Factors controlling diurnal variation in the isotopic composition of atmospheric water vapour observed in the taiga, eastern Siberia

Abstract

Deciduous forest covers vast areas of permafrost under severe dry climate in eastern Siberia. Understanding the water cycle in this forest ecosystem is quite important for climate projection. In this study, diurnal variations in isotopic compositions of atmospheric water vapour were observed in eastern Siberia with isotope analyses of precipitation, sap water of larch trees, soil water, and water in surface organic layer during the late summer periods of 2006, 2007, and 2008. In these years, the soil moisture content was considerably high due to unusually large amounts of summer rainfall and winter snowfall. The observed sap water d 18 O ranged from À17.9% to À13.3%, which was close to that of summer precipitation and soil water in the shallow layer, and represents that of transpired water vapour. On sunny days, as the air temperature and mixing ratio rose from predawn to morning, the atmospheric water vapour d 18 O increased by 1% to 5% and then decreased by about 2% from morning to afternoon with the mixing ratio. On cloudy days, by contrast, the afternoon decrease in d 18 O and the mixing ratio was not observed. These results show that water vapour that transpired from plants, with higher d 18 O than the atmospheric water vapour, contributes to the increase in d 18 O in the morning, whereas water vapour in the free atmosphere, with lower d 18 O, contributes to the decrease in the afternoon on sunny days. The observed results reveal the significance of transpired water vapour, with relatively high d 18 O, in the water cycle on a short diurnal time scale and confirm the importance of the recycling of precipitation through transpiration in continental forest environments such as the eastern Siberian taiga. Figure 4. Diurnal variations in d-excess and d 18 O of atmospheric water vapour (d 18 O V ), VPD, atmospheric mixing ratio of water vapour (Q) and air temperature (T) at 6 m height (dotted line) and 24 m height (solid line), and wind speed (thin solid line) and direction (cross) at 32 m height, observed on

Formation Chronology of Arsain Pingo, Darhad Basin, Northern Mongolia

“…The isotopic proportions of water‐derived hydrogen and oxygen were analyzed by the CO 2 /H 2 /H 2 O equilibration method using Delta V (Thermo Fisher Scientific, USA, manufactured in Germany) attached to a Gas Bench (Thermo Fisher Scientific, USA) at Hokkaido University, Japan (e.g. Ueta et al ., ). The data were expressed as δ D or δ 18 O values, defined as δ Sample (‰) = ( R Sample / R VSMOW − 1) × 1000, where R is the isotope ratio of water (D/H or 18 O/ 16 O), and subscripts Sample and VSMOW refer to samples and Vienna Standard Mean Ocean Water, respectively.…”

A soil water distillation technique using He-purging for stable isotope analysis

“…Studies of the isotopic distribution in atmospheric water vapour, precipitation, sap water, and soil water have led to an improved understanding of continental water circulation and enabled the estimation of the contribution of transpiration fluxes to atmospheric water vapour in forest ecosystems (Dawson et al, 2002;Li et al, 2007;Plamboeck et al, 1999;Sugimoto et al, 2003). The stable isotopic compositions of water in trees and their surrounding soil has revealed the primary water sources of trees (Busch et al, 1992;Dawson, 1998;Dawson and Ehleringer, 1991;Tsujimura et al, 2007;Ueta et al, 2012) and clarified the distinguishing characteristics of these sources for plants in permafrost ecosystems (Sugimoto et al, 2002).…”

Contribution of transpiration to the atmospheric moisture in eastern Siberia estimated with isotopic composition of water vapour

“…Under a high temperature condition, active plant transpiration is expected, which returns a water vapour with high δ 18 O to the atmosphere. Our previous study on diurnal variation of atmospheric water vapour has been revealed that plants transpire the water vapour with δ 18 O ranging from −17·9 to −13·3‰, which was much higher than those of atmospheric water vapour (Ueta et al ., ). Taking into consideration that the study site locates in the vast forested area in eastern Siberia, it is quite reasonable that plant transpiration was attributable to the high δ 18 O, when plant transpiration was active under warm condition.…”

“…(). Precipitation, plant sap water, soil water and water in surface organic layer were also sampled, and data for those were described in Ueta et al ., (in press) as well.…”

“…On the other hand, observations on longer time scale more than a few days revealed importance of different factors such as weather cycle (Lee et al ., ) or contribution of horizontal advection (Sato et al ., ). In eastern Siberia, we found a clear diurnal variation in the isotopic composition of atmospheric water vapour in taiga during summer period and revealed that the variation was caused by the contribution of plant transpiration and entrainment of free atmosphere in sunny days (Ueta et al ., ). However, we also found a large difference in the isotopic composition of water vapour from day to day and process of this variation on the longer time scale has not yet understood.…”