlation coefficient in winter is usually larger than that in summer. The daily modelling has good potential to investigate the daily or synoptic climate information in water isotopes. The findings are useful for understanding the applicability of isotope-enabled models in East Asia and the climate factors influencing the skill of isotope-enabled models on a daily basis. • This is the first systemic assessment of daily water vapour isotopes in East Asia using in-situ observations. • The models show better performances in monsoonal regions with warm and humid backgrounds, than cold and dry conditions. • The assessments provide the basis to interpret the skill of models in East Asia, and how it relates to climate conditions. [ABSTRACT FROM AUTHOR]

• The new observation and modelling provide a basis to understand the altitude effect in central Asian precipitation isotopes. • Below-cloud evaporation enhances the altitude effect for most areas in the arid mountain-basin system. • The periods with a larger gradient of altitude effect usually have higher temperature and more precipitation. The stable water isotopes of precipitation provide important information about the hydrological circulation. In the arid mountain-basin system in central Asia, the altitude effect of precipitation isotopes has been a controversial topic in recent years, but the sample availability in extreme environments constrains the accurate understanding of the relationship between altitude and stable isotopes in precipitation. Based on the observation of precipitation isotopes around the Tarim Basin covered by the world's second-largest shifting desert, we examined the relationship between altitude and isotope composition. There is an altitude effect of precipitation isotopes between the basin and the surrounding mountains, with the modelled gradient for annual mean δ 18O being approximately 1.96 ‰ per 1000 m, which is weaker than the observed gradient focusing on the oases (2.75 ‰ per 1000 m). The largest modelled difference in δ 18O between 1000–2000 m and 2000–3000 m above sea level occurs in August and September. The periods with a larger gradient of altitude effect usually have higher temperature and more precipitation. Across the westerlies-dominated central Asia, the below-cloud evaporation enhances the altitude gradient of precipitation isotopes for most areas. The findings are useful to understand the local and remote drivers of precipitation isotopes and the paleoaltimetry of stable isotopes in climate proxies. [ABSTRACT FROM AUTHOR]

The oxygen isotope compositions in atmospheric water including water vapor and precipitation have been widely used to trace moisture sources and to reconstruct past climates. However, the environmental controls of stable isotopes in atmospheric water depend on the time scales. Because of limited observations in arid Central Asia, factors controlling interannual variations in atmospheric water isotopes are still not clear. Using an isotope-enabled general climate model, we do not find at the annual scale the significant relationship between temperature and δ 18O values during 1979–2020 that is usually observed at the monthly scale. Under a warming background, there is no significant enriching trend in water isotopes. We found a strong positive correlation between westerly (and southerly) water vapor flux and δ 18O values on an interannual scale for the area between 35°N–50°N and 50°E–80°E in the upstream direction. High and low δ 18O years are characterized by different atmospheric circulations of mid-latitude Westerlies, and do not always correspond to the warm and cold years respectively. When the prevailing Westerlies are enhanced, the Westerlies circulation may carry more water vapor from the lower latitudes, leading to higher δ 18O values in Central Asia. The changes in Westerlies circulation are supported by the clustered backward trajectories during enriched and depleted years. The limited precipitation isotope observations also show similar findings as the simulations on an interannual scale. The interannual variations of stable isotopes in atmospheric water in arid Central Asia reflect the changes in the mid-latitude Westerlies circulation. This should be taken into account when interpreting oxygen isotope proxies of paleoclimate records. • Temperature effect of water isotopes exists on a seasonal scale, but is weakened on an annual scale in Central Asia. • On an annual scale, there are positive correlations between westerly (and southerly) water vapor flux and isotope value. • The critical area controlling interannual variability of water isotopes in the Tianshan Mountains is identified. [ABSTRACT FROM AUTHOR]

