Claus eco-hidrològiques per entendre la influència de les aigües subterrànies en rierols d'alta muntanya

The study of hydrology in the high mountains arouses interest given that they are the primary catchment areas, therefore, what happens here can be perceived downstream. In addition, in relative terms, the high mountain ecosystem is less disturbed by human action than other areas close to urban centers, as a result, they are ecosystems more sensitive to any change that occurs there, a condition attributed to the ecological study of the high mountain as suitable to be able to identify the various alterations caused by global change (climate and pollution). The various difficulties posed by sampling in these ecosystems, due to limited accessibility and difficulty in obtaining continuous and reliable data, especially due to the involvement of snowfall and the persistence of the snow cover that covers the ground, have as consequence of the fact that hydrological studies in the high mountains are scarce and are under constant review. Likewise, for the study of aquifers, there are other drawbacks that make sampling difficult, such as the transport of suitable equipment for drilling and that these are subject to very fluctuating recharge cycles as a result of the seasonal melting of the snow. This thesis tries to find out the influence of non-surface waters, as well as the dynamics and interactions that exist between the various hydrological compartments, in a high mountain headwater catchment, specifically in the Contraix catchment in the National Park of 'Aigüestortes and Estany de Sant Maurici. To this end, this work is composed of three studies. In the first study we apply a mixing model at an exit point of the catchment, specifically we apply the End-Member Mixing Analysis model, to quantify, both in absolute and relative terms, the contributions of each of the hydrological compartments to the surface flow, including the underground waters. In the second study we examine the qualitative relationship, in terms of hydrological connectivity, between the various hydrological compartments and the surface flow, performing an analysis of the Q-C hysteresis patterns recorded during several precipitation events, in two locations at different altitudes within the catchment and during the snow-free season. In the third, having verified the influence, in both quantitative and qualitative terms, that the non-surface hydrological compartments apply on the surface runoff, we intend to analyze the various biogeochemical processes of the subsurface hydrological compartment soil, to check their influence on the chemistry of the surface flow. For this purpose, two microcatchments have been selected, one with high soil cover, and the other without soil cover, and mass balances have been carried out as well as the chemical reconstructions of the surface flow in both microcatchments based on the chemistry of precipitation water, and finally the export of weathered elements (cations + Si), as well as the annual nitrogen and sulfur export flows, have been calculated. For all three studies, an automated sampling system was used that allowed us to obtain data at a high temporal resolution (minutes-hours), in order to capture in detail all discharge events. Firstly, in the first study four tracer solutes (Na+, Ca2+, DRSi and DOC) have been defined for the three potential hydrological contributors that make up the surface runoff: atmospheric water, soil water and deep underground water. Atmospheric waters have been characterized for being very dilute waters and having a low concentration of all these tracer solutes, soil waters for having a high concentration of all of them but with a high and characteristic concentration of DOC, and deep underground waters to also have a high concentration of all of them but highlighting the high concentrations of solutes from weathering (Na+, Ca2+ and DRSi). Likewise, the applied mixing model has shown a fairly constant contribution to the surface flow of groundwater throughout the year, around 50%, which increases during peak rain episodes that have become, at times punctual, higher than 70%. In the second study, through the application of a simple numerical model based on the observations of the hysteresis patterns, it was possible to verify that the hydrological connectivity between the surface flow and the different hydrological compartments is not the same along the climatic station. In an initial stage no clear connectivity was observed between the surface flow and the hydrological compartments, then in a second stage the hysteresis patterns suggest a connectivity of the surface flow exclusively with the soil compartment, then a third stage showed surface flow connectivity with both the soil and the aquifer, and finally a fourth stage showed surface flow connectivity with the aquifer rather constant and variable soil connectivity. Finally, in the third study we were able to verify that in areas with high soil coverage, runoff water has a higher concentration of Cl- than precipitation and water from areas without soil coverage. A possible cause is that an accumulation of Cl- occurs in the ion exchange complex of the soil, which ends up being released into the water of the saturated zone. In the same way, other ions can be the majority in the waters of areas with soil cover due to their retention in the ion exchange complex. It has also been shown that the presence of soil determines a greater export of the solutes that originate there during weathering (Ca2+, Na+, Mg2+, K+, DRSi and the fraction of carbonates and, above all, bicarbonates of the DIC) into the waters, probably because it stimulates the weathering processes of the rock. Finally, we have observed that in areas with soil cover there is a lower export of nitrogen and sulfur than in areas without soil cover. This thesis testifies that the hydrology in the high mountains is more complex than the vision of the so-called "Teflon basin" suggests (Williams et al., 2016), evidencing the need for more studies to be able to obtain a clear vision and summary of the hydrological behavior of high mountain catchment. In addition, it represents the first research work of this type in the Pyrenees area.