Drought Assessment in São Francisco River Basin, Brazil: Characterization through SPI and Associated Anomalous Climate Patterns

Abstract

The São Francisco River Basin (SFRB) is one of the main watersheds in Brazil, standing out for generating energy and consumption, among other ecosystem services. Hence, it is important to identify hydrological drought events and the anomalous climate patterns associated with dry conditions. The Standard Precipitation Index (SPI) for 12 months was used to identify hydrological drought episodes over SFRB 1979 and 2020. For these episodes, the severity, duration, intensity, and peak were obtained, and SPI-1 was applied for the longest and most severe episode to identify months with wet and dry conditions within the rainy season (Nov–Mar). Anomalous atmospheric and oceanic patterns associated with this episode were also analyzed. The results revealed the longest and most severe hydrological drought episode over the basin occurred between 2012 and 2020. The episode over the Upper portion of the basin lasted 103 months. The results showed a deficit of monthly precipitation up to 250 mm in the southeast and northeast regions of the country during the anomalous dry months identified through SPI-1. The dry conditions observed during the rainy season of this episode were associated with an anomalous high-pressure system acting close to the coast of Southeast Brazil, hindering the formation of precipitating systems.

Assessment of Precipitation and Hydrological Droughts in South America through Statistically Downscaled CMIP6 Projections

“…The SPI-12 index uses a time scale of 12 months. Besides identifying long-term rainfall patterns, SPI-12 can also be associated with flows, reservoir levels, and groundwater anomalies, helping to evaluate hydrological droughts [56,58]. More detailed information about the SPI calculation can be found in Santos et al [61] and Wilks [124].…”

“…The Standardized Precipitation Index (SPI), developed by McKee et al [57], quantifies the rainfall deficit or excess on different time scales. SPI on time scales greater than six months is employed to identify and characterize hydrological droughts that cause reduced soil moisture levels, river flows, groundwater recharge, and reservoir levels [56,58,59]. The SPI index proves advantageous because, besides allowing for evaluating drought impacts on different hydrological cycle components (using different time scales), it requires only rainfall data as input variables in the index computation [58].…”

“…SPI on time scales greater than six months is employed to identify and characterize hydrological droughts that cause reduced soil moisture levels, river flows, groundwater recharge, and reservoir levels [56,58,59]. The SPI index proves advantageous because, besides allowing for evaluating drought impacts on different hydrological cycle components (using different time scales), it requires only rainfall data as input variables in the index computation [58]. However, as SPI does not account for the temperature component, its analysis disregards evapotranspiration processes, which play an essential role in the hydrological cycle [56,58].…”

“…The SPI index proves advantageous because, besides allowing for evaluating drought impacts on different hydrological cycle components (using different time scales), it requires only rainfall data as input variables in the index computation [58]. However, as SPI does not account for the temperature component, its analysis disregards evapotranspiration processes, which play an essential role in the hydrological cycle [56,58]. Despite its limitations, several studies have employed the SPI-12 index (SPI index on a 12-month time scale) due to its simplicity of implementation to identify hydrological droughts in SA [6,56,58,[60][61][62][63][64][65][66].…”

“…However, as SPI does not account for the temperature component, its analysis disregards evapotranspiration processes, which play an essential role in the hydrological cycle [56,58]. Despite its limitations, several studies have employed the SPI-12 index (SPI index on a 12-month time scale) due to its simplicity of implementation to identify hydrological droughts in SA [6,56,58,[60][61][62][63][64][65][66].…”