Abstract
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 (dmoisture) and leaf water (dleaf) during the sunny days, and shallow soil water (dsoil) during the first sunny day after a rain event, showed strong diurnal patterns. There were significantly positive relationships between dleaf and RH and negative relationships between dmoisture and RH. The correlations of dleaf and dmoisture with T were opposite to their relationships with RH. In addition, we found opposite diurnal variations for dleaf and dmoisture during the sunny days, and for dsoil and dmoisture during the first sunny day after the rain event. The steady-state Craig–Gordon model captured the diurnal variations in dleaf, 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 dmoisture 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 dmoisture is generally much smaller compared with that of plant transpiration, but the influence could be large on a sunny day right after rainfall events.
Highlights
Measurements of water isotopic compositions provide insights into the study of hydrologic cycles, ecological processes, and palaeoclimates across multiple temporal and spatial scales (e.g., Brunel et al, 1992; Gat, 1996; Dawson et al, 2002; Newman et al, 2010; Wang et al, 2010, 2013, 2014; Zhang et al, 2011; Good et al, 2012)
The questions we addressed in this study are the following: (1) what are the diurnal patterns of d-excess in air moisture, leaves, roots, xylem and shallow soil water under different climatic and meteorological conditions? (2) What are the mechanisms of the observed patterns and their controlling factors? (3) How well do the widely used steady-state models capture the leaf d-excess dynamics?
The results showed that T, relative humidity (RH) and photosynthetically available radiation (PAR) varied significantly with the meteorological conditions and locations (Fig. 2)
Summary
Measurements of water isotopic compositions (e.g., δD, δ18O) provide insights into the study of hydrologic cycles, ecological processes, and palaeoclimates across multiple temporal and spatial scales (e.g., Brunel et al, 1992; Gat, 1996; Dawson et al, 2002; Newman et al, 2010; Wang et al, 2010, 2013, 2014; Zhang et al, 2011; Good et al, 2012). L. Zhao et al.: Diurnal variations in the d-excess of different water pools hydraulic redistribution (Dawson, 1993). The isotopic compositions of water from different areas are affected by specific meteorological processes, which provide a characteristic fingerprint of their origin (Clark and Fritz, 1997). Fewer investigations were conducted to measure simultaneously δD and δ18O of leaf water, xylem water, shallow soil water and air moisture, especially on the diurnal variations in these pools at ecosystem scale
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