Elsevier

Journal of Hydrology

Volume 572, May 2019, Pages 630-644
Journal of Hydrology

Research papers
The atmospheric water cycle of a coastal lagoon: An isotope study of the interactions between water vapor, precipitation and surface waters

https://doi.org/10.1016/j.jhydrol.2019.03.033Get rights and content

Highlights

Abstract

The interaction between the different components of the water cycle at ecosystem scale is of great interest in hydroecology. Stable isotopes of hydrogen and oxygen in the water molecule can be used to study these processes quantitatively due to their different physical properties. This study aims at quantifying the contribution of local moisture sources to the isotopic signature of water vapor in the specific estuary system of a coastal lagoon. Here we present a framework of the isotopic composition of the atmospheric water cycle components (water vapor and precipitation) for the coastal lagoon of Venice (Italy) and we discuss the interaction between the atmospheric component and the surface component of the local water cycle, i.e. the atmospheric moisture and the lagoon water. The Venetian Lagoon is an enclosed basin with low freshwater influx, resulting in the presence of large horizontal gradients of the isotopic composition of surface water. This feature allows to determine the sensitivity of the isotopic signal in near surface atmospheric moisture under a range of humidity conditions. We observed that when the lower atmosphere is close to water vapor saturation pressure condition, the isotopic composition of water vapor at the ground appears to be in isotopic equilibrium with lagoon surface water in a range of approximately 6–26 km from the sampling point. On the other hand, under unsaturated conditions, we estimated that lagoon evaporation flux accounts on average for ∼27% to the daily (06–18 Local Sidereal Time) total evapotranspiration surface flux. This is the first study that discusses the spatio-temporal variability of water vapor, precipitation and surface water isotopic composition for a coastal lagoon, thus including the major components of the local water cycle in an estuarine system with the isotope approach.

Introduction

Understanding the movements and paths of water through the different reservoirs of the water cycle is a central topic of hydrology. The terrestrial surface and the atmosphere exchange water in both gas and liquid form through evapotranspiration and precipitation processes. The isotopic composition of water represents an ideal tool to study these processes because it conserves the integrated history of the phase changes and of the mixing between different reservoirs. In this context, many studies used hydrogen and oxygen stable isotopes ratios in water vapor and precipitation to better understand the transport and the sources of water vapor in marine (Benetti et al., 2014, Steen-Larsen et al., 2015a, Steen-Larsen et al., 2015b, Steen-Larsen et al., 2014) and land ecosystems (Berkelhammer et al., 2013, Griffis et al., 2016, Jasechko et al., 2013, Yepez et al., 2003). The isotopic composition of water vapor was largely used in vegetated land ecosystem studies, to better understand the contribution of evapotranspiration flux within the Planetary Boundary Layer (PBL) (Lee et al., 2006, Wei et al., 2016) and the relative contribution of evaporation to transpiration on evapotranspiration flux (Wang et al., 2010, Yakir and Sternberg, 2000). The correct quantification of the evapotranspiration fluxes represents a key topic of these studies. Variations of these fluxes are expected because of the intensification of water cycle related to climate change (Huntington, 2006). Then, alteration in evapotranspiration could have implications on ecosystem services and feedback to regional and global climate (Jung et al., 2010). Plants transpiration seems to be the main component of water vapor flux over land, as highlighted by recent studies (Good et al., 2015, Jasechko et al., 2013). However, the relative contribution of transpiration to evapotranspiration can be influenced by several factor such as vegetation cover and free water and soil water availability. Moreover, it has been shown that local water vapor sources can significantly affect the isotopic composition of atmospheric water vapor within the PBL, especially if measurements are performed near the ground (Aemisegger et al., 2014, Delattre et al., 2015, Griffis et al., 2016, Wei et al., 2016).

An ecosystem with a characteristic isotopic composition of surface waters can be used to investigate the sensitivity of surface water vapor fluxes to nearby water sources. Coastal lagoons perfectly match the case since they are nearly enclosed systems, where freshwater mixes with seawater in a relatively small area. Moreover, the low energy regime of coastal lagoons makes this kind of estuary system feasible for the study of a number of processes operating in coastal water bodies (Adlam, 2014).

Noting that the isotopic composition of water vapor near the ground is affected by local water sources, this study aims at quantifying the lagoon’s evaporation flux influence on the isotopic composition of the total surface flux (i.e. the lagoon evaporation flux contribution relative to all evapotranspiration sources in the study area). Hence, this study has two objectives:

  • 1.