Different water sources exploited for public use have different exposure to risks associated with climatic and environmental change. Isotope ratios of tap water have previously been studied as a potential tool to link public supply waters with water source characteristics at local to continental scales, providing information on the footprint of and potential risks associated with the water sources used. Work that combines intensive spatial and temporal sampling with independent water management data has been limited, however. In this study, an extensive observation network was established during 2014–2016 to provide monthly tap water sampling across China. We show that the spatial distribution of annual mean tap water isotope ratios is generally consistent with that of local precipitation across China. We identify seasonal correlation between tap water and precipitation isotope ratios in south China, where use of surface water is prevalent. In contrast, relatively invariant tap water isotope ratios elsewhere in China, which are not correlated with seasonal variation of precipitation isotope ratios, can be attributed to use of groundwater or water from river basins with longer storage times. The tap water isotope signatures identified here could be widely applied to characterize water supplies and associated sustainability challenges in different regions worldwide. Key Points: A new nationwide network of tap water isotope data across China was establishedConnection between monthly stable isotopes in tap water and precipitation is identifiedDiagnostic patterns of tap water isotopes are associated with water resource use [ABSTRACT FROM AUTHOR]

The local meteoric water line (LMWL) reflects the relationship between stable oxygen and hydrogen isotopes in precipitation, and is usually calculated using an ordinary least squares regression (OLSR). When event-based data are used to calculate a LMWL, the differences in precipitation amount of samples are not considered using OLSR, which in turn may influence the representativeness of the LMWL for local hydrology. Small rain events occur widely in arid Central Asia (annual mean precipitation <150 mm), and where smaller precipitation has lower deuterium excess, this results in LMWLs with slopes and intercepts lower than the global average. Based on an observation network of isotopes in precipitation across the Tianshan Mountains in arid Central Asia, LMWLs for 23 stations are calculated from event-based data from 2012 to 2013 ( n  = 978), using ordinary least squares, reduced major axis and major axis regressions and their precipitation-weighted counterparts. For the northern slope and mountainous areas, the LMWL slope and intercept are close to the Global Meteoric Water Line (GMWL), but the slope and intercept are lower for the southern slope indicating the greater dominance of sub-cloud evaporation. The effect of moisture recycling in the irrigated areas on the northern slope also can be seen where the LMWL slopes are >8. Using a precipitation weighted regression method with event-based data (especially precipitation-weighted reduced major axis regression, PWRMA) is generally consistent with the OLSR regression using monthly data. However, event-based datasets provide a wider range of values to better constrain the regression than can be achieved using monthly data over a short period, providing a sounder basis for determining LMWLs for relatively short-term sampling campaigns in an arid setting. The use of the PWRMA regression is preferred for determining the LMWL for the Tianshan Mountains, and results in a regional meteoric water line of δ D = 7.9 δ 18 O + 10.16. [ABSTRACT FROM AUTHOR]

Based on stable hydrogen and oxygen isotope data (δ18O, δD) and meteorological observation data for complete hydrological annual precipitation from 2016 to 2017 in the monsoon marginal region of northern China (Fengxiang and Ningwu), the isotopic characteristics of precipitation and the sources of water vapor in these two regions combined were studied. The results showed that δ18O and δD values in the wet season (June through September) were higher than in the dry season (October to May of the following year) in Fengxiang and Ningwu. The intercept and slope of the meteoric water line in the two regions were somewhat low, revealing that the water vapor in the rainfall comes mainly from the tropical ocean. On a synoptic scale, significantly positive correlations among dry season precipitation, δ18O, and temperature manifested temperature effects, but in the wet season, the temperature effect was not significant. On a monthly scale, a relationship did not exist between the change in trend of the average value of monthly weighted δ18O in precipitation and the average temperature change value in the two regions. However, in the wet season, significantly negative relationships can be found between the average monthly weighted δ18O in precipitation and rainfall amount, which indicated a remarkable rainout effect. Further investigation revealed that continuous precipitation made the values of δ18O and δD more negative under the same source of water vapor (the rainout effect). Because the annual rainfall in the monsoon marginal region of Northern China is mainly made up of monsoon rainfall, the oxygen isotope index of geological and biological records, such as stalagmites and tree rings, which inherit meteoric water isotope information, can be used to reconstruct past rainfall changes in northern China. [ABSTRACT FROM AUTHOR]

