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. In this study, we use 5 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 a 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. We show that the importance of continental moisture recycling and the contribution of plant transpiration to the continental evaporation flux may be deduced from the d-relative humidity relation at the seasonal timescale as well as for individual events. The methodology and uncertainties associated with these estimates of the transpiration fraction of evapotranspiration are presented and the proposed novel framework is applied to individual events from our data set. 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 62 % is found albeit with a large event-to-event variability (0 % to 89 %) for continental Europe. During days of strong local moisture recycling a higher overall transpiration fraction of 76 % (varying between 65 % and 86 %) is found. These estimates are affected by uncertainties in the assumptions involved in our method as well as by parameter uncertainties. An average uncertainty of 11 % results from the strong dependency of the transpiration estimates on the choice of the non-equilibrium fractionation factor. Other uncertainty sources like the influence of boundary layer dynamics are probably large but more difficult to quantify. Nevertheless, such Lagrangian estimates of the transpiration part of continental evaporation could potentially be useful for the verification of model estimates of this important landatmosphere coupling parameter.

Simultaneous monitoring of stable oxygen isotope composition in water vapour and precipitation over the central Tibetan Plateau

“…During heavy rain events, however, local evapotranspiration is extremely weak (Huang and Wen, 2014), because clouds and precipitation cool the surface and moisten the boundary layer, leading to high relative humidities ( Fig. 2c and h) (Aemisegger et al, 2014). Therefore, effects of local evapotranspiration on the changes in water vapour δ 18 O can be ignored during such rainy periods, and the corresponding δ 18 O values in water vapour become more depleted (Fig.…”

Deuterium excess in the atmospheric water vapour of a Mediterranean coastal wetland: regional vs. local signatures

“…Except for some work based on cold trap sampling (Jacob and Sonntag, 1991;Williams et al, 2004;Angert et al, 2008), there are to our knowledge only three high time-resolution studies published from European sites (Iannone et al, 2010;Kurita et al, 2012;Aemisegger et al, 2014) and none at all treating Mediterranean areas. In addition, there is a need for dual tracer databases, in order to use the deuterium excess signal (d = δD − 8 × δ 18 O; Dansgaard, 1964) as an additional indicator of atmospheric processes.…”

The stable isotopic composition of water vapour above Corsica during the HyMeX SOP1 campaign: insight into vertical mixing processes from lower-tropospheric survey flights

“…Under the first-order approximation that these properties remain unaffected by the condensation history, the d-excess can serve as a tracer of moisture origin and source conditions (Merlivat and Jouzel, 1979;Aemisegger et al, 2014;Pfahl and Sodemann, 2014). Moisture over the Mediterranean Sea is known to have relatively high values of d-excess.…”