Precipitation Isotope Ratios Research Articles

Seasonal variations and altitude effect of oxygen and hydrogen stable isotope ratios of precipitation and spring water in an island of Seto Inland Sea

Seasonal variations and altitude effect of oxygen and hydrogen stable isotope ratios of precipitation and spring water in an island of Seto Inland Sea

Divergent roles of landscape and young streamflow fraction in stream water quality over seven catchments

Landscape patterns and water age are considered to play similar roles in influencing water quality, as human-caused landscape fragmentation usually leads to complex hydrological pathways and older water ages. The young streamflow fraction (Fyw), which can significantly alter water age, is believed to disproportionately impact water quality. Landscape and Fyw thus may play different roles in streamwater quality, but this assumption has not been examined. Here we examined the roles of Fyw (estimated by the amplitudes of seasonal cycles of oxygen isotope ratios in precipitation (Ap) and streamwater (As)) and landscape pattern (reflected by common landscape metrics) in streamwater quality over 7 forest-dominated catchments in eastern China. Landscape metrics, indicating land use arrangement, were closely related to multiple water quality parameters, suggesting a close association between stream water quality (sinks) and land use (sources). The Fyw had a positive relationship with ammonia nitrogen (R2 = 0.51, P = 0.044) and a negative relationship with nitrate nitrogen (R2 = 0.62, P = 0.022). The Fyw, determined by the heterogeneous-independent As/Ap, offers the possibility of modeling nitrogen compounds across multiple spatial scales. Importantly, the water quality parameters that significantly correlated with Fyw did not correlate with the landscape metrics. Differences in correlations of landscape and Fyw to water quality parameters imply that landscape and Fyw differed in their roles in water quality parameters, as well as the consideration for different water quality management strategies for different contaminants.

Water isotope ratios reflect convection intensity rather than rain type proportions in the pantropics.

Against the traditional view, a recently published theory argued that isotope ratios are higher in convective precipitation but lower in stratiform precipitation and proposed that isotope ratios reflect rain type proportions. This theory has been widely cited despite some early reservations. Whether the theory represents a faithful reflection of signals of water isotope ratios remains unclear. Here, we reassess its validity from different timescales and broader observations from the pantropics. Unexpectedly, our findings contradict the theory on daily, monthly, and even annual timescales. Pantropical precipitation isotope ratios remain strongly correlated to convection intensity but are independent of rain type proportions because stratiform precipitation isotope ratios cover a large range of values. We find that the theory has many serious weaknesses related to preferential data selection and suggest that new theories need to be validated at more locations on different timescales before gaining widespread acceptance.

Precipitation 17O‐Excess Altered During Tropical Convection: Evidence From Monsoon Cold Surges in Singapore

AbstractDespite the recent recognition of 17O‐excess as a promising new tracer for hydrological processes, our knowledge of the control mechanisms underlying 17O‐excess in tropical regions remains limited. To understand how microphysical processes during tropical convection affect precipitation isotope ratios, particularly 17O‐excess, in Singapore, we collected precipitation samples at minute intervals from six rain events associated with cold surges during the Northeast Monsoon seasons and analyzed their triple oxygen isotopes. Our results show that precipitation δ18O decreases in the convective zones and then gradually increases in the stratiform zones, while d‐excess exhibits an inverse trend. This correlation between δ18O and d‐excess indicates that rain evaporation plays a crucial role in regulating precipitation isotopes. Moreover, the rain events with a higher upstream rainout amount have lower δ18O and higher 17O‐excess values, suggesting that precipitation δ18O and 17O‐excess likely reflect the integrated upstream convective activity. Microphysical processes associated with upstream convection, such as rain evaporation and vapor recycling, are potential mechanisms that increase 17O‐excess values along moisture transport pathways for a rain event, and hence, undermine the effectiveness of 17O‐excess as a tracer of moisture source humidity. Contrary to the negative correlation observed in monthly precipitation, there is generally a positive correlation between d‐excess and 17O‐excess at the event scale. However, this correlation weakens as convective rain intensifies, suggesting that stronger convection can attenuate the positive correlation between d‐excess and 17O‐excess. Therefore, it is crucial to consider how tropical convection alters 17O‐excess when utilizing this tracer to interpret atmospheric dynamics and hydrological processes.

