The importance of city trees for reducing net rainfall: comparing measurements and simulations

An in situ tree interception experiment was conducted to determine the hydrological impact of a solitary standing Norway maple and a small-leaved lime in an urban environment. During the 2-year experiment, rainfall data were collected and divided into interception, throughfall and stemflow. With approximately 38 % of the gross precipitation intercepted by both trees, the interception storage was higher than for similar studies carried out in Mediterranean regions. The specialized forest interception models from Gash (first published in 1978) and Rutter (first published in 1971), as well as an adapted solitary tree version of the Water and Energy Transfer between Soil, Plants and Atmosphere model (WetSpa), were tested for their accuracy in modeling the measured interception storage. The models generally overestimated interception storage for small interception events (< interception storage) and underestimated interception storage for bigger interception events (> interception storage). Gash's method slightly outperformed WetSpa and Rutter for all events throughout seasons and trees. However, WetSpa showed better performance for rainfall events > 10 mm The similar performance of WetSpa and the Gash and Rutter models is noteworthy because the WetSpa interception model is part of a larger modeling framework that models the whole hydrological balance, whereas the Gash and Rutter methods are specialized standalone interception models. Thus, WetSpa is recommended to gain a more complete understanding of the impact of city trees on the full hydrological balance. This study emphasizes the potential effect of city trees on the whole hydrological balance via a combination of field data and simulation experiments using both specialized interception models (Gash and Rutter) and the relatively simple interception module of a holistic water balance model (WetSpa).