ABB LGR | Application of ABB LGR liquid water isotope analyzer

Chenghai Lake Basin

Shrinkage of plateau lakes under climate strength has drawn growing attention. Because of its intricate implication to hydro-meteorological condition and climate system, stable isotopes in precipitation (e.g. δ²H_p and δ¹⁸O_p) provide us a powerful tool to understand the climate-hydrologic dynamics in shrinking lakes. Most studies, however, focused on either the impact of large-scale atmosphere circulation and moisture sources or the local isotopic fractionation processes. So far, limited study has yet been carried out on the combined effect of both the regional atmospheric circulation and local isotopic fractionation processes on precipitation stable isotope variability.

Based on this, to fill the research gap, in this article “Isotopic dynamics of precipitation and its regional and local drivers in a plateau inland lake basin, Southwest China”, researchers from Tsinghua University, Natural Resources Institute Finland (LUCK) and Faculty of Agriculture and Forestry, University of Helsinki conducted related research in the Chenghai Lake Basin (26°27′-26°38′N, 100°38′-100°41′E), aiming to: (1) explore the temporal dynamic of large-scale convection activities and moisture sources driving the isotopic variability in the wet season; (2) reveal the potential mechanisms causing the spatial disparity of isotope compositions in precipitation at the north and the south shore of the lake.

During the observation period (April 2019 to October 2019), the authors sampled the daily rainfall amounts greater than 5 mm at two sites (Fig. 1) and collected them in pre-rinsed polyethylene bottles, tightly capped and coolly preserved until analysis in Lab. They used a liquid water isotope analyzer (LGR-DLT100) to determine the δ²H_p and δ¹⁸O_p compositions of precipitation and calculate the deuterium excess (d-excess) to investigate non-equilibrium effects on the isotopic composition of precipitation. They also collected the daily (7 days before forming precipitation in the Chenghai Lake basin) and monthly OLR values at the marine areas (25.0°-27.5° N, 100°-102.5° E) including the India Ocean (25.0°-27.5° N, 100°-102.5° E), Bay of Bengal (25.0°-27.5° N, 100°-102.5° E) and South China Sea (25.0°-27.5° N, 100°-102.5° E). In addition, they determined the origin of air masses and establish source-receptor relationships based the hybrid single particle Lagrangian integrated trajectory (HYSPLIT, version 4.0) model.

Location of study area and rainfall sampling sites. NS and SS represent rainfall sampling sites located at the north shore and south shore, respectively.

Based on backward trajectories of air masses obtained from HYSPLIT model, 68% of moisture came from δ¹⁸O depleted ocean (Indian Ocean, Bay of Bengal, South China Sea and Pacific Ocean), and the rainout process promoted the isotopic depletion when moisture arrived at the study basin. Evapotranspiration increased the heavy isotope ratios in precipitation originated from continents (northern China inland and western continents). The temporal dynamics of δ¹⁸O_p and δ²H_p were in phase with the convection activities intensity underlined the influence from largescale atmospheric circulation. Local meteorological factors played a secondary role in isotope variability. Precipitation amount-effect strongly affected isotope ratios while mild anti-temperature effect was observed at daily scale. Interestingly, the rainfall isotope ratios showed different mechanisms in govern at lake south shore and north shore, with a distance of 19 km in between. This south-to north difference can be explained by either lower 1.03% sub-evaporation in the south shore or 7% of recycled moisture contributing to precipitation in the north shore.

Temporal characteristics of δ¹⁸O_p (solid square), d-excessp (hollow triangle) and meteorological parameters, including relative humidity (black), air temperature (blue) (a) and precipitation amount (b) at 2 m above ground level in the Chenghai Lake basin. The P1- P5 periods were divided according to the variation trend of precipitation isotope values, precipitation amounts and the meteorological factors (precipitation, relative humidity and temperature).

The daily scattering of δ¹⁸O_p and δ²H_p in precipitation and the linear fitted local meteoric water lines (LMWLs). The bubble size represents the d-excess_p value scale. The grey shadow ellipse region is affected by the sub-cloud evaporation.

Boxplot showing the distribution of δ¹⁸O_p, δ²H_p and d-excess_p of the south (SS) and the north precipitation (NS) of daily-based datasets in P2 (a-c), P4 (d-f), P5 (g-i) and the whole P1-P5 period (j-l).

This study investigated the drivers of the temporal and spatial change of precipitation isotope composition, and threw light on the climate-hydrological isotope construction. The regional convection activities and moisture recycling drove temporal isotopic variations, and sub-cloud evaporation or recycled moisture controlled the spatial pattern during the wet season in the Chenghai Lake basin, Southwest China. Marine moisture contributed to 68% of local precipitation and moisture from the Indian Ocean (including OIO, OBB and OSP) was the predominant contributor. The minimum δ¹⁸O_p during the monsoon season was associated with strong convective activity, and the rainout effect depleted the isotope composition along the moisture transporting pathway. These findings discover the driving forces for δ¹⁸O_p variation and provide solid interpretations for hydro-climate change in Southwest China. The authors think that more robust conclusions could be obtained with a larger set of samples. Further work for at least 48 months continuous isotopic precipitation records in the Chenghai Lake basin is encouraged.

6376420452627898998291907.pdf