Isotopic evidence for seasonal water sources in tree xylem and forest soils

AbstractForest trees greatly influence both the routing of water downward into the subsurface and the re‐routing of water upward through water uptake and transpiration. To reveal how the subsurface soil water pools used by trees change across seasons, we analysed 2 years of stable isotope ratios of precipitation, soil water from different depths (using both bulk sampling and suction‐cup lysimeters), and xylem in a mixed beech and spruce forest. Precipitation as well as mobile and bulk soil waters all showed a distinct seasonal signature; the seasonal amplitude decreased with depth, and mobile soil waters fluctuated less than bulk soil waters. Xylem water signatures in both tree species were similar to the bulk soil water signatures and rather different from the mobile soil water signatures. The beech and spruce trees had different isotope ratios, suggesting the use of different water sources, and these differences were larger under dry antecedent conditions than wet antecedent conditions. Despite these differences, both species predominantly transpired waters with a winter‐precipitation isotopic signature throughout the summer, including during wet conditions when more recent precipitation was available. Over most of the sampling dates, the fraction of recent precipitation (i.e. from the preceding 30 days) in xylem water was low, despite both species typically demonstrating the use of both shallow and deeper soil waters. These results provide evidence that the soil water storages used by these trees are largely filled in winter and bypassed by recent precipitation, implying long residence times.

Irrigation‐induced evaporative water loss in a glacially derived soil site

AbstractWorldwide, significant agricultural activities take place where groundwater is used for irrigation and water losses occur due to evaporation. Water loss through evaporation is affected by the soil type, local climate and most likely crop type. Previous studies from arid regions demonstrated the utility of oxygen and hydrogen isotopes in estimating evaporative water loss during return flow back to an aquifer. This study examined the region around Kalamazoo, Michigan, United States, where the soil is formed from glacial sediments and the region experiences a temperate climate. The present study is applicable to agricultural practices in many parts of the world that are in a temperate climate zone. The study area primarily uses centre‐pivot systems supplied by wells. Water samples were taken periodically from wells close to centre‐pivot irrigation systems. Knowledge of isotope ratios in precipitation, the primary source of groundwater recharge is a prerequisite for this study. The distribution of oxygen and hydrogen in local precipitation, the so‐called Local Meteoric Water Line, is available based on several years of measurements. An evaporative model of the isotope data indicates water loss from evaporation to be in the range of 14% and 6%. This is less than what was estimated for arid region zones and is perhaps also due to the well‐drained nature of the regional soil which promotes rapid infiltration lowering evaporation. Evaporative loss was greater at wells near cornfields than at wells that supplied other crop types. Little expected correlation between the groundwater isotope data and chloride concentration is attributable to an external input of chloride from road salt used in winter months.

Relationship between isotope ratios in precipitation and stream water across watersheds of the National Ecological Observation Network

AbstractThe timescales associated with precipitation moving through watersheds reveal processes that are critical to understanding many hydrologic systems. Measurements of environmental stable water isotope ratios (δ2H and δ18O) have been used as tracers to study hydrologic timescales by examining how long it takes for incoming precipitation tracers become stream discharge, yet limited measurements both spatially and temporally have bounded macroscale evaluations so far. In this observation driven study across North American biomes within the National Ecological Observation Network (NEON), we examined δ18O and δ2H stable water isotope in precipitation (δP) and stream water (δQ) at 26 co‐located sites. With an average 54 precipitation samples and 139 stream water samples per site collected over 2014–2022, assessment of local meteoric water lines and local stream water lines showed geographic variation across North America. Taking the ratio of estimated seasonal amplitudes of δP and δQ to calculate young water fractions (Fyw), showed a Fyw range from 1% to 93% with most sites having Fyw below 20%. Calculated mean transit times (MTT) based on a gamma convolution model showed a MTT range from 0.10 to 13.2 years, with half of the sites having MTT estimates lower than 2 years. Significant correlations were found between the Fyw and watershed area, longest flow length, and the longest flow length/slope. Significant correlations were found between MTT and site latitude, longitude, slope, clay fraction, temperature, precipitation magnitude, and precipitation frequency. The significant correlations between water timescale metrics and the environmental characteristics we report share some similarities with those reported in prior studies, demonstrating that these quantities are primarily driven by site or area specific factors. The analysis of isotope data presented here provides important constraints on isotope variation in North American biomes and the timescales of water movement through NEON study sites.

