Isotopic Composition Of Atmospheric Vapor Research Articles

Intra-annual variations in isotopic composition of atmospheric water vapour in a semi-arid monsoon region: observations and inferences

The isotopic composition of atmospheric water vapour dynamically changes due to the complex interplay between the moisture-contributing sources (precipitation, evaporating soil moisture, transpiration, evaporating water bodies, etc.), operating ecohydrological processes, and the climatic conditions. In the present study, the isotopic composition of different sources of water was sampled; air moisture (n = 47) twice a month for the period December, 2021 to January, 2023; transpiration samples (n = 76); evaporation and rain samples (n = 12 each). The samples were analyzed using the mass spectrometry to investigate the dynamic nature of the isotopic composition of the atmospheric vapour and the factors influencing it. The research findings showed that the isotopic composition of atmospheric moisture rapidly depleted after the onset of the monsoon, indicating the replacement of pre-monsoon local vapour by monsoon moisture of depleted isotopic composition. The process of evaporation from the soil surface during the post-monsoon period was found to progress parallel to other soil hydrological processes, such as capillary action and diffusion-driven downward flux of enriched water molecules. This resulted in the broadening of an isotopically enriched layer of soil moisture in the vadose zone with time, and accordingly, there was transpiration of this enriched soil moisture from the deeper layers of soils with time by plants/trees. Evaporation of soil water and the transpiration of plants and trees of enriched water vapour gradually altered the isotopic composition of the atmospheric water vapour composition from the depleted isotopic composition of the monsoon period to an enriched isotopic composition. The transpiration of enriched water vapour from deep-rooted trees indicated more prolonged arid climate conditions. The findings of the study are useful in understanding the regional hydrological and hydroclimatic processes which can be implemented in agro-climatic studies and water resource management.

A modified isotope-based method for potential high-frequency evapotranspiration partitioning

To better understand water and energy cycles, numerous efforts to partition evapotranspiration (ET) into evaporation (E) and transpiration (T) have been made over the recent half century. One of the analytical methods is the isotopic approach. The isotopic composition of ET (δET) is a crucial parameter in the traditional isotope-based ET partitioning model, which however, has considerable uncertainty and high sensitivity. Here we proposed a modified T fraction in total ET (FT) calculation using Keeling plot slope (k), the atmospheric vapor concentration (Cv), and the isotopic composition of atmospheric vapor (δv), to avoid the direct use of δET. Following the traditional method, we used the Craig-Gordon model for the isotopic composition of evaporation (δE) and chamber method for the isotopic composition of transpiration (δT) in our modified method. The modified FT calculation method (FT (m)) can be applied at a 15-min time scale using the average values (FTi (mp)) and at a 1 Hz time scale for high-frequency method (FTi). The modified method was verified by both theoretical derivations and field observations. FTi (mp) was equivalent to those using the traditional isotopic method at a 15-min time scale. However, FTi eliminated the highly sensitive parameter δET, and redistributed the sensitivity of δET into three less sensitive parameters. Additionally, FTi has two main advantages. First, the high-frequency method avoids the extrapolation of the Keeling plot regression line intercept. Second, the high-frequency method can produce a 95% confidence interval of FT in a measurement cycle (e.g., 15 min). The calculated confidence interval was different from that of traditional uncertainty analysis. The high-frequency method might be useful when investigating evapotranspiration partitioning under short-term extreme weather events and flush agricultural irrigation.

Comparison of atmospheric water vapour δ18O and δ2H estimated using evaporation pan, rainfall equilibrium and continuous measurements

For a period of 16 months in Sydney, Australia, the variations of 2H/1H and 18O/16O in atmospheric vapour (δ2HA and δ18OA) were estimated using an evaporation pan method as well as using the isotopic precipitation-equilibrium approach. These calculations were then compared with δ2HA values measured at 10 m above ground surface using a Fourier Transform Infrared Spectrometer (FTIR). As pan isotopic composition was available on a weekly time scale, the evaporation rates were measured daily, and the atmospheric variables were available hourly, the weekly time scale was used to calculate the arithmetic averages of the atmospheric variables that were used in the estimation of the pan-derived δ2HA.Good agreement (r = 0.7, P-value = 0.00) was found between the pan-derived and the FTIR measured δ2HA for weekly intervals, although individual differences ranged from −25.0 to 20.4‰, with the absolute difference averaging 8.0‰. A sensitivity analysis showed that the determination of δ2HA is most sensitive to air temperature, relative humidity and the isotopic composition of the pan water.While the precipitation-equilibrium approach only appears to be representative of atmospheric conditions close to times of precipitation events, the pan-derived isotopic composition of atmospheric vapour was found to be closer to the FTIR averages over longer periods including intervals with no precipitation. Overall, this means that the pan method is far more effective for uninterrupted estimation of δ2HA and δ18OA of atmospheric water vapour, as required for water budget studies, than the precipitation-equilibrium method, and it is more cost effective and robust than continuous measurement.

