Caracterización de las amenazas geo-climáticas en los Andes centrales y semi-áridos de Chile y Argentina (30-33 °S)

Abstract

The extreme geo-climatic events constitute an important hazard in the Central and Semi-Arid Andes of Argentina and Chile, assiduously generating fatalities and significant economic losses. Due to this situation, it was worked on the aspects that were considered more urgent for the quantification of their current and future hazard. Due to the scarcity of meteorological information and recorded extreme geo-climatic events, these topics were developed in areas of the region where the necessary base information is densified and therefore the probability of finding novel results is greater. First, the probabilities for the triggering of hyper-concentrated flows, and debris and mud flows in the middle basin of the Elqui River (Chile) were determined. The objective was achieved collecting, for a period of fourteen years, the precipitation events generating high-discharge flows, as well as the larger precipitation events that did not generate this process. For each of these events, data of peak one-hour storm precipitation, temperature (representing the zero-isotherm altitude) and antecedent precipitation of one, five and ten days were collected from three meteorological stations. Initially, an ordinal logistic regression model for each antecedent precipitation was fitted, but all were discarded due to the low significance of these variables in the generation of the models. This result allowed to hypothesize that the high discharge flows of the region would be triggered by in-channel entrainment and shallow failure planes, and not by deep failure planes. Subsequently a new model with the remaining variables, peak precipitation and temperature, was performed, which was statistically validated. From this, it was considered prudent to take as thresholds for the occurrence of hyper-concentrated flows, and debris and mud flows, their respective probabilities of 50 %. For these thresholds, the model had an efficiency in the prediction of high-discharge flows of 90 %. Finally, the partial correlation coefficients of each predictor variable of the final model with respect to the dependent were calculated, establishing that the temperature has greater influence than the peak one-hour storm precipitation. Through the Difunta Correa watershed, representative of periglacial area of the Semi-Arid Andes, were investigated the thermo-radiative characteristics, the possible water sources, and the current and future frequency of debris flows triggered from rapid melt of seasonal snow, and / or ice within the active layer. Information was collected on three temporal clusters of debris flows during which no rains and major earthquakes occurred. The thermo-radiative conditions of each cluster were analysed through nearby stations that cover the entire watershed altitudinal range Snow covers were calculated using the closest satellite images before and after each cluster in order to evaluate the snowmelt contribution for each. The frequency of melting-driven debris flows, for the remainder of the 21st century was evaluated by calculating the trends of climatic variables that control them. The results indicate that debris flows present several patterns such as: lag of some hours between the warmest daily hours and their triggering, occurrence in clusters of three to five days during the early summer, and accelerated increase of temperature during the previous days to the clusters beginnings. In addition, it was inferred that the water of debris flows can come from the melt of seasonal snow as well as of shallow ice within the active layer. Lastly, due to the positive trend of the maximum temperature of warmer trimester and the high inter-annual variability of precipitation, a frequency increase is expected, followed by a possible decrease due to the negative and positive trends of precipitation and mean annual temperature, respectively. Finally, meteorological information and locations and dates of 569 landslides and snow avalanches were analysed, in order to advance in the detection and compression of temporary changes in the extreme geo-climatic events of the Eastern Central Andes. Initially, a characterization was carried out, that allowed to divide the extreme geo-climatic events in zones and seasons with different climates. Then, the trends of these events were calculated taking precautions for their non-systematic monitoring (selection of the series type, year of beginning and type of trend made). The frequencies of the extreme geo-climatic events triggered by precipitation for the different zones and seasons of the annual cycle were stationary or presented low positive trends. The positive changes were related to changes in precipitation, since both variables showed significant correlations, and greater increases in the Atlantic climate dominance sector than in the Pacific dominance. From these trends it was known that the increase in the record of extreme geo-climatic events triggered by precipitation since the second half of the 20th century was mainly due to increases in monitoring. In addition, from the study it was deduced that despite the warming that the region suffers, the decrease in the extreme geo-climatic events triggered by snowfall since the end of the 1990s was due solely to a reduction in precipitation. This is because the winter warming occurred exclusively on the days without precipitation; on days with precipitation an oceanic air mass usually enters which has not had significant thermal changes since at least 1974. Based on the results and conclusions obtained throughout the Thesis, it was deduced that although the relationship between the extreme geo-climatic activity and the climate is clear, it varies considerably depending on the type of landslide or snow avalanche, the type of triggering causal factor, the climate and the land properties. Understanding this relationship in its entirety implies knowing the interaction between atmospheric dynamics and the geotechnical, physical and morphometric properties of the terrain, so it can be considered a multidisciplinary study.