Hydro-Climatic Shifts and Catchment Transformation: Modelling Streamflow Responses in Lake Bogoria Basin, Kenya

Climate variability and land use land cover (LULC) changes are critical drivers of regional water balance, ultimately determining water quantity within river systems. This study sought to determine the impacts of rainfall variability, temperature variability, and LULC changes on the volume of water flowing through rivers draining into Lake Bogoria in Baringo County, Kenya, from 1981 to 2020. We established five Soil and Water Assessment Tool (SWAT) models for the time periods 1981, 1991, 2001, 2011, and 2020, inputting topographic, climate, LULC, and soil data. Primary data was also collected from the Endorois community and key informants from relevant ministries. Our analysis revealed that rainfall increased from 1290.3 mm in 1981 to 1853.6 mm in 2020. The mean temperature exhibited a slight warming trend, rising from 23.77°C in 1981 to 23.85°C in 2020. Land use land cover analysis depicted a net increase in surface area under water by 7.17 km², grasslands by 1.22 km², shrublands by 7.92 km², agricultural land (rain-fed and irrigated) by 8.06 km², Prosopis species by 0.74 km², peri-urban areas by 0.28 km², and bare areas by 0.62 km² over the forty years. Conversely, tree cover decreased substantially by 25.83 km². The model performance was robust, achieving a coefficient of determination (R²) of 0.78 and a Nash-Sutcliffe Efficiency (NSE) of 0.75, indicating good and very good performance, respectively. The total simulated discharge volumes of the Waseges, Kipsirian, and Ngiriki rivers increased from 8,230,008 m³ in 1981 to 17,629,946 m³ in 2020. We conclude that the combined effects of increasing rainfall and extensive deforestation overshadowed the impact of rising temperatures and the expansion of agricultural land, resulting in a net increase in river discharge volumes at the watershed scale. However, this masks a critical shift towards a more volatile hydrological regime with potential for more severe dry-season water scarcity.