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

摘要

Deuterium excess (d-excess) of airmoisture is traditionally considered a conservative tracer of oceanicevaporation conditions. Recent studies challenge this view and emphasize theimportance of vegetation activity in controlling the dynamics of air moistured-excess. However, direct field observations supporting the role ofvegetation in d-excess variations are not well documented. In this study, wequantified 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 hourlyintervals during three extensive field campaigns at two climaticallydifferent locations within the Heihe River basin, northwestern China. Theecosystems at the two locations range from forest to desert. The resultsshowed that with the increase in temperature (T) and the decrease inrelative humidity (RH), the δD–δ18O regression lines ofleaf water, xylem water and shallow soil water deviated gradually from theircorresponding local meteoric water line. There were significant differencesin d-excess values between different water pools at all the study sites. Themost positive d-excess values were found in air moisture (9.3‰) andthe 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 sunnydays, and shallow soil water (d_soil) during the first sunny dayafter a rain event, showed strong diurnal patterns. There were significantlypositive relationships between d_leaf and RH and negativerelationships between d_moisture and RH. The correlations ofd_leaf and d_moisture with T were opposite to theirrelationships with RH. In addition, we found opposite diurnal variations ford_leaf and d_moisture during the sunny days, and ford_soil and d_moisture during the first sunny day afterthe rain event. The steady-state Craig–Gordon model captured the diurnalvariations ind_leaf, with small discrepancies in the magnitude. Overall, thisstudy provides a comprehensive and high-resolution data set of d-excess ofair moisture, leaf, root, xylem and soil water. Our results provide directevidence that d_moisture of the surface air at continentallocations can be significantly altered by local processes, especially planttranspiration during sunny days. The influence of shallow soil water ond_moisture is generally much smaller compared with that of planttranspiration, but the influence could be large on a sunny day right afterrainfall events.