Stable isotopes of atmospheric water vapour and precipitation in the northeast Qinghai‐Tibetan Plateau

Abstract

Stable water isotopes (δ18O and δ2H) are an important source signature for understanding the hydrological cycle and altered climate regimes. However, the mechanisms underlying atmospheric water vapour isotopes in the northeast Qinghai‐Tibetan Plateau of central Asia remain poorly understood. This study initially investigated water vapour isotopic composition and its controls during the premonsoon and monsoon seasons. Isotopic compositions of water vapour and precipitation exhibited high variability across seasons, with the most negative average δ18O values of precipitation and the most positive δ18O values of water vapour found during the premonsoon periods. Temperature effect was significant during the premonsoon period but not the monsoon period. Both a higher slope and intercept of the local meteoric water line were found during the monsoon period as compared with in the premonsoon period, suggesting that raindrops have been experienced a greater kinetic fractionation process such as reevaporation below the cloud during the premonsoon periods. The δ2H and δ18O signatures in atmospheric water vapour tended to be depleted with the occurrence of precipitation events especially during the monsoon period and probably as a result of rainout processes. The monthly average contribution of evaporation from the lake to local precipitation was 35.2%. High d‐excess values of water vapour were influenced by the high proportion of local moisture mixing, as indicated by the gradually increasing relative humidity along westerly and Asian monsoon trajectories. The daily observation (observed ε) showed deviations from the equilibrium fractionation factors (calculated ε), implying that raindrops experienced substantial evaporative enrichment during their descent. The average fraction of raindrops reevaporation was estimated to be 16.4± 12.9%. These findings provide useful insights for understanding the interaction between water vapour and precipitation, moisture sources, and help in reconstructing the paleoclimate in the alpine regions.

Hydrological functioning of thawing soil water in a permafrost‐influenced alpine meadow hillslope

“…The LMWL had a slope of 8.4 and an intercept of 23.2, which were both higher than the slope and the intercept of the global meteoric water line (GMWL) (8.0 and 10, respectively). This might be a result of the nonequilibrium fractionation during the transport of atmospheric water vapor and an elevation effect (Wu et al., 2019). The more gentle slopes of the mobile soil water line (MSWL) and the bulk soil water line (BSWL) compared with the LMWL indicated nonequilibrium fractionation and the influence of evaporation (Dansgaard, 1964) (Figure 2).…”

Vegetation History and Precipitation Changes in the NE Qinghai‐Tibet Plateau: A 7,900‐years Pollen Record From Caodalian Lake

“…The pattern of changes in δ 18 O ostr and δD wax in the Qinghai Lake during the Holocene disagree with the P ann record, indicating that δ 18 O p and δD p are not solely controlled by precipitation amount. The range of factors controlling δ 18 O p and δD p in the region is complex, and factors other than precipitation, such as moisture sources and raindrop re‐evaporation, could also contribute to δ 18 O p and δD p (Cui et al., 2015; Henderson et al., 2010; Tian et al., 2003; H. W. Wu et al., 2019). For example, there were multiple moisture sources in the Qinghai Lake basin during the Holocene, as mentioned in Section 5.3; however, it is unclear how these different moisture sources affected the fluctuations in δ 18 O p and δD p .…”

“…The pattern of changes in δ 18 O ostr and δD wax in the Qinghai Lake during the Holocene disagree with the P ann record, indicating that δ 18 O p and δD p are not solely controlled by precipitation amount. The range of factors controlling δ 18 O p and δD p in the region is complex, and factors other than precipitation, such as moisture sources and raindrop re-evaporation, could also contribute to δ 18 O p and δD p (Cui et al, 2015;Henderson et al, 2010;Tian et al, 2003;H. W. Wu et al, 2019).…”

Isotopic fractionation induced by a surface effect influences the estimation of the hydrological process of topsoil

“…The surface effect could be quantified by comparing the isotopic levels of soil water and unconfined labelling water. If there is no surface effect as a result of clay or OM, the δ 18 O and δ 2 H content in soil water and the unconfined labelling water should be identical after reaching equilibrium (Penchenat et al, 2020; Wu et al, 2019) and vice versa. These two pools may complicate the positional relationship between SWL and LMWL and thus, affect estimations of the hydrological processes in topsoil.…”