Response of stemflow as a function of various characteristics of the precipitation event

The amount of rainfall intercepted by vegetation is usually estimated by considering the amounts of precipitation, throughfall and stemflow. As stemflow values most often present only a minor fraction of the partitioned rainfall, they are frequently neglected. In addition, stemflow development during the event and under different conditions is also rarely analysed. At the study plot in Ljubljana, Slovenia, rainfall partitioning components and rainfall event characteristics have been measured since 2014. This database with high frequency measured data was used to analyse how different rainfall event properties influence the development of stemflow measured under the birch tree (Betula pendula Roth.).

156 rainfall events with observed stemflow were selected. For each event a figure showing increase of rainfall and stemflow during the event was prepared. The figures were grouped according to their similarity using a hierarchical clustering approach. For each group the significant event characteristics were analysed. Certain influence on the response of the stemflow was observed for rainfall amount and its intensity, duration of dry period before the event, as well as for average air temperature and air humidity during the event. The figures showing the situation for rainfall events with the smallest rainfall amounts and the lowest intensities were grouped in the cluster 1. The cluster 2 combined stemflow events with negligible response to rainfall development. These events delivered less than 20 mm of rainfall, while their duration was on average 5 hours, which is significantly less than duration of the events, grouped in the clusters 3 and 4. The average air temperature for events, grouped in cluster 2, was quite high as 65% of the events were observed during leafed phenophase. These events were also characterised with generally quite long dry periods before the event. The events merged in the cluster 3 showed noticeable response to rainfall development as the stemflow dynamics followed the increase of the rainfall. These events were characterized by an average of 30 mm of rainfall, reaching up to 102 mm per event. Also rainfall intensity was quite high and similar to rainfall intensities, significant for events grouped in cluster 4. It consisted of events with the strongest stemflow response, which coincide also with the largest amounts of rainfall on average per event. However, air temperature was the lowest and air humidity was the highest during the events, grouped in the cluster 4, which corresponds to mainly leafless phenophase.