Drought-Rich Periods Are More Likely Than Flood-Rich Periods in Brazil

Streamflow exhibits persistent decadal variability; however, it is unclear if the magnitude and spatial extent of these variabilities are symmetric for droughts and floods. Here, we examine drought-rich and flood-rich periods in 319 streamflow gauges in Brazil from 1940 to 2020. Drought- and flood-rich periods are detected by computing annual streamflow minima and maxima time series and using scan statistics to verify if events exceeding a threshold follow a Bernoulli process. We contrast streamflow time clustering with rainfall, evaporation, water abstraction, the Atlantic Multidecadal Oscillation (AMO), and the Pacific Decadal Oscillation (PDO). We detected a higher spatial frequency of drought- than flood-rich periods. For 5-year return period thresholds, drought-rich periods are observed in 81% of the basins, 16.7 times the false positive rate (4.8%) and 4.7 times flood-rich periods (17%). This asymmetry is linked with sharp increases in water abstractions since the 1990s and a higher prevalence of rainfall-poor periods (41% of gauges) compared to rainfall-rich (22% of gauges), which we interpret as being further amplified into drought-rich periods due to an interannual persistence of water storage deficits. Brazil experienced a dry period until the 1960s, extensive flooding in the 1980s, and record low flows from the 2000s onward. Drought and flood-rich periods are well aligned with rainfall clustering, water abstractions, the AMO and PDO. Droughts-rich periods are more frequent in shorter time scales (several years to one decade) and flood-rich periods in longer time scales (a few decades). Our findings highlight the nonlinearity and asymmetry of drought and flood change at decadal scales.