Sub-cloud rain evaporation in the North Atlantic winter trade winds derived by pairing isotopic data with a bin-resolved microphysical model

Abstract. Sub-cloud rain evaporation in the trade wind region significantly influences the boundary layer mass and energy budgets. Parameterizing it is, however, difficult due to the sparsity of well-resolved rain observations and the challenges of sampling short-lived marine cumulus clouds. In this study, sub-cloud rain evaporation is analyzed using a steady-state, one-dimensional model that simulates changes in drop sizes, relative humidity, and rain isotopic composition. The model is initialized with relative humidity, raindrop size distributions, and water vapor isotope ratios (e.g., δDv, δ18Ov) sampled by the NOAA P3 aircraft during the Atlantic Tradewind Ocean–Atmosphere Mesoscale Interaction Campaign (ATOMIC), which was part of the larger EUREC4A (ElUcidating the RolE of Clouds–Circulation Coupling in ClimAte) field program. The modeled surface precipitation isotope ratios closely match the observations from EUREC4A ground-based and ship-based platforms, lending credibility to our model. The model suggests that 63 % of the rain mass evaporates in the sub-cloud layer across 22 P3 cases. The vertical distribution of the evaporated rain flux is top heavy for a narrow (σ) raindrop size distribution (RSD) centered over a small geometric mean diameter (Dg) at the cloud base. A top-heavy profile has a higher rain-evaporated fraction (REF) and larger changes in the rain deuterium excess (d=δD-8×δ18O) between the cloud base and the surface than a bottom-heavy profile, which results from a wider RSD with larger Dg. The modeled REF and change in d are also more strongly influenced by cloud base Dg and σ rather than the concentration of raindrops. The model results are accurate as long as the variations in the relative humidity conditions are accounted for. Relative humidity alone, however, is a poor indicator of sub-cloud rain evaporation. Overall, our analysis indicates the intricate dependence of sub-cloud rain evaporation on both thermodynamic and microphysical processes in the trade wind region.

A coupled equilibrium boundary layer model with stable water isotopes and its application to local water recycling

The contribution of evapotranspiration (ET) to regional precipitation, known as “local water recycling”, is a key process in the water cycle that can affect water management. However, the amount of planetary boundary layer (PBL) moisture that arises from ET is highly uncertain due to complex atmosphere and land surface conditions. In this study, an idealized two-layer equilibrium boundary layer model was coupled with a stable water isotope module including HDO and H218O to constrain PBL growing season water transport processes. The model was validated using turbulent heat fluxes and isotope ratios of water vapor (δm) and precipitation (δP) measured at a cropland site and a nearby tall tower in the Upper Midwest, United States. The results show that the PBL equilibrium features of δm and δP are well-constrained by thermal and moisture equilibrium in the PBL. For this study region, the summer values of rain evaporation fraction (f) and local water recycling ratio (LRR) are estimated to be 0.09 and 0.29 ± 0.12, respectively. The summer LRR values for the years 2006–2010 were 0.35, 0.36, 0.17, 0.29, and 0.29, respectively. The small value of LRR in 2008 corresponded to a drought condition with the lowest precipitation and second lowest ET among the five years. The summer magnitude of the amount effect is −2.8‰ (mm day−1)−1 and −0.8‰ (mm day−1)−1 for HDO and H218O, respectively. The local water recycling is identified as a significant factor influencing the continental isotope effect. Cropland has likely changed the regional LRR by −7.6 to 19.5% under different pre-agriculture land use scenarios. The feedback processes revealed here indicate that local water recycling is expected to be weakened under drought conditions, but it will be enhanced if irrigation is applied more intensely with more frequent drought events as the climate continues to warm.

Seasonal Variations in Triple Oxygen Isotope Ratios of Precipitation in the Western and Central United States.

Triple oxygen isotope ratios offer new opportunities to improve reconstructions of past climate by quantifying evaporation, relative humidity, and diagenesis in geologic archives. However, the utility of in paleoclimate applications is hampered by a limited understanding of how precipitation values vary across time and space. To improve applications of , we present , d-excess, and data from 26 precipitation sites in the western and central United States and three streams from the Willamette River Basin in western Oregon. In this data set, we find that precipitation tracks evaporation but appears insensitive to many controls that govern variation in , including Rayleigh distillation, elevation, latitude, longitude, and local precipitation amount. Seasonality has a large effect on variation in the data set and we observe higher seasonally amount-weighted average precipitation values in the winter (40 ± 15 per meg [± standard deviation]) than in the summer (18 ± 18 per meg). This seasonal precipitation variability likely arises from a combination of sub-cloud evaporation, atmospheric mixing, moisture recycling, sublimation, and/or relative humidity, but the data set is not well suited to quantitatively assess isotopic variability associated with each of these processes. The seasonal pattern, which is absent in d-excess and opposite in sign from , appears in other data sets globally; it showcases the influence of seasonality on values of precipitation and highlights the need for further systematic studies to understand variation in values of precipitation.

