Resumen de Ecohydrology of mediterranean headwater catchments. The role of forest in the redistribution and isotopic modification of water fluxes
The present dissertation aims to analyse the role of forest cover on the redistribution of water fluxes and improve the current knowledge on hydrological functioning of Mediterranean headwater catchments. This study has been carried out in the Vallcebre research catchments, an area representative of these Mediterranean mountain environments. Continuous measurements of rainfall, throughfall, stemflow, runoff, meteorological data and stable isotopes of water have been used to investigate hydrological processes at different spatio-temporal scales.
At the plot scale, the findings obtained from a Scots pine (Pinus sylvestris L.) and downy oak (Quercus pubescens Willd.) forest plots have shown that stemflow, despite being only a small portion of the incident precipitation, is a substantial source of water and particulate matter at the base of trees. Stemflow is the result of a complex combination of biotic and abiotic factors, it increases with the event size but the duration of rainfall, intensity or the evaporative demand highly influence its temporal dynamics. In addition, we have found the size of trees to be the main factor producing differences among individuals of each species. However, between species, main stemflow differences have been attributed to different bark storage capacities and different evaporation rates. Besides, through the analysis of the particles contained in throughfall and stemflow, we have observed that the interaction between particulate matter and vegetative surfaces affects the size and the retention of particles. In general, the presence of leaves in oaks increases the size of particles, and needles of pines enhance its retention. We have also found that Saharan dust events are a substantial source of particulate matter in the study area.
Isotopic differences among rainfall, throughfall and stemflow have been observed. Fractionation processes are more evident for events of low rainfall amount, when canopies are not completely saturated. They can be caused by a mixture of factors, for example, evaporation is more likely to have a higher impact at the beginning of rainfall, however, under low evaporation conditions, isotopic exchange (between water and vapour) may acquire more relevance. In addition, for rainfall events with temporal variations of the isotopic composition, the retention of part of the final portion of rainfall on leaves and stems can also produce isotopic differences in both directions, enrichment or depletion.
At the catchment scale we have found that, in addition to the isotopic changes produced by canopy interception processes, the isotopic composition of rainfall also varies along an elevation gradient. Throughout the Can Vila catchment and for several runoff events, the effect of the spatio-temporal variability of the input isotopic signal on hydrograph separation results has been tested. Results have shown that although the Isotopic Hydrograph Separations are dominated by pre-event water, for some floods, the pre-event water contribution can differ significantly depending on the single location of the input isotopic signal used. Comparing hydrograph separation results obtained using different single input signals, with results obtained using a catchment scale input isotopic signal, we could determine the most representative sampling location and define a “smart” sampling strategy for improving Isotopic Hydrograph Separations at the small catchment scale.
Overall, findings gathered in the present dissertation highlight the role of stemflow as a preferential flow path of water and nutrients that can enhance biogeochemical processes at the base of trees during rainfall events. Results also emphasize that the isotopic variability of rainfall, due to canopy interception processes and elevation gradients, has to be taken into account for a better understanding of the hydrological processes in Mediterranean headwater catchments.
