Quantifying heterogeneity in ecohydrological partitioning in urban green spaces through the integration of empirical and modelling approaches

Abstract

Urban green spaces (UGS) can help mitigate hydrological impacts of urbanisation and climate change through precipitation infiltration, evapotranspiration and groundwater recharge. However, there is a need to understand how precipitation is partitioned by contrasting vegetation types in order to target UGS management for specific ecosystem services. We monitored, over one growing season, hydrometeorology, soil moisture, sapflux and isotopic variability of soil water under contrasting vegetation (evergreen shrub, evergreen conifer, grassland, larger and smaller deciduous trees), focussed around a 150-m transect of UGS in northern Scotland. We further used the data to develop a one-dimensional model, calibrated to soil moisture observations (KGE’s generally > 0.65), to estimate evapotranspiration and groundwater recharge. Our results evidenced clear inter-site differences, with grassland soils experiencing rapid drying at the start of summer, resulting in more fractionated soil water isotopes. Contrastingly, the larger deciduous site saw gradual drying, whilst deeper sandy upslope soils beneath the evergreen shrub drained rapidly. Soils beneath the denser canopied evergreen conifer were overall least responsive to precipitation. Modelled ecohydrological fluxes showed similar diversity, with median evapotranspiration estimates increasing in the order grassland (193 mm) < evergreen shrub (214 mm) < larger deciduous tree (224 mm) < evergreen conifer tree (265 mm). The evergreen shrub had similar estimated median transpiration totals as the larger deciduous tree (155 mm and 128 mm, respectively), though timing of water uptake was different. Median groundwater recharge was greatest beneath grassland (232 mm) and lowest beneath the evergreen conifer (128 mm). The study showed how integrating observational data and simple modelling can quantify heterogeneities in ecohydrological partitioning and help guide UGS management.

Assessing land use effects on ecohydrological partitioning in the critical zone through isotope‐aided modelling

“…The applied model is derived from the one‐dimensional, plot‐scale ecohydrological model EcoHydroPlot (see Stevenson et al, 2023 for full details). Briefly, the model simulates the ecohydrological partitioning of water into green and blue fluxes of evaporation (E), transpiration (Tr) and groundwater recharge (Re).…”

“…The model simulates canopy storage with precipitation inputs, evaporation loss, a maximum storage threshold, and resulting throughfall. Our version of the model simulates canopy storage with a slightly different workflow as Stevenson et al (2023) as we had a different climatic setting, and this improved model simulations. Here, the available canopy storage was set as a maximum of interception storage and further limited by a throughfall threshold (if exceeded by interception storage, throughfall was allowed).…”

“…This was partly motivated by the increasing need to quantify the water “footprint” of alternative land uses following the 2018 drought which resulted in substantially reduced crop yields in some areas and increased forest tree mortality (Hänsel et al, 2022; Kleine, Tetzlaff, Smith, Dubbert, & Soulsby, 2021; Moravec et al, 2021). We used the lumped, low‐parameter, EcoHydroPlot model, which is easy to parameterise for different vegetation cover types (Stevenson et al, 2023), added isotope tracer simulations, and applied it for plot‐scale simulations of four different dominant land use and soil units at the DMC catchment. Such a simple, one‐dimensional, plot‐scale model with a parsimonious concept also has high potential to be used in communicating to stakeholders to underpin management decisions as model concepts are easy to follow.…”

High‐resolution in situ stable isotope measurements reveal contrasting atmospheric vapour dynamics above different urban vegetation

“…Others found higher ET and older groundwater recharge beneath urban trees, but more marked soil evaporative losses under grassland (Gillefalk et al, 2021). By integrating simple modelling and observational water isotope data, Stevenson et al (2023) 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 green spaces in Berlin showed that trees were more dependent on deeper, older soil and groundwater sources, whereas grass very probably recycled shallow, younger soil water in transpiration (Marx et al, 2022).…”

“…Others found higher ET and older groundwater recharge beneath urban trees, but more marked soil evaporative losses under grassland (Gillefalk et al, 2021). By integrating simple modelling and observational water isotope data, Stevenson et al (2023) 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.…”