Terrestrial moisture contributed by surface evaporation and transpiration, also known as recycled moisture, plays an important role in hydrological processes especially across arid central Asia. The stable hydrogen and oxygen isotopes can be used for water budget analysis to calculate the contribution of recycled moisture to precipitation between two locations along the moisture flow. Based on a three-component isotopic mixing model, the moisture recycling in oasis stations of arid central Asia during summer months is assessed. At large oases of Urumqi, the proportional contribution of recycled moisture to local precipitation is approximately 16.2%, and the mean proportions of surface evaporation and transpiration are 5.9% ± 1.5% and 10.3% ± 2.2%, respectively. At small oases like Shihezi and Caijiahu the contribution of recycled moisture is less than 5%, and the proportion of surface evaporation is much less than that of transpiration. The vegetative cover in arid central Asia is generally sparse, but the evapotranspiration contribution to precipitation cannot be ignored at the widely distributed oases. The oasis effect shows great variability depending on locations and water availability for evapotranspiration. [ABSTRACT FROM AUTHOR]

• Precipitation isotope characteristics of a leeward oasis in central Asia were identified. • Moisture recycling ratio ranged between 17.0% and 63.9% in the central Asian oasis. • Bayesian isotopic mixing model is promising in quantifying moisture recycling. Locally recycled moisture from transpiration and surface evaporation is of great importance in the terrestrial hydrological cycle, especially in the widely distributed oases across arid central Asia. Quantitative assessment of the proportional contribution of recycled moisture to local precipitation, i.e., the recycling ratio, is useful to understand the land-air interaction as well as the anthropogenic impact on the regional water cycle. Here we analyzed the stable hydrogen and oxygen isotopes in precipitation samples collected at six stations across the Kaxgar-Yarkant Oasis in the western Tarim Basin of central Asia from April 2018 to June 2020. Using this data, the moisture recycling ratio in this typical oasis was assessed using a Bayesian three-component isotopic mixing model. For the plain stations, the annual weighted mean δ 18O value in precipitation ranged from −5.94 ‰ to −1.46 ‰, and the mountain station has a lower annual mean precipitation isotopic ratio. The average recycling ratio during the summer months ranged between 17.0 % and 63.9 % for each sampling station in the Kaxgar-Yarkant Oasis, and the proportional contribution from transpiration ranged from 15.1 % to 61.3 %. The contribution of plant transpiration to local precipitation is much larger than that of surface evaporation. The recycled portion in total precipitation amount may increase the local precipitation under an oasis expansion background but is insufficient to change the arid background. In addition, the Bayesian isotopic mixing model is promising to determine the recycling ratio in an arid setting, and provides more spatial details than the climate reanalysis-based calculation. [ABSTRACT FROM AUTHOR]

In arid northwest China where the precipitation intensity is relatively low, the stable hydrogen and oxygen isotopes (δ 2H and δ 18O) in precipitation are usually impacted by the sub-cloud evaporation. To understand the sub-cloud evaporation effect under an arid climate, we used the hourly meteorological data at 14 stations in the Kaxgar-Yarkant River Oasis, a rainshadow oasis of northwest China, and estimated the monthly and hourly isotopic change in falling drops from the cloud base to the ground. The results showed that the hourly meteorological observations are an effective way to assess the spatiotemporal pattern of sub-cloud evaporation effect. Across the 14 stations, the annual mean changes in deuterium excess below cloud base ranged from −19.1‰ to −6.8‰. The impacts of sub-cloud evaporation during spring and autumn are larger than those during winter and weaker than those during summer. The exponential regression (R 2 = 0.96), instead of linear regression (R 2 = 0.85), is better at describing the relationship between the raindrop remaining fraction and the isotopic changes from the cloud base to the ground. The sensitivity analysis of isotopic changes to different relative humidity scenarios shows that the drying scenario may lead to slightly larger sensitivity than the wetting scenario. The impact of sub-cloud evaporation on stable isotope compositions in precipitation may be underestimated when low precipitation events (especially less than 1 mm/h) are not sampled. • Hourly meteorological records are effective to assess sub-cloud evaporation effect on precipitation isotopes. • Arid mountains show a strong sub-cloud evaporation effect like nearby low-lying plains. • The impact of sub-cloud evaporation may be underestimated when low precipitation events are not sampled. [ABSTRACT FROM AUTHOR]

