Spatiotemporal changes in precipitation concentration in São Francisco River basin

We analyzed the temporal and spatial variability of precipitation concentration in the São Francisco River basin (SFRB), Brazil, on annual, and seasonal scales using the precipitation concentration index (PCI). The PCI was calculated from a high-resolution daily precipitation gridded dataset for the period 1961–2022. Annual and seasonal PCI values were used to investigate significant trends by applying the Modified Mann–Kendall (MMK) test and to assess the impact of climate change by comparing two 31-year climate periods (1961–1991 and 1992–2022). The results showed that precipitation concentration varies spatially across the SFRB, reflecting regional climate delineation and the distribution of wet and dry months. On an annual scale, the upper, middle and sub middle SFRB regions displayed irregular and highly irregular rainfall distribution while the lower SFRB region showed moderate precipitation concentration. In summer and autumn most of SFRB areas display moderate rainfall distribution, while in spring both moderate and irregular rainfall distribution were found in the middle and sub middle SFRB regions. In winter, irregular rainfall distribution was present in the upper SFRB, highly irregular rainfall distribution in the middle SFRB, moderate rainfall distribution in the sub middle SFRB and uniform distribution in the lower SFRB. Across almost the entire SFRB moderate rainfall distribution was found in wet season and highly irregular distribution in dry season. The most significant change from the first to the second period was found in the seasonal rainfall regime: in winter, the area with low precipitation concentration, indicating uniform rainfall distribution, increased to 100% in the Lower SFRB region (wet season and subhumid climate); in spring, the area with irregular rainfall distribution increased from 23 to 77% in the Middle SFRB (transition from dry to wet season and semiarid climate). The MMK test, applied for the entire period (1961–2022), showed that in summer and spring, a significant positive trend in PCI values was observed for large areas of the SFRB (57% and 58%, respectively), indicating that rainfall distribution has become more irregular. This irregularity can affect energy generation, agricultural planning and production, and water resources management. In both wet and dry seasons, positive trends were found across the central and northern areas of the middle, sub-middle, and lower SFRB, indicating an increasing risk of floods and droughts in these regions.