Baseline Assessment of Water Quality, Quantity, and Sediment Transport in the Kabutie Watershed, Upper Yala River Catchment, Nandi County, Kenya: Implications for Watershed Health and Drinking Water Security

Abstract

Watershed degradation resulting from soil erosion and organic pollution threatens water security and ecosystem health in many rural catchments of Kenya. The Kabutie watershed, a key sub-catchment of the Upper Yala River in Nandi County, supplies drinking water to Kapsabet Town and surrounding communities, yet baseline information on its hydrology and water quality has been lacking. This study established baseline conditions of water quality, stream discharge, and sediment transport across the Kabutie watershed to support future monitoring and management interventions. A cross-sectional survey was conducted in July 2025 at fourteen monitoring sites. Stream discharge was measured using an Acoustic Doppler Velocimeter. Physicochemical parameters including temperature, pH, electrical conductivity, total dissolved solids, turbidity, and Dissolved oxygen were measured in situ using calibrated field probes. Total suspended solids were determined using APHA standard methods, and sediment loads were estimated from discharge and suspended sediment concentrations. Stream discharge ranged from 0.004 to 1.306 m³/s. Turbidity exceeded the Kenyan drinking water standard of 25 NTU at 85.7% of sampling sites, with Kapyagan Stream recording the highest value (301 NTU). TSS exceeded the recommended limit of 30 mg/L at 42.9% of sites, reaching 282 mg/L at Kapyagan. Dissolved oxygen levels were below the recommended 80% saturation threshold for aquatic life at all sites, with critically low values observed at Kaptilalon Stream (45.5%). Sediment loads ranged from 0.004 to 15.008 tons/day, with the Kabutie River exporting an estimated 5,478 tons of sediment annually. In contrast, pH, conductivity, and TDS remained within acceptable limits across all sites. The Kabutie watershed is experiencing significant degradation driven by soil erosion and organic pollution. Kapyagan Stream, a major drinking water source, was identified as the most vulnerable site. Immediate riparian restoration, livestock exclusion, pollution-source investigations, and long-term watershed monitoring are recommended to safeguard water quality and drinking water security.