The PhD student Juan Pinos defends his thesis titled "Multi-approach investigation of the dynamics of rainfall partitioning fluxes and their implications in a Mediterranean environment"
The PhD student Juan Pinos, from the Surface Hydrology and Erosion group, will defend his thesis on 24 January at 11:00h in Sala de Grados (C1/070-1) at Universitat Autònoma de Barcelona.
Title: Multi-approach investigation of the dynamics of rainfall partitioning fluxes and their implications in a Mediterranean environment
Directors: Pilar Llorens and Jérôme Latron
Thesis Committee: Antonio del Campo, Ilja van Meerveld, Teresa Gimeno
Abstract:
Rainfall interception by forest canopies prevents rainfall from immediately reaching the soil, redistributing it in the form of throughfall and stemflow. The aim of the present research was to investigate the role of forest cover on the spatial and temporal distribution of both flows in a Mediterranean forest. The dissertation aimed to fill some knowledge gaps by using novel ecohydrological approaches in a Scots pine forest in the Vallcebre research catchments, a representative area of Mediterranean middle mountain environments. High temporal resolution monitoring of rainfall, throughfall, stemflow, meteorological variables, and soil water content, as well as water sampling of the different fluxes for the determination of the stable water isotopes signatures in the period 2018-2020, were used to improve the mechanistic understanding of forest-water interactions.
With this information, we investigated the throughfall partitioning into splash throughfall, free throughfall, and canopy drip by means of rainfall and throughfall drop size distributions, and their relationships with the observed shift in their isotopic composition. Throughfall types were characterized according to different rainfall classes based on duration and intensity. We found that during rainfall events, splash throughfall was greater at the beginning of the events, as well as the vapour pressure deficit. This initial stage coincided also with the greater throughfall isotopic enrichment with respect to rainfall isotopic signal. Higher splash throughfall and vapour pressure deficit indicated that the isotopic enrichment was caused by splash droplet evaporation. Moreover, isotopic differences among rainfall, throughfall, and stemflow were analysed. Throughfall was more enriched than rainfall and stemflow more than throughfall. Seasonal differences in isotopic shift were also observed, with throughfall being more depleted during the growing season whereas stemflow was more enriched. Meteorological variables did not show a relationship with either throughfall or stemflow isotopic shifts. Isotopic fractionation was caused by a combination of factors occurring throughout the event, with evaporation being more relevant during the initial stage of the events, whereas canopy selection processes (i.e., the possibility of intercepted rainfall to be retained and transmitted asynchronously) dominated towards the end of the events. Nevertheless, intra-event mixing of waters could occur during the same event.
Stemflow is a spatially-concentrated and chemically-enriched water flux input, altering soil moisture and chemistry in near-stem areas when infiltrating in the vicinity of tree trunks. While many studies have examined stemflow production among tree species, no known research examined the stemflow distribution around the trunk before reaching the ground. Moreover, only a few studies explored its infiltration dynamics (the double funnelling effect). The results of our study clearly show a non-uniform distribution of stemflow down in the trunk surface, meaning that stemflow distribution shows preferential flow paths. These flow paths are related to biotic factors (trunk lean, bark morphology, and tree neighbourhood), and to a lesser extent to abiotic factors (rainfall intensity peaks). By conducting a field experiment simulating stemflow, labelled with a dual-tracer approach, we were able to determine the influence of stemflow in the soil moisture response at the base of the tree. We observed that stemflow primarily infiltrated along the surface of coarse roots and through macropores, and demonstrate, by a set of metrics, the prevalence of preferential flow. Knowledge of above- and below-ground stemflow distribution and its implications is therefore emphasized.
The insights and findings presented in this dissertation will enable forest hydrologists to better characterize the hydrological processes in Mediterranean forest landscapes. In addition, future research lines that could extend and complement our results are also discussed.
