Short-term stable isotopic composition variations of near-surface atmospheric water vapor in four semiarid areas (Binxian, Guyuan, Wujiachuan, Yuzhong) in interior northwestern China

Abstract

This study used an in situ method to monitor short-term atmospheric water vapor concentrations and stable isotopic composition (δ18Ov and δDv) variations at four sites in semiarid areas of northwestern China during 2014. On a spatial scale, we distinguish a south-to-north difference in atmospheric water vapor concentration. The spatial patterns of water vapor lines (WVLs) may be associated with regional setting. Smaller slopes of WVLs in Guyuan and Yuzhong may reflect dew evaporation, and a steeper slope of WVL in Wujiachuan may be representative of atmospheric water vapor isotopic composition in equilibrium with that of precipitation. On a temporal scale, δ18Ov, δDv, and dv exhibited variable patterns, indicating various influences of entrainment, local evapotranspiration (ET), and dew evaporation. A deceasing trend of atmospheric water vapor δ18Ov and δDv in Wujiachuan indicates that the entrainment effect surpassed that of evaporation; an increasing trend in Guyuan and Yuzhong may be attributed to the effect of evaporation surpassing that of entrainment. Based on a precipitation event at Guyuan, the lateral air mass transported from Wujiachuan may be the reason for rapid recovery of atmospheric vapor isotopic composition to the baseline value and a looping pattern between atmospheric water vapor mixing ratio (w) and δ18Ov observed at Guyuan. Linear correlation coefficients of δ18Ov with w varied from 0.02 to 0.88 at four study sites. Location differences may be the reason for the capability of w to explain why the variability of atmospheric water vapor isotopic composition was greater at Wujiachuan than at Binxian. Dew evaporation after the precipitation event may be the major reason for eliminating the relationship between w and δ18Ov at Guyuan and Yuzhong. Although synoptic conditions were similar at the four sites, δv variations may be controlled by ET, entrainment, and dew formation at different sites and times.