Quantifying the effects of urban vegetation on water partitioning in complex cityscapes: the potential of isotope-based ecohydrological models

Abstract:

The continued global acceleration of urbanisation increasingly requires sustainable, adaptive management strategies for land and water use in cities. Although the effects of buildings and sealed surfaces on urban runoff generation and local climate are well known, much less is known about how these artificial influences integrate with water partitioning in urban green spaces. In particular, little is quantitatively known about how different types of urban green spaces regulate the partitioning of evaporation, transpiration and groundwater recharge and how this contrasts with the effect of sealed surfaces. To address this crucial issue, we integrated field observations with advanced, isotope-based ecohydrological modelling at the plot scale in Berlin, Germany. Measurements of soil moisture, sap flow, and stable isotopes in precipitation, soil water and groundwater have been made over the course of one growing season. Additionally, an eddy flux tower at the site continuously collects various hydroclimate data. These data have been used as input to, and for calibration of, the process-based ecohydrological model EcH₂O-iso. The model also tracks stable isotope ratios and water ages in various stores (e.g. soils and groundwater) and fluxes (evaporation, transpiration and recharge). EcH₂O-iso has successfully been used to describe the effects of vegetation cover on water partitioning in a number of studies but this is the first implementation in an urban setting. It shows that ecohydrological water use by vegetation type increases in the order trees > shrubs ≈ grass, mainly through higher interception. Accordingly, trees can reduce groundwater recharge by >50%, but provide cooling latent heat transfers to the atmosphere. Similarly, ages of stored water and fluxes are generally greater under trees than shrub and grass. The model also shows how the interface between sealed surfaces and green infrastructure creates edge effects in form of infiltration hot spots. The results, which form the basis for ongoing upscaling study, show that urban green spaces play an important role in urban hydrology. Consequently, it is clear that vegetation management needs to be considered in sustainable water and land use planning in urban areas to build resilience in cities to climatic and other environmental change.