Assessment of Throughfall Drivers Under Pine Canopies Across Different Urban Climates

Abstract

Throughfall plays a significant role in hydrological processes, defining the effective rainfall available for soil moisture and runoff generation under vegetation. This study presents the first cross-climate comparison of the drivers of throughfall under black pine trees (Pinus nigra Arnold) in urban environments. Open rainfall and throughfall were measured at two experimental sites in the city of Ljubljana, Slovenia (temperate continental), and Sopron, Hungary (humid continental). To determine which of the climatic and canopy-related variables influence the event throughfall percentage (Tf) across three periods (i.e., whole study, growing and dormant periods), regression tree (RT) and boosted regression tree (BRT) models were used. The total rainfall and mean Tf recorded during the study period from September 2023 to September 2024 were 1591.2 mm and 45% ± 21.5% in Slovenia, respectively. Conversely, Hungary experienced 767.2 mm and 50% ± 27.2% . Both models confirmed rainfall (RA) as the primary driver of Tf across the three examined periods in both sites. Furthermore, the BRT model confirmed rainfall intensity as the secondary influential variable, specifically during Slovenia's growing and Hungary's dormant periods. Contrarily, the RT model showed relative humidity (RH) and leaf area index as the secondary variables defining the Tf over the whole study period at both sites, with variations across the growing and dormant periods. Under a scenario that included only the atmospheric variables, the BRT model identified RH as the most significant driver across all periods and both sites. Similarly, the RT model recognised RH as the primary variable during the three periods in Hungary and the entire study period in Slovenia. Moreover, air temperatures influenced Tf in Slovenia's growing and dormant periods. The findings indicated throughfall as a climate-sensitive parameter, emphasising the significance of these results in hydrological models susceptible to climate variability and in areas characterised by comparable climatic conditions.