Investigation of the intercepting performance of urban trees at a rainfall event basis

Despite the widespread concerns of increasing precipitation extremes caused by climate change, the application of urban trees as a nature-based solution against pluvial flooding are still often overlooked and undervalued due to its limited practical value in case of extreme rainfall occurrences. Hence, this study investigated the rainfall interception performance of open-grown birch (Betula pendula Roth.) and pine (Pinus nigra Arnold) trees in an urban park of Ljubljana, Slovenia (Europe) on an event basis using disdrometer data. In addition, we closely analysed the rainfall microstructure under both types of trees during the extreme storm event on September 29, 2021 and compared it to average values from the past. During this single storm event with a cumulative rainfall of 87.6 mm within 7.6 hours, birch trees intercepted 19.5% of the gross rainfall, which is lower than its average interception of 35.9% during the period of 2014-2017. On the other hand, the canopy of pine trees was able to intercept 47.0% of rainfall – higher than its average retention capacity of 37.1% during extreme events in the past (>40 mm). The total number of raindrops recorded for this event under the birch tree crown is 362,556 with a largest diameter of 13 mm, where the throughfall from birch and pine trees constitute 75% and 53% of the rainfall, respectively. For small to moderate rainfall events that occurred from August to October 2021 with an average number of 167,329 raindrops, the amount of rainfall intercepted by birch and pine tree canopies corresponds to 23.8% and 55.7%, respectively. The results show that on an event basis, both types of trees perform effectively in intercepting low to medium magnitudes of precipitation. Results also reveal that pine trees can intercept substantially more rainfall even during the high-magnitude events, which emphasizes the importance of proper selection of trees species with higher rainfall interception capacity to leverage its benefits in an urban setting during extreme conditions.

Acknowledgments: Results are part of the CELSA project entitled “Interception experimentation and modelling for enhanced impact analysis of nature-based solution” and research programme P2-0180 supported by the Slovenian Research Agency (ARRS).