Improving temperature reconstructions from ice-core water-isotope records

Abstract. Oxygen and hydrogen isotope ratios in polar precipitation are widely used as proxies for local temperature. In combination, oxygen and hydrogen isotope ratios also provide information on sea surface temperature at the oceanic moisture source locations where polar precipitation originates. Temperature reconstructions obtained from ice-core records generally rely on linear approximations of the relationships among local temperature, source temperature, and water-isotope values. However, there are important nonlinearities that significantly affect such reconstructions, particularly for source region temperatures. Here, we describe a relatively simple water-isotope distillation model and a novel temperature reconstruction method that accounts for these nonlinearities. Further, we examine in detail many of the parameters, assumptions, and uncertainties that underlie water-isotope distillation models and their influence on these temperature reconstructions. We provide new reconstructions of absolute surface temperature, condensation temperature, and source region evaporation temperature for all long Antarctic ice-core records for which the necessary data are available. These reconstructions differ from previous estimates due to both our new model and reconstruction technique, the influence of which is investigated directly. We also provide thorough uncertainty estimates for all temperature histories. Our reconstructions constrain the pattern and magnitude of polar amplification in the past and reveal asymmetries in the temperature histories of East and West Antarctica.

The NEON Daily Isotopic Composition of Environmental Exchanges Dataset

The National Ecological Observatory Network (NEON) provides open-access measurements of stable isotope ratios in atmospheric water vapor (δ2H, δ18O) and carbon dioxide (δ13C) at different tower heights, as well as aggregated biweekly precipitation samples (δ2H, δ18O) across the United States. These measurements were used to create the NEON Daily Isotopic Composition of Environmental Exchanges (NEON-DICEE) dataset estimating precipitation (P; δ2H, δ18O), evapotranspiration (ET; δ2H, δ18O), and net ecosystem exchange (NEE; δ13C) isotope ratios. Statistically downscaled precipitation datasets were generated to be consistent with the estimated covariance between isotope ratios and precipitation amounts at daily time scales. Isotope ratios in ET and NEE fluxes were estimated using a mixing-model approach with calibrated NEON tower measurements. NEON-DICEE is publicly available on HydroShare and can be reproduced or modified to fit user specific applications or include additional NEON data records as they become available. The NEON-DICEE dataset can facilitate understanding of terrestrial ecosystem processes through their incorporation into environmental investigations that require daily δ2H, δ18O, and δ13C flux data.

Variability and Controls on δ18O, d‐excess, and ∆′17O in Southern Peruvian Precipitation

AbstractThe isotopic composition of precipitation is used to trace water cycling and climate change, but interpretations of the environmental information recorded in central Andean precipitation isotope ratios are hindered by a lack of multi‐year records, poor spatial distribution of observations, and a predominant focus on Rayleigh distillation. To better understand isotopic variability in central Andean precipitation, we present a three‐year record of semimonthly δ18Op and δ2Hp values from 15 stations in southern Peru and triple oxygen isotope data, expressed as ∆′17Op, from 32 precipitation samples. Consistent with previous work, we find that elevation correlates negatively with δ18Op and that seasonal δ18Op variations are related to upstream rainout and local convection. Spatial δ18Op variations and atmospheric back trajectories show that both eastern‐ and western‐derived air masses bring precipitation to southern Peru. Seasonal d‐excessp cycles record moisture recycling and relative humidity at remote moisture sources, and both d‐excessp and ∆′17Op clearly differentiate evaporated and non‐evaporated samples. These results begin to establish the natural range of unevaporated ∆′17Op values in the central Andes and set the foundation for future paleoclimate and paleoaltimetry studies in the region. This study highlights the hydrologic understanding that comes from a combination of δ18Op, d‐excessp, and ∆′17Op data and helps identify the evaporation, recycling, and rainout processes that drive water cycling in the central Andes.

