Hydro-climatic trend analysis and qualitative vulnerability assessment under scenarios of climate change projections (RCP 4.5 and RCP 8.5) in the Lower Akagera sub-catchments, Rwanda

Water held in shallow aquifers underpins domestic supply, livestock watering and small-scale irrigation across eastern Rwanda, where surface sources are seasonal and unevenly distributed. The Lower Akagera sub-catchments face rising abstraction alongside a warming and increasingly erratic climate, yet the area has no operational observation network and, until now, no published assessment connecting the measured water-table record to projected climate. This study pursued two objectives: to characterise historical hydro-climatic trends over 1981–2024, and to assess qualitatively how the vulnerability of the shallow aquifer system may evolve to 2063 under two greenhouse gas emission pathways. Measured water-table depths at Rwinkwavu, together with temperature and rainfall series from NASA POWER, were tested for monotonic change using the Mann–Kendall test with the Sen's slope estimator, and Spearman's rho was applied as an independent rank-based check. Future climate was represented by four CMIP6 global climate models—MPI-ESM1-2-HR, GFDL-ESM4, ACCESS-CM2 and CanESM5 forced by the SSP2-4.5 and SSP5-8.5 pathways, which correspond to the radiative forcing levels formerly designated RCP 4.5 and RCP 8.5. Projections were obtained from the Copernicus Climate Data Store and bias-corrected against the 1991–2020 observational baseline. Water-table depth increased significantly across the record, moving from roughly 10–11 m in the early 1980s to 15–16 m by 2024—a net deepening of about 4–5 m over 44 years (Mann–Kendall Z = −2.87, p = 0.004; Spearman's ρ = −0.41). Over the same period mean air temperature rose at approximately +0.12 °C/year, driven principally by night-time minima increasing at +0.10 °C/year against only +0.01 °C/year for daytime maxima, while annual rainfall totals ranged between 580 mm and 1,795 mm about a mean of 1,086 mm. The decline in water level is consistent with this warming and with the pronounced rainfall variability; reduced effective recharge is a plausible mechanism, though it was not measured directly. Projections to 2063 indicate warming under both pathways, more pronounced under the high-emission trajectory. Mean minimum temperature is projected at 16.83 °C under SSP2-4.5 against 17.11 °C under SSP5-8.5, while extreme maxima reach 36.68 °C and 38.38 °C respectively, a difference of 1.70 °C. Mean daily precipitation is marginally higher under SSP5-8.5 (2.98 mm/day against 2.86 mm/day), yet the moderate pathway produces more intense individual events (43.32 mm/day against 41.35 mm/day). On the balance of these indicators, SSP2-4.5 presents conditions less unfavorable to aquifer replenishment than SSP5-8.5, principally because lower thermal extremes and reduced thermal variability moderate evaporative demand. This inference rests on the relationship between climate forcing and recharge reported in comparable semi-arid settings; it is not the output of a groundwater flow simulation. Set against a documented recharge rate of only 2–20 mm/year and renewable reserves classified as very low, the findings indicate that the shallow aquifer system of the Lower Akagera is highly exposed to climate-driven stress. Strengthened observation networks, climate-smart agricultural practice, managed aquifer recharge, reduction of the 41.6% non-revenue water loss, and transboundary coordination are therefore warranted, though the present analysis does not by itself quantify the benefit of any individual measure.