Urban tree water demand: Comparison of modeling results to measured sapflow and soil moisture data

Current shifts in rainfall and temperature regimes towards dryer and hotter periods in central Europe have caused substantial water stress for urban trees.  To be able to adapt water supply to urban trees under a changing climate,  a quantification of evapotranspiration and water availability becomes necessary and is at the same time, very challenging in the heavily modified urban environments. Both processes are influenced by soil sealing and complex shading patterns of the surrounding street canyon.  

For five urban street trees in the city center of Berlin, evapotranspiration rates and water availability was monitored in a field campaign (sapflow measurement and soil moisture in different depth) during the vegetation period of 2022. The monitoring results were then used to test a hydrological urban tree model with an integrated shading model which specifically takes into account the shading and sealing variability of the surrounding built environment.

Both measured and modelled data a  show that potential evapotranspiration rates were significantly larger for trees with full sun exposure compared to shaded trees. At sites with full sun exposure, the increased evapotranspiration also reduced soil moisture content faster; at the same time measured actual evapotranspiration was reduced by up to 2/3 during water stress periods.

In conclusion, the comparison showed that our model is a promising option to obtain information on water availability and to improve water management for urban trees under different shading and sealing environments in heavily modified cities. The tool will be further developed to be used by local authorities and practitioners to identify water shortage periods and hot spots  within the city to optimize irrigation efforts.

Key words: urban trees, evapotranspiration (ET), water availability, water stress, water management,  urban environment, shading