Dinâmica pluvial na bacia hidrográfica do Rio Chapecó: influência do El Niño oscilação sul nas áreas de risco de enchente/inundação (1975-2021)

Abstract

The El Niño Southern Oscillation (ENSO) is a worldwide phenomenon, and, in Brazil, it has distinct characteristics in each region. In the South of the country, this weather pattern causes an increase (El Niño) or reduction (La Niña) in annual precipitation totals. Therefore, it is possible to relate this increase in the amount of precipitation to episodes of floods and inundations that have occurred over the years in the region. Several authors have already studied the relationship between ENSO and inundation episodes. However, in western Santa Catarina, these studies are still scarce. To expand the studies on this theme, the spatial focus of this research is the hydrographic basin of the Chapecó River (BHRC), which for decades has accumulated social and economic losses with hydrological events. BHRC is found on the Araucaria plateau, more specifically in the Midwest of Santa Catarina. The Chapecó River is 248 km long, draining an area of 8,190 km², passing through 30 municipalities. These municipalities are considered small, mostly with less than 10 thousand inhabitants. The period of the research is from 1975 to 2021, with the goal of obtaining a significant series, with more than 30 years and with as few flaws as possible. Based on the above, this research aims to characterize the rainfall regime in the BHRC and show variations in the areas of flood/inundation risk in ENSO years. The specific objectives are: (i) to characterize (qualitatively and quantitatively) the rainfall regime; (ii) identify the time of recurrence of extreme precipitation events; (iii) map the areas at risk of flooding/inundation; and (iv) prepare risk maps for the most affected municipalities. The text is divided into five chapters. The first consists of the bibliographic reference on the research theme, looking to define the main concepts. The second includes the methodological assumptions of the research. The third addresses the area of study, presenting physical aspects. The fourth includes the characterization of the rainfall regime (total annual and monthly), the statistical parameters, the classification of standard years, and the time of recurrence of extreme events (five-day period). Finally, in the fifth chapter, information about the fieldwork and the identification of flood/inundation risk areas in the municipalities of Coronel Freitas, Nova Itaberaba, Águas de Chapecó, Saudades, Modelo, and Quilombo are presented. The BHRC showed great variability of rainfall over the years. Wetter periods coincide with El Niño, while dry years are generally unrelated to La Niña. Thus, there is a pattern of precipitation in moderate and strong El Niño episodes, but not in the dry periods linked to La Niña. The largest precipitation events (major floods) had a recurrence time of more than 20 years, reaching up to 127 years in the rainfall season of Saudades, in the event of July 1983. The identification of risk areas was based on the mapping conducted by CPRM, after the floods of 2014 and 2015. In the municipalities of Coronel Freitas, Saudades, Nova Itaberaba and Águas de Chapecó, the risk areas pointed out in the thesis are similar to the respective mapping. However, in the municipalities of Modelo and Quilombo, the research found risk areas greater than those presented in the CPRM mapping, which was confirmed by recent hydrometeorological events (between 2020 and 2023) in the respective municipalities. ENSO events result in a negligible increase in flood/inundation risk areas. It is noted that, in episodes of strong El Niño, the probability of hydrometeorological disasters increases, given the excess of rainfall in these years, forming the necessary conditions for such events.