Assessment of climate variability impacts on water quality using SPI and GIS approaches in the Mubuku River catchment, Uganda

The Mubuku River catchment in western Uganda is highly sensitive to hydroclimatic variability, yet quantitative evidence linking rainfall anomalies to water quality degradation remains limited. This study presents a decadal (2014-2024) integrated assessment combining the Standardized Precipitation Index (SPI), rainfall deviation analysis, GIS-based spatial interpolation, and laboratory evaluation of physicochemical and microbial parameters. Results reveal significant temporal and spatial deterioration in water quality, with Total Dissolved Solids increasing by ~35%, hardness rising from 300 to 400 mg/L, and nitrate concentrations exceeding 50 mg/L at multiple sites. Microbial contamination also intensified, with fecal coliform counts increasing from 50 to 75 CFU/100 mL, surpassing WHO permissible limits. SPI analysis identified recurrent drought years (2014, 2016, 2023, 2024) alongside high-runoff periods, demonstrating a dual vulnerability framework where both precipitation deficits and excesses exacerbate contamination. Spatial analysis highlighted critical hotspots (WS4, WS6, WS8), where pollutant concentrations were 40-60% higher than the catchment average. The findings further indicate that drought conditions enhance mineral dissolution and baseflow contamination, while wet periods accelerate runoff-driven pollutant transport from agricultural and urban sources. This study contributes a novel climate-water quality linkage framework for tropical catchments by integrating hydroclimatic indices with spatial water quality assessment. The results provide actionable insights for targeted interventions, including hotspot-based management, seasonal regulation strategies, and improved wastewater control, to enhance water security and climate resilience.