The role of dew and radiation fog inputs in the local water cycling of a temperate grassland during dry spells in central Europe

Abstract

Abstract. During dry spells, non-rainfall water (hereafter NRW) mostly formed from dew and fog potentially plays an increasingly important role in temperate grassland ecosystems with ongoing global warming. Dew and radiation fog occur in combination during clear and calm nights, and both use ambient water vapor as a source. Research on the combined mechanisms involved in NRW inputs to ecosystems is rare, and distillation of water vapor from the soil as a NRW input pathway for dew formation has hardly been studied. Furthermore, eddy covariance (EC) measurements are associated with large uncertainties on clear, calm nights when dew and radiation fog occur. The aim of this paper is thus to use stable isotopes as tracers to investigate the different NRW input pathways into a temperate Swiss grassland at Chamau during dry spells in summer 2018. Stable isotopes provide additional information on the pathways from water vapor to liquid water (dew and fog) that cannot be measured otherwise. We measured the isotopic composition (δ18O, δ2H, and d=δ2H-8⋅δ18O) of ambient water vapor, NRW droplets on leaf surfaces, and soil moisture and combined them with EC and meteorological observations during one dew-only and two combined dew and radiation fog events. The ambient water vapor d was found to be strongly linked with local surface relative humidity (r=-0.94), highlighting the dominant role of local moisture as a source for ambient water vapor in the synoptic context of the studied dry spells. Detailed observations of the temporal evolution of the ambient water vapor and foliage NRW isotopic signals suggest two different NRW input pathways: (1) the downward pathway through the condensation of ambient water vapor and (2) the upward pathway through the distillation of water vapor from soil onto foliage. We employed a simple two-end-member mixing model using δ18O and δ2H to quantify the NRW inputs from these two different sources. With this approach, we found that distillation contributed 9 %–42 % to the total foliage NRW, which compares well with estimates derived from a near-surface vertical temperature gradient method proposed by Monteith in 1957. The dew and radiation fog potentially produced 0.17–0.54 mm d−1 NRW gain on foliage, thereby constituting a non-negligible water flux to the canopy, as compared to the evapotranspiration of 2.7 mm d−1. Our results thus underline the importance of NRW inputs to temperate grasslands during dry spells and reveal the complexity of the local water cycle in such conditions, including different pathways of dew and radiation fog water inputs.

Physics-informed machine learning for understanding rock moisture dynamics in a sandstone cave

“…Based on the SHAP values of scheme #1, precipitation can be excluded as an input variable for the LSTM network. Among the three directly monitored variables that have contributions to rock water content, air relative humidity and air temperature determines the vapor concentration and the dew point temperature (Nguyen et al, 2014), and whether the wall temperature is below the dew point temperature determines whether vapor condensation could occur (Ferná ndez-Corté s et al, 2006;Gabrovšek et al, 2010;Li et al, 2021). Because water 265 vapor is the direct source of condensation water and whether the wall temperature is below the dew point Vapor concentration and dew point temperature are both functions of actual vapor pressure, which is determined by air temperature, saturated vapor pressure and relative humidity.…”

The role of radiative cooling and leaf wetting in air–leaf water exchange during dew and radiation fog events in a temperate grassland

“…Using the averages of g L under each method of LT1-LT4, the respective air-leaf water exchange amount for Lolium, Taraxacum, and Trifolium was 0.048-0.055 mm night − 1 , 0.028-0.032 mm night − 1 , and 0.041-0.064 mm night − 1 , respectively (Table 3). Furthermore, our previous study in Li et al (2021) estimated that NRW input was 0.26 mm night − 1 during N2 night. Therefore, the respective air-leaf water exchange amount for Lolium, Taraxacum, and Trifolium was 17-21%, 11-12%, and 16-25% of the total NRW input during N2 night (Table 3).…”

“…The isotope composition of atmospheric water vapor (δ 18 O v and δ 2 H v ) was measured using a cavity ring-down laser spectrometer (L2130-i, Picarro Inc., Santa Clara, CA, USA) during N1, N2 and N3 events in 2018 (see details in Li et al, 2021 andAemisegger et al, 2012), which was not available in 2019. Atmospheric air was pulled into the L2130-i cavity through a PTFE intake hose, and a PTFE-filter inlet (FS-15-100 and TF50, Solberg International Ltd., Itasca, IL, USA) fixed at 6 m agl.…”

“…During dry spells, non-rainfall water (NRW), mainly dew and fog, are additional water sources (besides rainfall water) for plants across many ecosystems (Dawson, 1998;Feild and Dawson, 1998;Agam and Berliner, 2006). However, little research has been done on the ecohydrological relevance of NRW inputs to temperate grasslands (Jacobs et al, 2006;Li et al, 2021). With global warming, dry seasons are predicted to become drier (IPCC, 2021), hence NRW is expected to have an increasing hydrological significance.…”

“…Condensation on the instrument's humidity sensor under such conditions will lead to large uncertainties of measured results for leaf conductance. Eddy-covariance (EC) measurements with high time resolution are used to estimate leaf conductance (Bonan et al, 2014), but with dew or fog occurrences, the EC method tends to underestimate the latent heat flux by about 2/3 (Jacobs et al, 2006), and yields unrealistic values of latent heat flux if dew condenses on or fog droplets drip off on the optical window of the often employed open-path Infrared Gas Analyzer (Li et al, 2021). Furthermore, in contrast to the case of estimating leaf conductance during the evaporation process (i.e., a loss of water from internal leaf into the atmosphere), leaf conductance estimates during the condensation process with a net water transfer from atmosphere onto or into a leaf are much more challenging to obtain.…”