Vertical distribution of stable isotopic composition in atmospheric water vapor and subsurface water in grassland and forest sites, eastern Mongolia
Introduction
The stable isotopic composition of atmospheric moisture is intimately related to its origin and phase changes, and the isotopic ratios of atmospheric water vapor provide critical information about the mechanisms of evaporation from the earth’s surface and subsequent transport in the atmosphere (Gat and Matsui, 1991, He and Smith, 1999). Generally, the stable hydrogen and oxygen isotopes in precipitation are used for tracing the transport of water vapor in the atmosphere (e.g., Yamanaka et al., 2002, Yoshimura et al., 2003, Kurita et al., 2003).
Ehhalt, 1974, Taylor, 1984 measured the profile of the deuterium isotope ratio in atmospheric water vapor in the troposphere and showed a high correlation between the specific humidity and isotope ratio in the lower and middle troposphere, suggesting continuous depletion of heavy isotopes through the Rayleigh fractionation process. Moreover, Smith (1992) observed the isotopic ratio in water vapor and cloud ice in the troposphere over the ocean, and found detraining fractionated cloud vapor to be the primary source of atmospheric water vapor in the middle troposphere, whereas lofted ice carried up by convective clouds and then detrained into the environment provides atmospheric vapor in the upper troposphere. Gedzelman (1988) performed a modeling study to interpret the deuterium isotope ratio in water vapor above the atmospheric boundary layer (ABL) using experimental data. However, these previous studies did not take into account the processes occurring within the ABL.
He and Smith (1999) measured deuterium and oxygen 18 isotopic ratios at multiple levels in the ABL using an aircraft at altitudes up to 3 km, and showed constant δD and δ18O in the mixed region of the ABL and sharply decreasing values near the top. They also discussed thermodynamic mixing of air near the surface and air from the free atmosphere with turbulence in the ABL and its effect on the isotopic ratio profiles in water vapor. Thus, data of δD and δ18O throughout the ABL are essential in elucidating the source and transport process of water vapor existing over the earth’s surface. The evaporation of water vapor from the ground surface and transpiration of water vapor via vegetation are also important components contributing to atmospheric water vapor in the ABL. Moreover, partitioning of evapotranspiration into evaporation and transpiration is necessary for investigating the source of atmospheric water vapor, and for this purpose, the stable isotopic composition has been used (Moreira et al., 1997, Wang and Yakir, 2000, Yepez et al., 2003).
Mongolia is situated in the north-eastern region of the Eurasian Continent, and in recent years has experienced climatic warming and drying. The mean air temperature in winter has increased by approximately 5 °C during the past 30 years (Yasunari et al., 1998). Moreover, previous estimates of monthly average precipitation and evapotranspiration using the atmosphere-water budget method from 1979 to 1993 showed that the mean precipitation is nearly equal to the mean evapotranspiration in summer, suggesting that precipitation is formed by evapotranspired water vapor in this region, and that recycling of water plays an important role (Yasunari et al., 2000). However, no evidence was provided to support this interpretation.
In north-eastern Asia including Mongolia, as a consequence of the steep meridional gradient in climatic conditions, a distinct ecotone of forest–steppe–desert is formed (Sugita et al., 2006). At the interface of forest and steppe, the northern part of mountain slopes is often covered with forest, while the south facing slope is covered with grass only. In such regions, different sources and processes of transport of atmospheric water vapor might therefore cause distinctive stable isotopic compositions of water vapor in the ABL over the forest and steppe.
In this study, we focus on the difference in the water vapor source and evapotranspiration phenomena between forest and steppe regions in Mongolia. The objectives are to determine the stable isotopic composition of deuterium and oxygen 18 in the water vapor at multiple altitudes in the ABL, and to evaluate the contribution of transpiration in evapotranspiration occurring in the forest and grassland regions.
Section snippets
Study area
The present study was performed in a region situated from N 48° 30′ to N 46° 30′ and E 108° 15′ to E 110° 45′ in eastern Mongolia (Fig. 1), corresponding to an area of headwater to mid-stream in Kherlen River basin. The annual precipitation ranges from 150 mm in the south-eastern region to 300 mm in the headwater region, though more than 70% falls during three months from June through August and snowfall is scarce (Sugita et al., 2006). The vegetation ranges from steppe in the mid-stream region to
Methods
Intensive sampling of atmospheric water vapor, soil water and precipitation was performed at FOR and KBU from 8th to 19th June, 18th to 24th July, 18th to 23rd August and 29th September to 4th October, 2003. Air temperature, relative humidity, latent heat flux, soil water content and precipitation have been monitored since March, 2003 (see Sugita et al., 2006, for details of the measurement system).
Daily and monthly precipitation were sampled at MNG and KBU sites from October 2002 through
The stable isotopic composition of meteoric water and soil water
The relationship between δ18O and δD in the meteoric water and soil water is shown in Fig. 4, and the average values are listed in Table 1. All data distributed along the local meteoric water line in the Kherlen River basin except for the soil water in the grassland site (KBU). The soil water showed a higher isotopic ratio (−19‰ < δ18O < 0‰) than the atmospheric water vapor (−32‰ < δ18O < −16‰) in both KBU and FOR. Moreover, the soil water in the grassland site tended to show a higher isotopic ratio
Conclusions
The stable isotopes of deuterium and oxygen 18 in precipitation, atmospheric water vapor and subsurface water were investigated in a forest site and grassland site in a semi-arid region in the Kherlen River basin, eastern Mongolia. Atmospheric water vapor was sampled throughout the ABL up to a height of 1000 m using an aircraft plus sampling on the ground. Meteorological factors were also monitored at both sites.
The δ18O of precipitation in the grassland and forest sites showed clear seasonal
Acknowledgements
The authors would like to thank Dr. D. Azzaya, Dr. G. Davaa, and Dr. D. Oyunbaatar of the Institute of Meteorology and Hydrology, the National Agency for Meteorology, and Hydrology and Environment Monitoring of Mongolia, for facilitating local logistics. This study was supported by the Japan Science and Technology Agency through a grant under the Core Research for Evolutional Science and Technology (CREST) program funded by the RAISE project. Partial support was also obtained from the Global
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