Deuterium excess reveals diurnal sources of water vapor in forest air

Abstract

An understanding of atmospheric water vapor content and its isotopic composition is important if we are to be able to model future water vapor dynamics and their potential feedback on future climate change. Here we present diurnal and vertical patterns of water isotope ratios in forest air (δ(2)H(v) and δ(18)O(v)) not observed previously. Water vapor observed at three heights over 3 consecutive days in a coniferous forest in the Pacific Northwest of the United States, shows a stratified nocturnal structure of δ(2)H(v) and δ(18)O(v), with the most positive values consistently observed above the canopy (60 m). Differences between 0.5 m and 60 m range between 2-6‰ for δ(18)O and 20-40‰ for δ(2)H at night. Using a box model, we simulated H(2)O isotope fluxes and showed that the low to high δ(2)H(v) and δ(18)O(v) profiles can be explained by the vapor flux associated with evaporation from the forest floor and canopy transpiration. We used d-excess as a diagnostic tracer to identify processes that contribute to the diurnal variation in atmospheric moisture. Values of d-excess derived from water vapor measurements showed a repeated diel pattern, with the lowest values occurring in the early morning and the highest values occurring at midday. The isotopic composition of rain water, collected during a light rain event in the first morning of our experiment, suggested that considerable below-cloud secondary evaporation occurred during the descent of raindrops. We conclude that atmospheric entrainment appears to drive the isotopic variation of water vapor in the early morning when the convective boundary layer rapidly develops, while evapotranspiration becomes more important in the mid-afternoon as a primary moisture source of water vapor in this forest. Our results demonstrate the interplay between the effects of vegetation and boundary layer mixing under the influence of rain evaporation, which has implications for larger-scale predictions of precipitation across the terrestrial landscape.

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

“…6). The same trends were also found in urban settings (New Haven and Beijing), agricultural settings (Rosemount and Luancheng), forest (Borden Forest) and grassland (Duolun) (Welp et al, 2012), at one Beijing site (Wen et al, 2010), and in the Pacific Northwest (Lai and Ehleringer, 2011). These results showed that d moisture diurnal variation is not a pattern unique to any particular location or vegetation type, and the diurnal pattern of d moisture may suggest that d moisture is not a conserved tracer of humidity conditions in the marine moisture source region (Welp et al, 2012), and is strongly controlled by local evaporation and transpiration.…”

“…The lowest d moisture values are found near the ground (1.5 ‰) at high altitudes during September (S1-Sep). The low values may be related to the atmospheric entrainment contribution, as atmospheric entrainment has been found to be responsible for the low d-excess values observed in the Pacific Northwest (Lai and Ehleringer, 2011). In our study, during the sunny days, the d moisture values vary diurnally, showing a clear and robust pattern of the highest d moisture values at midday, and the lowest d moisture values at night at all the sites (Fig.…”

“…The main moisture sources of local air moisture come from canopy transpiration, soil evaporation and atmospheric entrainment (Lai and Ehleringer, 2011). If d moisture is a conservative tracer of conditions in the moisture source region, we would not expect it to vary with local relative humidity unless there is a local source of moisture for patterns of d leaf (Fig.…”

“…In addition, modeling simulations also showed that plant transpiration plays an important role in diurnal d-excess variations (Welp et al, 2012), which contradicts the conventional understanding. Based on isotopic observations from a US Pacific Northwest temperate forest and a modeling exercise, Lai and Ehleringer (2011) concluded that atmospheric entrainment appears to drive the isotopic variation in water vapor in the early morning when the convective boundary layer develops rapidly, while evapotranspiration becomes more important in mid-afternoon as a primary moisture source of water vapor in the studied forest. These authors therefore also cast some doubts on whether continental water vapor d-excess can be used as a conserved tracer of environmental conditions during evaporation at the moisture source.…”

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

“…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. Indeed, d is often considered as a indicator of evaporation conditions, but its interpretation in continental vapour remains complex since it is affected by the multiple vapour sources, combining the initial oceanic vapour and evapotranspiration from different continental water sources (Gat et Matsui, 1991;Vallet-Coulomb et al, 2008;Lai and Ehleringer, 2011;Welp et al, 2012;Jouzel et al, 2013;Aemisseger et al, 2014).…”