Spatial–seasonal effect sizes in river water quality in the Zambezi Basin: from a DPSIR-informed diagnosis to SWOT–TOWS-based strategic synthesis

ABSTRACT Six-step graphical abstract illustrating the progression from spatial–seasonal water-quality monitoring to strategic management of the Bua River: Analytical progression from spatial–seasonal physicochemical monitoring and η²-based effect-size assessment to water-quality vulnerability profiling, SWOT–TOWS strategic synthesis and climate-resilient management of the Bua River within the Zambezi Basin. (1) Monitoring covers upstream (Mchinji), midstream (Dowa) and downstream (Ntchisi) locations across cold–windy, hot–dry and rainy–wet seasons. (2) Factorial generalised linear models (GLMs) quantify Site, Season and Site × Season effects using eta-squared (η²) effect sizes. (3) The resulting water-quality vulnerability profile distinguishes interaction-sensitive parameters, season-dominated parameters, spatial hotspots and comparatively resilient or vulnerable conditions. (4) SWOT categorises catchment strengths (ecological assets and relatively resilient conditions), weaknesses (observed physicochemical vulnerabilities and monitoring/management limitations), opportunities (restoration, improved monitoring, policy alignment and climate-smart management) and threats (catchment degradation, erosion, nutrient loading, land-use intensification and climate variability). (5) TOWS translates these factors into four strategic pathways: SO (protect and leverage existing strengths), WO (use opportunities to address vulnerabilities), ST (use ecological and institutional assets to reduce emerging threats) and WT (prioritise preventive interventions in vulnerable reaches and seasons). (6) The intended outcome is climate-resilient Bua River management within the Zambezi Basin. River-water quality in strongly seasonal catchments cannot be understood adequately from spatial averages alone because hydroclimatic conditions can alter both the magnitude and direction of longitudinal patterns due to interactions. We utilised physicochemical water quality data from across three different sections of Malawi's Bua River in three distinct hydroclimatic seasons. Aware of the existing interactions, physicochemical parameter-specific factorial GLMs, eta-squared (η2) effect sizes and standardised Site × Season profiles were used to distinguish spatial, seasonal and interaction-sensitive responses. Site× Season effects were largest for temperature (η2 = 0.676), oxygen saturation (η2 = 0.666), pH (η2 = 0.519), dissolved oxygen (η2 = 0.486) and chlorophyll-a (η2 = 0.417), whereas Season dominated turbidity (η2 = 0.818), TDS (η2 = 0.802), ammonium (η2 = 0.625), EC (η2 = 0.617) and soluble reactive phosphorus (η2 = 0.586). Moving from environmental diagnosis to strategic synthesis, an evidence-informed SWOT–TOWS framework translated these findings into management priorities centered on protecting comparatively resilient reaches, targeting season-sensitive hotspots, reducing sediment and nutrient connectivity, strengthening riparian and water-quality monitoring, and incorporating hydroclimatic variability into catchment planning. The approach provides an interaction-aware basis for climate-responsive freshwater management in the Bua and comparable seasonal river systems in the wider Zambezi River Basin.