ABSTRACT Graphical abstract of Koga Reservoir showing seasonal differences in water quality. Dry season has high temperature and dissolved oxygen with low nutrients and turbidity, while wet season shows monsoon runoff with high nutrients, high turbidity, and lower temperature and oxygen. Central reservoir image illustrates inflow, outflow, and dominant processes including watershed runoff, hydrodynamics, and sediment-nutrient coupling. Reservoirs in tropical highland regions are increasingly affected by sedimentation, nutrient enrichment, and hydrological variability. However, their seasonal dynamics remain insufficiently understood. This study evaluates the spatial and seasonal variability of water quality and identifies dominant controlling processes in the Koga Reservoir, Northwestern Ethiopia. Water samples were collected from eleven geo-referenced sites during the dry (February 2025) and wet (August 2025) seasons and analyzed for physicochemical parameters and nutrients. Results showed significant seasonal variation (p ≤ 0.021), with wet-season runoff elevating nitrate (5.81 mg L−1), phosphate (5.94 mg L−1), ammonium (7.62 mg L−1), turbidity (859.5 NTU), and total suspended solids (272.5 mg L−1). In contrast, temperature decreased while dissolved oxygen remained below 5 mg L−1 at several sites, indicating ecological stress. Spatially, inflow and intake zones consistently exhibited elevated nutrient and sediment concentrations, whereas mid-reservoir areas showed attenuation through dilution and settling. Multivariate analysis identified two dominant processes that explained ∼70% of the variability: nutrient-sediment enrichment driven by watershed runoff and thermal-oxygen dynamics influenced by temperature and turbulency. Cluster analysis delineated three functional zones: inflow-dominated, transitional, and buffered central regions. These findings highlight the need for catchment management, erosion control, and adaptive reservoir operation to sustain irrigation efficiency and ecosystem health.
Ayenew et al. (Thu,) studied this question.