Predicting Water Supply and Evapotranspiration of Street Trees Using Hydro-Pedo-Transfer Functions (HPTFs)

Abstract

The climate, soil properties, groundwater depth, and surrounding settings in cities vary to a tremendous extent, which all lead to different growing conditions and health for street trees. Because of climate change, the availability of water in cities will undergo changes in the next decades. As urban trees have a very positive influence not only on microclimate but also on biodiversity and life quality in general, they need to be protected. Thus, we need to know how to measure and calculate the availability of water for street trees to optimize their site conditions and water supply. This study presents Hydro-Pedo-Transfer Functions (HTPFs) for predicting water supply and actual evapotranspiration of street trees for varying urban conditions. The HTPFs are easy to use, and the input parameters can either be mapped easily or taken from local climate agencies or soil surveys. The first part of the study focuses on the theoretical background and related assumptions of the HTPFs for predicting water supply, and on obtaining the potential and actual evapotranspiration of urban street trees using easily available data. The second part gives information and exemplifies how this input data can be measured, mapped, or predicted. Calibration of the HTPFs were done using the sap-flow measurements of three Linden trees (Tilia cordata). Exemplarily, the HTPF scenarios for the varying urban site conditions of Berlin are presented. The water supply and actual evapotranspiration of the street trees severely depend on the local climate (summer rainfall and potential evapotranspiration), site conditions (catchment area, soil available water, and degree of sealing), and on the tree characteristics (species, age, and rooting depth). The presented concept and the equations build a good and flexible frame that is easy to program using a spreadsheet tool or an R script. This tool should be tested and validated also for other cities and climate regions.

Impact of shading on evapotranspiration and water stress of urban trees

“…The model was parametrized for the most abundant tree species in Berlin, Tilia cordata (SenUVK, 2021), more specifically for a T. cordata with an age of 15–30 years, a crown size of 16 m 2 , a rooting depth of 1.5 m and a crop coefficient of = 1.4–1.6 (Wessolek & Kluge, 2021). The pheno‐multiplier ( = 0.5, for first 2 weeks of leaf development and last 2 weeks of leaf shedding) was added to describe the stage of development throughout the vegetation period.…”

“…In conclusion, existing water models for urban trees do not (Vico et al, 2014) or only partially include the effects of shading (Hörnschemeyer et al, 2021;Wessolek & Kluge, 2021). None of the studies considered varying shading patterns typically found within street canyons, which affect global radiation and potential evapotranspiration on a diurnal time scale.…”

“…However, the effects of shading on evapotranspiration were not considered in calculating the water balance. Wessolek and Kluge (2021) developed hydro-pedotransfer functions (HPTFs) to calculate the annual water demand, potential ET (ETp), actual ET (ETa) and water stress of street trees. The function applies to different tree ages, species, soil sealings, soil types and global radiation is reduced with the SVF.…”

“…Current measures, such as manual irrigation, are limited and only applied during extreme conditions in temperate zones in Central Europe (David et al, 2018; Dickhaut & Eschenbach, 2018; SenUVK, 2021). To improve irrigation management and avoid the risk of vitality losses and mortality of urban trees, it is necessary to clearly understand urban tree water demand, considering shading patterns and site‐specific effects (McDowell et al, 2008; Schütt et al, 2022; Wessolek & Kluge, 2021).…”

“…Evapotranspiration (ET) is a key variable to quantify water flow and stress at the atmosphere–soil–plant interface (Allen et al, 2006; Olmedo et al, 2016). In many urban tree models, evapotranspiration is also used to evaluate the general cooling effect (Pace et al, 2021), including a reduction of global radiation (Grylls & van Reeuwijk, 2021; Hörnschemeyer et al, 2021; Wessolek & Kluge, 2021) or the occurrence of water stress (Revelli & Porporato, 2018; Rötzer et al, 2021; Vico et al, 2014). To our best knowledge up to date, no monitoring study has quantified the effects of shading on the water stress of street trees.…”