Enhancing Understanding of the Hydrological Cycle via Pairing of Process‐Oriented and Isotope Ratio Tracers

AbstractThe hydrologic cycle couples the Earth's energy and carbon budgets through evaporation, moisture transport, and precipitation. Despite a wealth of observations and models, fundamental limitations remain in our capacity to deduce even the most basic properties of the hydrological cycle, including the spatial pattern of the residence time (RT) of water in the atmosphere and the mean distance traveled from evaporation sources to precipitation sinks. Meanwhile, geochemical tracers such as stable water isotope ratios provide a tool to probe hydrological processes, yet their interpretation remains equivocal despite several decades of use. As a result, there is a need for new mechanistic tools that link variations in water isotope ratios to underlying hydrological processes. Here we present a new suite of “process‐oriented tags,” which we use to explicitly trace hydrological processes within the isotopically enabled Community Atmosphere Model, version 6 (iCAM6). Using these tags, we test the hypotheses that precipitation isotope ratios respond to parcel rainout, variations in atmospheric RT, and preserve information regarding meteorological conditions during evaporation. We present results for a historical simulation from 1980 to 2004, forced with winds from the ERA5 reanalysis. We find strong evidence that precipitation isotope ratios record information about atmospheric rainout and meteorological conditions during evaporation, but little evidence that precipitation isotope ratios vary with water vapor RT. These new tracer methods will enable more robust linkages between observations of isotope ratios in the modern hydrologic cycle or proxies of past terrestrial environments and the environmental processes underlying these observations.

Stable isotope record of super typhoon Lekima (2019)

Stable isotope ratios (δ18O and δD) in atmospheric precipitation and water vapor have the potential to diagnose synoptic-scale weather processes. Here we present high-frequency measurements of stable isotopes in water vapor and rainfall during super typhoon Lekima of 2019 in Shanghai, Eastern China. Our results indicate that both water vapor and rainfall exhibit large fluctuations in δD, with the magnitude of 94.69‰ and 57.88‰, respectively. These fluctuations reflect changes in typhoon-induced atmospheric and precipitation processes. Water vapor content and its δD values before Lekima are generally high due to stable atmospheric conditions that effectively suppress vapor condensation and isotope fractionation. During Lekima, isotope ratios in both water vapor and rainfall are complex and generally follow an inverted U-shaped pattern due to the combined effects of convection intensity, moisture convergence and rain evaporation. After Lekima, dry continental (northerly) air mass and subsidence cause a substantial decrease in both water vapor content and δD over the study region. Our study sheds light on the influence of super typhoon on isotope behaviors of water vapor and rainfall, and may have some implications for paleoclimate reconstruction of typhoon activity.

Soil water use sources and patterns in shrub encroachment in semiarid grasslands of Inner Mongolia

Soil water sources for shrub encroachment in arid and semiarid regions have not been fully understood. This study used stable δ2H and δ18O isotope ratios of plants, soil water and precipitation, coupled with proportional similarity index of species to identify relations between shrub encroachment and water sources for four shrub and grass species at various shrub encroachment stages in a semiarid grassland of Inner Mongolia. We found that shallow rooted grasses (Agriophyllum squarrosum and Stipa bungeana) predominantly used shallow soil water (0–20 cm) during growing season. In contrast, deeply-rooted semi-shrubs (Artemisia ordosica and Artemisia sphaerocephala) used middle and deep soil water in July but shallow and middle soil depth water in May and September. Plants competition for soil water in different shrub encroachment stages were different depending on plant water use sources. The highest plant competition existed in the A. ordosica community in semi-fixed sandy land, followed by A. squarrosum community in shifting sandy land, A. ordosica community in fixed sandy land, and S. bungeana community in fixed sandy land. Competition for soil water and differences of water use sources of grass and shrub species were crucial in driving shrub encroachment in semiarid grasslands. Communities dominated by A. ordosica were not stable, with an increase in surface soil water availability, the dominance was replaced by S. bungeana. Our findings on the linkage between plant water use sources and patterns and shrub encroachment shall help future arid and semiarid grassland management under changing climate conditions.