Pan-derived isotopic composition of atmospheric vapour in a Mediterranean wetland (Rhône River Delta, France)†

A continuous record of atmospheric vapour isotopic composition (δA) can be derived from the isotope mass balance of a water body submitted to natural evaporation. In this paper, we present preliminary results of the application of this method to a drying evaporation pan, located in a Mediterranean wetland, during a two-month summer period. Results seem consistent with few atmospheric vapour data based on the assumption of isotopic equilibrium with precipitation, but we observed a shift between pan-derived δA and the composition of vapour samples collected by cold trapping. These results suggest that further investigations are necessary to evaluate the effect of diurnal variations of atmospheric conditions on the applicability of the pan-evaporation method, and on the representative of grab atmospheric samples. We also propose a sensitivity analysis for evaluating the impact of the different measured components on δA calculation, and show an improvement in the method efficiency as the pan is drying.

Observations of hydrogen and oxygen isotopes in leaf water confirm the craig-gordon model under wide-ranging environmental conditions

The Craig-Gordon evaporative enrichment model of the hydrogen (deltaD) and oxygen (delta(18)O) isotopes of water was tested in a controlled-environment gas exchange cuvette over a wide range (400 per thousand deltaD and 40 per thousand delta(18)O) of leaf waters. (Throughout this paper we use the term "leaf water" to describe the site of evaporation, which should not be confused with "bulk leaf water" a term used exclusively for uncorrected measurements obtained from whole leaf water extractions.) Regardless of how the isotopic composition of leaf water was achieved (i.e. by changes in source water, atmospheric vapor deltaD or delta(18)O, vapor pressure gradients, or combinations of all three), a modified version of the Craig-Gordon model was shown to be sound in its ability to predict the deltaD and delta(18)O values of water at the site of evaporation. The isotopic composition of atmospheric vapor was shown to have profound effects on the deltaD and delta(18)O of leaf water and its influence was dependent on vapor pressure gradients. These results have implications for conditions in which the isotopic composition of atmospheric vapor is not in equilibrium with source water, such as experimental systems that grow plants under isotopically enriched water regimes. The assumptions of steady state were also tested and found not to be a major limitation for the utilization of the leaf water model under relatively stable environmental conditions. After a major perturbation in the deltaD and delta(18)O of atmospheric vapor, the leaf reached steady state in approximately 2 h, depending on vapor pressure gradients. Following a step change in source water, the leaf achieved steady state in 24 h, with the vast majority of changes occurring in the first 3 h. Therefore, the Craig-Gordon model is a useful tool for understanding the environmental factors that influence the hydrogen and oxygen isotopic composition of leaf water as well as the organic matter derived from leaf water.

Geochemical evidence for groundwater behavior in an unconfined aquifer, south Florida

Five well sites have been investigated along an east-west transect across the surfical aquifer system (SAS) of south Florida. Differences between rainfall during wet seasons (June–October) and evaporation during dry seasons (November–May) give surface waters of this region isotopically light ( δ 18 O −22‰ and δ D −7.6‰ ) and heavy ( δ 18 O +4.2‰ ) compositions, respectively. Surface waters and shallow groundwaters are enriched in 18O and D to the west, which is consistent with westward decrease in equal excess of rainfall. In the shallow portion of the SAS (less than 20 m, Biscayne sub-aquifer) heterogeneous stable isotopic compositions occur over short spans of time (less than 90 days), reflecting seasonal changes in the isotopic composition of recharge and rapid flushing. Homogeneous stable isotopic compositions occur below the Biscayne sub-aquifer, marking the zone of delayed circulation. Surface evaporation calculated from a stable isotope evaporation model agrees with previously published estimates of 75–95% by physical evaporation measurements and water budget calculations. This model contains many parameters that are assumed to be mean values, but short-term variability in some of these parameters may make this model unsuitable for the application of yearly mean values. For the Everglades, changes in the isotopic composition of atmospheric vapor during the dry season may cause the model to yield anomalous results when annual mean values are used. Chloride-enriched waters (more than 280 mg 1 −1) form a plume emanating from the bottom central portion of the transect. Elevated chloride concentration and light stable isotopic composition ( δ 18 O ≈ −2‰ , δ D ≈ −8‰ ) suggest this plume is probably caused not by salinity of residual seawater in the aquifer, but by leakage from the minor artesian water-bearing zone of the Floridan aquifer system. Stable isotope values from Floridan aquifer groundwater plot close to the meteoric water line, in the same area as Everglades rainfall. These Floridan waters are interpreted to have originated in central Florida some 25 000–132 000 years ago, indicating that meteoric conditions in the Florida peninsula have changed little since late Pleistocene time.

Estimating Evaporation Using Stable Isotopes: Quantitative Results and Sensitivity Analysis for Two Catchments in Northern Canada

The stable isotope-mass balance method can provide useful water balance information in ungauged catchments. The method has been used to evaluate evaporation and water balance at two contrasting sites in northern Canada. Areally weighted evaporative discharge from an 850 km2 tundra catchment in south-central District of Keewatin is estimated to be about 7 % of total water discharge (≈ 16 mm/yr), compared to about 19 % (≈ 65 mm/yr) from a 300 km2 forested watershed in the Upper Mackenzie Valley. Lakes in both watersheds exhibit broad ranges of evaporation/inflow ratios related to local water balance. The potential errors in the estimates are evaluated through consideration of possible variations in basin storage, humidity, and the isotopic composition of atmospheric vapour.