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:
  1. investigate dynamics in δv within two contrasting urban vegetation types to understand what types of landcover enhance moisture fluxes back to the atmosphere.
  2. investigate these changes in relation to related ecohydrological dynamics of soil moisture storage, sap flow rates and biomass accumulation.
  3. 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.