Dynamics of nutrient content and water quality for irrigation along the Sebakwe river within the Zibagwe sub-catchment

Abstract

Irrigation water quality is a critical determinant of agricultural sustainability in semi-arid smallholder farming regions where river systems serve as the primary source of water for crop production. This study investigated the spatial and seasonal dynamics of nutrient concentrations and physicochemical water quality parameters along the Sebakwe River within the Zibagwe Sub-catchment, KweKwe District, and Midlands Province of Zimbabwe, evaluating the river's suitability for smallholder irrigation against Food and Agriculture Organisation (FAO) guidelines. Water and riverbed sediment samples were collected monthly from six sampling sites along a 19-kilometre longitudinal transect over eleven months, encompassing both dry and wet hydrological seasons. A total of 198 water samples and 48 composite sediment samples were analysed for pH, Electrical Conductivity (EC), Total Dissolved Solids (TDS), Sodium Adsorption Ratio (SAR), and concentrations of Sodium (Na⁺), Calcium (Ca²⁺), and Magnesium (Mg²⁺) using Atomic Absorption Spectrophotometry (AAS). Data were analysed using one-way ANOVA, Tukey's HSD post-hoc tests, Pearson correlation, and Principal Component Analysis (PCA) in R Studio. Results revealed statistically significant upstream-to-downstream deterioration across all monitored parameters (p < 0.001), driven by smallholder irrigation return flows, livestock farming, artisanal gold mining, and urban effluent discharges. Sodium exhibited the most pronounced spatial amplification, recording an 11-fold increase from Site 1 (0.23 meq/L) to Site 6 (2.52 meq/L) during peak dry season. Dry season concentrations were significantly elevated relative to wet season values, with EC increasing by 52.8% and Na⁺ by 78.3% between seasons. Benchmarking against FAO Irrigation and Drainage Paper No. 29 confirmed the river maintains an "Excellent — No Restriction" classification for salinity, sodicity, and specific ion toxicity throughout the monitoring period. The principal agronomic concern was episodic dry season alkalinisation at lower reach sites, where pH peaked at 8.98, exceeding the FAO operational limit of 8.4 and risking soil micronutrient immobilisation for sensitive crops.