The local meteoric water lines (LMWLs) reflect water sources and the degree of sub-cloud evaporation at a specific location. Lanzhou is a semi-arid city located at the margin of the Asian monsoon, and the isotope composition in precipitation around this region has aroused attention in hydrological and paleoclimate studies. Based on an observation network of stable isotopes in precipitation in Lanzhou, LMWLs at four stations (Anning, Yuzhong, Gaolan and Yongdeng) are calculated using the event-based/monthly data and six regression methods (i.e., ordinary least squares, reduced major axis, major axis regressions, and their counterparts weighted using precipitation amount). Compared with the global meteoric water line, the slope and intercept of LMWL in Lanzhou are smaller. The slopes and intercepts calculated using different methods are slightly different. Among these methods, precipitation-weighted least squares regression (PWLSR) usually had the minimum average value of root mean sum of squared error (rmSSEav), indicating that the result of the precipitation weighted method is relatively stable. Higher precipitation amount and lower air temperature result in larger slopes and intercepts on an annual scale, which is out of accordance with the summertime. [ABSTRACT FROM AUTHOR]

The proportional contribution of recycled moisture to local precipitation is a geographically dependent parameter that cannot be ignored in water budgets. Stable hydrogen and oxygen isotopes are sensitive to environmental changes and can be applied to investigate the modern water cycle. In this study, a three-component mixing model is used to calculate the contribution of different water vapors (advection, evaporation and transpiration) to summer precipitation in Lanzhou city, Northwest China. The results show that for all sampling sites in Lanzhou, the contribution of advection vapor to precipitation is the largest, followed by the plant transpiration vapor, and the contribution of surface evaporation water vapor is usually the least, with the average values of 87.96%, 9.1% and 2.9%, respectively. The spatial differences of plant transpiration vapor are generally larger than those of advection vapor and surface evaporation vapor, and the high values appear in Yongdeng, Daheng and Gaolan. [ABSTRACT FROM AUTHOR]

Stable isotopes (e.g., δ2H and δ18O) in tap water are important tools to understand the local climate or environment background, water sources and the state of regional water supply. Based on 242 tap water samples, 35 precipitation samples and 24 surface water samples gathered in the urban area of Lanzhou, the basic spatiotemporal characteristics of isotopes in tap water, their connection with isotopes in other water bodies and change during the process from raw water to tap water are discussed in detail, combining the information of local tap water supply and water source. It can provide reliable help for understanding the isotope characteristics of local tap water, regional water supply management and determination of tap water source of in a small area. Except for the establishment of a new data set of isotopes in tap water with complete time series and uniform spatial distribution of sampling sites, other results show that: (1) The Local Tap Water Line (LTWL) of Lanzhou is δ2H = (6.03 ± 0.57) δ18O + (−8.63 ± 5.44) (r2 = 0.41, p < 0.01). (2) For seasonal variations, δ2H and δ18O in tap water both are higher in autumn and lower in spring. The diurnal and daily variations of isotopes in tap water are not large. As for spatial variations, the monthly mean values of δ2H and δ18O in tap water at each sampling site show little difference. The isotopes in tap water collected from one single sampling site can be considered as a representative for isotopes in tap water in the area with a single tap water source. (3) Isotopes in tap water show weak connection with precipitation isotopes, but exhibit good connection (consistent seasonal variation, similar numerical range, small numerical difference and high correlation) with isotopes in surface water, which is the direct water source. Isotopes in water change little from raw water to tap water. Isotopic composition of tap water in Lanzhou can be used as a representative of isotopes in surface water. [ABSTRACT FROM AUTHOR]