Chirripó hydrological research site: Advancing stable isotope hydrology in the Central American Páramo

AbstractThe Chirripó hydrological research site (CHRS) is located within the Chirripó National Park, Costa Rica (between 3100 and 3820 m asl) whereby ~100 km2 are covered by Páramo, a high‐elevation tropical grassland ecosystem. A lake district with approximately 30 lakes of glacial origin is also protected in this area. The CHRS has been monitored since April 2015 with the aim of establishing the first water isotope baseline for the Central American Páramo. At a regional scale, the water isotope ratios (δ2H and δ18O) in precipitation and surface water at CHRS are useful for describing the governing moisture transport from the Caribbean Sea and Pacific Ocean and the complex rainfall producing systems across the N–S mountain range of Central America. These data are also providing unique information about the evaporation and water balance conditions of tropical glacial lakes and the formation of orographic and convective precipitation in high‐elevation tropical ecosystems. Current data sets from CHRS include continuous lake water temperature and meteorological conditions (i.e., precipitation amount, air temperature and relative humidity), as well as water stable isotopes in precipitation, stream water, and lake water (daily to biweekly sampling frequency). Stream water is collected at several locations across the topographic gradient whereas lake water is sampled in the three main lake systems of CHRS. CHRS serves as a reference site for conducting pilot isotopic research in high‐elevation ecosystems to advance the atmospheric, hydrogeological and ecohydrological studies in these understudied biomes. All data from April 2015 to November 2020 are publicly available.

Convection, Terrestrial Recycling and Oceanic Moisture Regulate the Isotopic Composition of Precipitation at Srinagar, Kashmir

AbstractThe moisture transport and precipitation in the western Himalayas is an interplay of two atmospheric systems: Western Disturbances (WDs) and Indian Summer Monsoon (ISM). WDs primarily transport moisture from the Mediterranean Sea and the Atlantic Ocean, while ISM transports moisture from the Arabian Sea and the Bay of Bengal. Local moisture sources also contribute to the regional precipitation budget. The moisture sources can be distinguished by measuring dual isotopic signatures in precipitation. Oxygen and hydrogen isotopic ratios measured in daily precipitation samples collected at Srinagar, Kashmir from March 2015–April 2017, allowed delineating the role of factors like large‐scale moisture transport processes, and local meteorological factors such as temperature, precipitation amount and relative humidity on the precipitation isotope ratios and its effect on the meteoric water line parameters. Isotopic data were further used to estimate the percentage contributions of distinct moisture sources across different seasons and validation of an isotope enabled general circulation model (iEGCM). We found that the time integrated, large‐scale convection over several days, constitutes a major factor governing the isotopic composition of precipitation, while the role of local meteorological parameters is minimal. A box model to simulate the moisture transport and estimate the oceanic versus terrestrially recycled moisture sources to the regional precipitation reveals Arabian sea to be the key moisture source. Finally, on comparing the observed isotopic composition with iEGCM simulation, discrepancies are noted. Results from the present study will enable improving the interpretation of regional paleo‐climate data derived from various proxy records.

A Statistical Method for Generating Temporally Downscaled Geochemical Tracers in Precipitation

AbstractSampling intervals of precipitation geochemistry measurements are often coarser than those required by fine-scale hydrometeorological models. This study presents a statistical method to temporally downscale geochemical tracer signals in precipitation so that they can be used in high-resolution, tracer-enabled applications. In this method, we separated the deterministic component of the time series and the remaining daily stochastic component, which was approximated by a conditional multivariate Gaussian distribution. Specifically, statistics of the stochastic component could be explained from coarser data using a newly identified power-law decay function, which relates data aggregation intervals to changes in tracer concentration variance and correlations with precipitation amounts. These statistics were used within a copula framework to generate synthetic tracer values from the deterministic and stochastic time series components based on daily precipitation amounts. The method was evaluated at 27 sites located worldwide using daily precipitation isotope ratios, which were aggregated in time to provide low resolution testing datasets with known daily values. At each site, the downscaling method was applied on weekly, biweekly and monthly aggregated series to yield an ensemble of daily tracer realizations. Daily tracer concentrations downscaled from a biweekly series had average (+/- standard deviation) absolute errors of 1.69‰ (1.61‰) for δ2H and 0.23‰ (0.24‰) for δ18O relative to observations. The results suggest coarsely sampled precipitation tracers can be accurately downscaled to daily values. This method may be extended to other geochemical tracers in order to generate downscaled datasets needed to drive complex, fine-scale models of hydrometeorological processes.

Estimation of Environmental Behavior in Recent Years Based on the Viewpoint of Tritium (T) Concentration and Oxygen, Hydrogen Stable Isotope Ratios (δ<sup>18</sup>O, δD) in Precipitation

Estimation of Environmental Behavior in Recent Years Based on the Viewpoint of Tritium (T) Concentration and Oxygen, Hydrogen Stable Isotope Ratios (δ<sup>18</sup>O, δD) in Precipitation