Observed trends and variability of seasonal extreme rainfall indices and projected changes in Rwanda

Rwanda like other East African nations, has experienced significant social and economic impacts due to extreme precipitation events. This study analyzes trends and variability in extreme rainfall indices in Rwanda for 1983-2021 and projects changes under RCP scenarios using CORDEX-CORE models for 2026-2066 and 2066-2100 periods. Precipitation indices were computed following the methodology of the Expert Team on Climate Change Detection and Indices (ETCCDI). The considered six extreme indices are consecutive wet days (CWD), heavy precipitation days (R10mm), and very heavy precipitation days (R20mm), maximum one-day precipitation amount (RX1day), precipitation from very wet days (R95pTOT), and total annual precipitation (PRCPTOT). Results indicate that for both long rain (LR) and short rain (SR), CWD, R10mm and R20mm are more variable in the eastern region and RX1day, R95pTOT and PRCPTOT in western region. Spatial trends are heterogeneous countrywide. For country-averaged extremes, Rx1day shows statistically significant (at alpha = 0.05) negative trend (-0.204 mm/year) for LR. During LR, CWD is projected to significantly increase (at p < 0.05) in Zones III, IV, and VI under RCP2.6 for both 2026-2060 and 2066-2100 and in Zones IV and VI under RCP8.5 for 2026-2060. For the period 2066-2100, an increase is projected in Zones II, III, IV, and VI under RCP8.5. Conversely, a decrease is projected in Zone V across all periods and scenarios. R10mm and R20mm are projected to decrease in all climatic zones under both RCP2.6 and RCP8.5 for both 2026-2060 and 2066-2100. Similarly, PRCPTOT is projected to decrease in almost all climatic zones for 2026-2060 under both scenarios. For 2066-2100, a decrease is expected in Zones I, II, III, IV, and VI under RCP2.6 and in Zones II, III, and IV under RCP8.5. During SR, CWD, RX1day, R95pTOT, and PRCPTOT are projected to increase in all climatic zones under both RCP2.6 and RCP8.5 for both 2026-2060 and 2066-2100. R20mm is projected to increase in Zones I, II, III, and IV under RCP2.6 and in Zones I, II, III, IV, and V under RCP8.5 for 2026-2060. For 2066-2100, an increase is projected in Zones I, II, III, IV, and VI under RCP2.6 and in all zones under RCP8.5. Giveth that the economy of Rwanda depends on rainfall, and its complex topography, understanding trends and variability and projected changes of extreme precipitation is key for the agriculture, water resources, disaster risk reduction and infrastructure sector. These findings are crucial for agricultural planning, disaster preparedness, and guiding adaptation strategies for Rwanda's socio-economic sectors.