Dissertation Defense: Characterization of anomalous climatic conditions in the La Plata Basin using the SPI and SPEI índices

Changes in the water-climate balance have contributed to an increase in the intensity and frequency of extreme climate events in river basins in South America (SA). Precipitation (PRE) and potential evapotranspiration (PET, which takes temperature into account) are the main variables regulating the water balance. Dry and wet climate events can occur when there is a change in this balance. Climate indices such as the Standardized Precipitation Index (SPI) and the Standardized Precipitation-Potential Evapotranspiration Index (SPEI) are indicators that make it possible to classify and describe parameters (such as duration, severity and peak) on different accumulation scales (1 and 12 months, for example) of dry and wet climate events. The La Plata Basin (LPB), which is transboundary and considered the second largest river basin in SA, can be divided into sub-basins according to the main contributing rivers: Paraná, Paraguay and Uruguay. The main objective of this work is to investigate anomalous climatic conditions of drought and wet periods in the LPB sub-basins using the SPI and SPEI climatic indices and compare which one is more sensitive at accumulation scales 1 and 12. For 1980-2018, PRE and PET data (calculated using the Penman-Monteith method) were provided by the Climate Research Unit (CRU), Time-Series Version 4.05. A climatological analysis of PRE and PET was carried out for each sub-basin of the LPB and statistical techniques were applied to detect linear trends (t-student) and Sen's Slope (Mann-Kendall) and linear relationships (Pearson correlation) between the series (PRE, PET, SPI and SPEI) at scales 1 and 12. Using the SPEI index on a monthly scale as a reference, the comparison with the SPI values in the moderate, severe and extreme categories was explored using the errors (bias and root mean square error - RMSE) and the Mann-Whitney-Wilcoxon test for 90%. The dry/wet episodes identified by the SPEI and SPI were paired and compared according to duration, severity and peak parameters. Finally, the SPI and SPEI were synthesized to determine which is more sensitive to dry/wet conditions at scales 1 and 12 for each sub-basin of the LPB. Among the main results were: (i) a positive trend of increasing PET in all sub-basins; (ii) the PRE and PET series are more similar in Uruguay and lower in Paraná (significant correlation of 0.9); (iii) there is a climatic trend towards drier conditions (decreasing) associated with the trend of increasing PET only in the Paraná and Paraguay sub-basins; (iv) the proportion of dry and wet episodes identified with SPI and SPEI at scales 1 and 12 is similar. Furthermore, only in the Paraná sub-basin is the bias statically significant at the 90% confidence level, indicating a tendency for the SPEI to overestimate SPI values. The duration parameter is similar between SPEI and SPI, except for dry events on the monthly scale in Paraná (SPEI indicates longer duration). Both indices showed similar results for severity, except for wet episodes at scale 1 over Uruguay (SPEI overestimated). The SPI overestimated the peak of dry episodes on both scales and wet episodes on scale 12, while the SPEI overestimated the peak of wet episodes on scale 1. The suggestion of the index for each situation analyzed is considered an important indicator for climate and hydrological planning, and can contribute to various sectors, including the feasibility of hydroelectric potential in the LPB region.