Deuterium excess as a proxy for continental moisture recycling and plant transpiration

Abstract

Abstract. Studying the evaporation process and its link to the atmospheric circulation is central for a better understanding of the feedbacks between the surface water components and the atmosphere. Stable water isotopes are ideal tools to investigate surface evaporation as they are naturally available tracers of water phase changes in the atmosphere. The strength of isotope fractionation processes depends on environmental conditions such as relative humidity and temperature. In this study, we use five months of deuterium excess (d) measurements at the hourly to daily timescale from a cavity ring-down laser spectrometer to characterise the evaporation source of low-level continental water vapour at the long-term hydrometeorological monitoring site Rietholzbach in northeastern Switzerland. To reconstruct the phase change history of the air masses in which we measure the d signature and to diagnose its area of surface evaporation we apply an established Lagrangian moisture source diagnostic. With the help of a correlation analysis we investigate the strength of the relation between d measurements and the moisture source conditions. Temporal episodes with a duration of a few days of strong anticorrelation between d and relative humidity as well as temperature are identified. The role of plant transpiration, the large-scale advection of remotely evaporated moisture, the local boundary layer dynamics at the measurement site and recent precipitation at the site of evaporation are discussed as reasons for the existence of these modes of strong anticorrelation between d and moisture source conditions. The relation between d in atmospheric water vapour at the measurement site and the relative humidity conditions at the location of evaporation exhibits distinct characteristics for land surface evaporation and ocean evaporation. We show that the importance of continental moisture recycling and the contribution of plant transpiration to the continental evaporation flux can be deduced from the d-relative humidity relation at the seasonal timescale as well as for individual events. The slope of the relation between d and the diagnosed moisture source relative humidity provides a novel framework to estimate the transpiration fraction of land evapotranspiration at the local to continental scale. Over the whole analysis period (August to December 2011) a transpiration fraction of the evapotranspiration flux over the continental part of the moisture source region of 63% is found albeit with a large event-to-event variability (0% to 99%) for continental Europe. During days of strong local moisture recycling a higher overall transpiration fraction of 82% (varying between 65% and 94%) is found. Such Lagrangian estimates of the transpiration part of continental evaporation could potentially be useful for the verification of model estimates of this important land-atmosphere coupling parameter.

Plant Water δD and δ18O of Tundra Species from West Greenland

“…However, two pieces of evidence support this new precipitation estimate. First, O, and more positive d values are typically the result of (A) colder, drier air at the time of initial evaporation from sea water and/or (B) additional moisture recycling over land (Merlivat and Jouzel, 1979;Aemisegger et al, 2014;Pfahl and Sodemann, 2014). The d value for our precipitation estimate is 19‰, which is in good agreement with the available evidence.…”

Stable isotopes of atmospheric water vapour and precipitation in the northeast Qinghai‐Tibetan Plateau

“…a). Upon the rain's cessation, clouds clear, the ground heats up again, and RH decreases and partly due to warming, partly due to reduced humidity (Aemisegger et al, ). In this case, local evapotranspiration contributed to changes in water vapour δ 18 O, which quickly returned to relatively enriched values, as shown in Figure a (Deshpande et al, ).…”

Continental scale variation in 17O-excess of meteoric waters in the United States

“…Deviations from this line are expressed as d-excess, where d-excess = dD À 8 Â d 18 O (Dansgaard, 1964) and used to evaluate regional aridity, salinity of seawater, and storm tracks (Gat and Carmi, 1987;Swart et al, 1989;Sjostrom and Welker, 2009;Aemisegger et al, 2014).…”