Although stable isotopes of hydrogen and oxygen in surface waters (especially in river waters) are useful tools to understand regional hydrological processes, relevant information at some upper reaches of large rivers in western China is still limited. During 2016–2017, we focused on the Liujiaxia Reservoir along the upper Yellow River, where we collected surface water samples at two locations, above and below the dam (identified as "lake water" and "river water"). The results show that the heavy isotopes in lake and river waters are enriched during the warm months, when the river discharge is large, and depleted during the cold months. The slopes of the water line (δ2H versus δ18O) for both the lake and river waters were lower than that of the global mean, due to evaporation. The different d values of the lake and river water reflect the regional evaporation and water sources. [ABSTRACT FROM AUTHOR]

• Clarifying the response relationship between mountain precipitation and irrigation in oasis with multisource data. • The climate background information and evidence of precipitation recycling contained in stable isotopes. • Impacts of irrigation and local hydrological cycle driven by mountain-valley winds on precipitation. • The oasis and irrigation influence the local precipitation through offset, remote, combination, and delayed effects. Water is one of the scarcest resources in arid regions, and irrigation is an important means to improve crop yield and ensure food security. The Xinjiang features an integrated irrigation agriculture and oasis economy paradigm. However, the understanding of the impact mechanism and quantitative analysis of irrigation on precipitation is inadequate now. With the aim of clarifying the response relationship between mountain precipitation and irrigation in oasis, we clarify and quantify the moisture sources of precipitation using the Hybrid Single Particle Lagrangian Integrated Trajectory (HYSPLIT) model and multisource data (including field-collected isotopes data, C-Isoscape, ERA5, and GDAS). The results obtained show: (1) Isotope analysis provides valuable insights into the climate background and precipitation recycling. The Local Meteoric Water Line's lower slope and intercept than Global Meteoric Water Line suggest a severe arid climate and strong sub-cloud evaporation. The intercept (13.44) of Local Meteoric Water Line in Mountain Areas exceeds the global average, indicating significant moisture recycling in mountain areas (MA). The d-excess in MA is prominent year-round, highlighting the influence of recycled moisture. (2) The oasis and irrigation influence the local precipitation through offset, remote, combination, and delayed effects. The offset effect hinders precipitation formation in irrigation areas (IA) due to the counteracting irrigation-induced humidification and cooling effect. Through the interaction of westerly circulation and local mountain-oasis hydrological cycle, irrigation-induced moisture is transported to MA and uplifted, which is remote effect. The combination effect synergistically enhances precipitation intensity, especially in intricate MA, through the combined influence of terrain's forced uplift and convective cells. Furthermore, the mountainous precipitation influenced by the mountain-oasis hydrological cycle often exhibits a delayed effect. (3) Based on HYSPLIT model tracking results, the contribution ratio of oasis and irrigation induced evapotranspiration to summer precipitation in MA is approximately 19.60%, ranging from 13.68% to 28.49%. The findings provide insights into the mechanisms of irrigation and water resource management in the northern slope of Tianshan Mountains. [ABSTRACT FROM AUTHOR]

Based on the 420 samples of precipitation and related meteorological parameters obtained from the four sampling sites (Yongdeng, Gaolan, Lanzhou and Yuzhong) in Northwest China from April 2011 to February 2013, the influence of sub-cloud secondary evaporation effect on stable isotopes in precipitation was analyzed. Four main factors affecting the secondary evaporation were precipitation, air temperature, water vapor pressure, and relative humidity. The results showed that sub-cloud secondary evaporation had a significant effect on isotopes when the rainfall amount was small, but the correlation was not significant for snowfall or heavy rainfall. As the temperature increased, the secondary evaporation was enhanced. Water vapor pressure greatly impacted the sub-cloud secondary evaporation of the rain, but had less influence on the snow events. Relative humidity showed an influence on d-excess value, as well as the slope and intercept of the δD-δ 18 O correlation equation of light rainfall, but had a small impact when snow occurred. The estimated secondary evaporation rate was generally lower in winter and higher in summer, and spatially varied depending on locations. During the summer monsoon period (June to September), the secondary evaporation rate was estimated to be between 5.90% and 10.50% for each station with the mean value of 8.30%, and during the winter monsoon period (October to May), the rate was between 3.20% and 5.62%, with the average value of 4.54%. [ABSTRACT FROM AUTHOR]