The patterns and implications of diurnal variations in the d-excess of plant water, shallow soil water and air moisture

Abstract. Deuterium excess (d-excess) of air moisture is traditionally considered a conservative tracer of oceanic evaporation conditions. Recent studies challenge this view and emphasize the importance of vegetation activity in controlling the dynamics of air moisture d-excess. However, direct field observations supporting the role of vegetation in d-excess variations are not well documented. In this study, we quantified the d-excess of air moisture, shallow soil water (5 and 10 cm) and plant water (leaf, root and xylem) of multiple dominant species at hourly intervals during three extensive field campaigns at two climatically different locations within the Heihe River basin, northwestern China. The ecosystems at the two locations range from forest to desert. The results showed that with the increase in temperature (T) and the decrease in relative humidity (RH), the δD–δ¹⁸O regression lines of leaf water, xylem water and shallow soil water deviated gradually from their corresponding local meteoric water line. There were significant differences in d-excess values between different water pools at all the study sites. The most positive d-excess values were found in air moisture (9.3‰) and the most negative d-excess values were found in leaf water (−85.6‰). The d-excess values of air moisture (d_moisture) and leaf water (d_leaf) during the sunny days, and shallow soil water (d_soil) during the first sunny day after a rain event, showed strong diurnal patterns. There were significantly positive relationships between d_leaf and RH and negative relationships between d_moisture and RH. The correlations of d_leaf and d_moisture with T were opposite to their relationships with RH. In addition, we found opposite diurnal variations for d_leaf and d_moisture during the sunny days, and for d_soil and d_moisture during the first sunny day after the rain event. The steady-state Craig–Gordon model captured the diurnal variations in d_leaf, with small discrepancies in the magnitude. Overall, this study provides a comprehensive and high-resolution data set of d-excess of air moisture, leaf, root, xylem and soil water. Our results provide direct evidence that d_moisture of the surface air at continental locations can be significantly altered by local processes, especially plant transpiration during sunny days. The influence of shallow soil water on d_moisture is generally much smaller compared with that of plant transpiration, but the influence could be large on a sunny day right after rainfall events.