    Determine the isotopic composition and the sources of the main components of a lagoon’s water cycle, namely: the atmospheric moisture, the surface water and precipitation;

  • 2.

    Determine how representative are the local water vapor sources on the moisture isotopic signal at ground level. This is translated as a length scale and as a contribution of lagoon evaporation flux to the total surface flux under saturated and unsaturated vapor pressure conditions, respectively.

We measured the isotopic composition of atmospheric water vapor and precipitation in the inland boundary of the Venetian Lagoon, a shallow coastal lagoon in northern Italy, for one year. A complete survey of the lagoon surface water was also performed during the study period to determine the mean isotopic composition of the lagoon, its spatial variability and the processes driving this physical quantity.

Variations of the isotopic ratio of water are discussed in this work using the delta notation [‰] (Craig, 1961):δ=RSampleRStandard-11000where R is isotopic ratio of the sample and of the standard water (Vienna Standard Mean Ocean Water, VSMOW), for D/H (δD) and 18O/16O (δ18O). The secondary order parameter called d-excess [‰] (Dansgaard, 1964) is used to investigate non-equilibrium effects on the isotopic composition of water vapor and precipitation:d-excess=δD-8δ18O

Section 3 focuses on the main results regarding the water cycle components investigated in this study, their main sources and the main processes affecting their isotopic composition (objective 1). In Section 4, the significance of local water vapor sources in local atmospheric moisture is discussed considering different relative humidity and wind conditions (objective 2).

The interaction between the different components of the water cycle at ecosystem scale is a field of study largely explored in hydroecology (e.g. Lai et al., 2006, Yakir and Sternberg, 2000). However, no works are available for coastal lagoons considering precipitation, water vapor and surface waters together. This is the first study where the isotopic composition of moisture and surface water are investigated for the Venetian Lagoon. Therefore, this study represents the first attempt of including the major components of the water cycle in an estuarine system, paving the way for future studies about the interaction between the moisture in lower troposphere and the surface of transitional waters.

Section snippets

Study area

The Venetian Lagoon is a shallow coastal lagoon in northern Italy connected to the Adriatic Sea by three main inlets (Bocche di Porto). The position of the Venetian Lagoon in the Italian setting and a detailed map of the study area are reported in Fig. 1a and b, respectively. The surface of the lagoon is ∼550 km2, 11% of the surface is permanently covered with water, 80% consists of tidal shallows and salt marshes strongly affected by the tide, the remaining 9% of the surface is represented by

The isotopic composition of water in the lagoon hydrosphere

A summary of the isotopic composition of water vapor, precipitation and surface waters of the study area is given in Fig. 2. The LMWL was calculated from monthly precipitation data. In general, d-excess < 10‰ for precipitation can be considered a signature of raindrops evaporation during the descent from the cloud base to the ground. For Venice, mean ± SD d-excess is 8 ± 5‰ (monthly samples), therefore many precipitation events might have undergone partial evaporation. To discriminate the

Isotopic equilibrium between water reservoirs

When the air above the water surface is saturated with respect to the water surface temperature (RHSST ≈ 100%), vapor and liquid phases tend to isotope equilibrium and net surface evaporation flux is close to zero. As can be noted in Fig. 3.b, in some cases high mean RHSST (>85%, n = 18) was recorded during water vapor sampling. Since RHSST > 85% was observed with (n = 4) or without (n = 14) precipitation, water vapor should be close to isotopic equilibrium with precipitation or with surface

Conclusions

Stable isotopic composition of the water molecule is a powerful tool to study evapotranspiration and precipitation processes, allowing to partition water fluxes between the atmosphere and the terrestrial surface. We presented the first study of the atmospheric water cycle and its interaction in the lower PBL of the coastal lagoon of Venice (Italy) that includes observations of water vapor, precipitation and surface water, by answering to the following questions: (i) to what extent the isotopic

Declaration of interest

None.

Acknowledgements

Authors are grateful to the three anonymous referees for their valuable comments which improved the quality of this manuscript significantly. Authors are grateful to Dr. Ilaria Crotti for the English language support and the writing assistance.

Funding

This work is part of the PhD project of Daniele Zannoni, scholarship by Ca’ Foscari University.

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