1. Introduction
Urban green spaces mediate trade-offs between “green” and “blue”
water fluxes with potential for high evapotranspiration (ET) rates. They
potentially mitigate the Urban Heat Island (UHI) effect, but on the
other hand might reduce groundwater recharge and stream flow generation
. Understanding, quantifying and optimizing this partitioning across
urban critical zones is increasingly important in the face of increased
urban growth and climatic warming. In addition, wider benefits of urban
green spaces – or green infrastructure – are increasingly recognized;
these include the potential to enhance infiltration and ameliorate urban
storm runoff to increase local biodiversity , to provide social
functions through improved health for local residents and to improve
water security in terms of sufficient provision of good water quality .
Consequently, as one component of an evidence base for wider urban
planning, the trade-offs between higher ET rates and groundwater
recharge , as well as the linked uncertainties, are an increased focus
for research (e.g. BMUB ).
Water stable isotopes have proved valuable tools that can help resolve
the partitioning of incoming precipitation into different components of
ET fluxes or to constrain biosphere-atmosphere feedbacks between
atmospheric vapor and ET, and thus have high potential to contribute to
a scientific evidence-base for managing urban green spaces . Water
isotopes have also been shown to be a useful tracer to understand
processes and linkages across the critical zone and the
soil-plant-atmosphere continuum in different geographic regions although
critical zone studies in urban areas are still relatively rare . Use of
isotopes includes tracking the effects of evaporation in isotopic
fractionation and in identifying the effects of seasonality of water
sources for different vegetation types . Numerous isotope studies have
used soil water or river water isotopes to assess evaporative effects ,
whilst others have related the composition of xylem water to potential
sources of root water uptake . However, studies using high-resolution
data to investigate how evaporation and/or transpiration affect the
isotopic composition of atmospheric vapour (δv) at the
surface boundary layer are especially rare for urban areas (e.g. Gorski
et al., ).
The onset of relatively inexpensive cavity ring-down spectroscopes
(CRDS) has revolutionized the field of isotope studies allowing
efficient tracing of isotopic transformations across the atmospheric
water cycle , quantifying ecohydrological interactions and the origin of
atmospheric moisture (i.e. evaporation or condensation; Gao et al., ).
Recent developments in using in-situ measurements of stable water
isotopes are making use of non-destructive online monitoring techniques
and are increasingly advanced . In terms of analyzing
δv, grab samples or refrigerated traps for offline
analysis in the laboratory were already used in the 1990s with rapidly
accelerating progress in recent years . Today, CRDS techniques have been
shown to be useful for measuring δv at continuously
high-resolution and thus, enabling real-time analysis of
δv which can give more novel insights than precipitation
alone . For example, the technique has been successfully deployed for
monitoring sub-tropical sub-cloud raindrop evaporation ; for testing
vapour equilibrium assumption for δ18O cellulose
estimates ; diurnal and intra-seasonal variations in evaporative signals
at different heights above the Greenland ice sheet ; and to characterise
variation in δv and their controlling factors during
extreme precipitation events . To date, however, hardly anyin-situ studies have assessed δv dynamics in the
urban atmospheric boundary.
Previous isotopic studies have reported contrasting ecohydrological
partitioning under different land use types in urban green spaces . A
study in Scotland assessed land use influences on isotopic variability
revealing that urbanisation, intensive agriculture and responsive soils
caused rapid cycling of precipitation to stream water . Others found
higher ET and older groundwater recharge beneath urban trees, but more
marked soil evaporative losses under grassland . By integrating simple
modelling and observational water isotope data, Stevenson et al.
quantified the heterogeneities in urban ecohydrological partitioning and
found that median ET increased from grassland, to evergreen shrub, to
larger deciduous forest through to larger conifer trees, with
groundwater recharge behaving contrary. Mixing models applied to
different Berlin green spaces showed that trees were more dependent on
deeper, older sub-soil and groundwater sources, whereas grass very
probably recycled shallow, younger soil water in transpiration . Such
isotopic information of water fluxes through the critical zone can be
used in ecohydrological models that can resolve ET into its component
parts. However, to do this, the isotopic gradient at the
atmospheric-land interface is usually defined in models assuming
δv is in equilibrium with current or recent rainfall .
Despite now being logistically possible, monitoring δvin-situ at different heights and above vegetation canopies is
still rare. Braden-Behrens et al. demonstrated the value of directin-situ eddy covariance measurements of δv in the
surface boundary layer. Despite standard model assumptions of an
equilibrium between δv and precipitation,
δv can be out of equilibrium with local water sources
and can show gradual depletion with altitude . High-resolutionin-situ monitoring of δv allows testing of such
equilibrium assumptions, but so far, very few studies have tested this
with in-situ ambient data .
Here, we conducted a “proof of concept” study to assess the changing
isotopic composition of δv over a 2.5 months period in
an urban green space with contrasting landcover. We deployed a laser
spectrometer in the field for continuous in-situ monitoring of
δv in the urban surface boundary layer. Our overarching
research question was whether we can generate data with in-situreal-time sequential monitoring that increases our understanding of
origins of atmospheric moisture and its link to water partitioning by
contrasting urban vegetation. Our specific objectives were to:
- investigate dynamics in δv within two contrasting
urban vegetation types to understand what types of landcover enhance
moisture fluxes back to the atmosphere.
- investigate these changes in relation to related ecohydrological
dynamics of soil moisture storage, sap flow rates and biomass
accumulation.
- assess the extent of equilibrium between vapour and precipitation.
Based on these assessments, we discuss the future value, challenges and
potential in gaining and processing such high-resolution data to improve
understanding of ET partitioning at different heights in the atmosphere
above different types of landcover in urban green spaces, which would be
important for increased process understanding across urban critical
zones.