Analytical Investigation of Streamflow Variability and Low-Flow Frequency in the Upper Osun River Basin, Osun State

Understanding streamflow variability and low-flow frequency is essential for water resources planning, particularly as the Osun River basin in South West Nigeria faces increasing water demand and climate-related hydrological change. This study analyses streamflow variability and estimates low-flow frequency characteristics for the Upper Osun River basin. Daily streamflow records from three gauging stations (1980 to 2023) were compiled and subjected to stationarity testing using the Mann-Kendall test and Pettitt change point detection. Low-flow frequency analysis applied FDC, Q95, Q90, and 7-day 10-year low flow methods to characterise minimum flow conditions. Seasonal trend decomposition isolated climatic and land use change contributions to observed trends. Double mass curve analysis with rainfall identified periods of altered rainfall-runoff relationship consistent with land use change effects. Results indicate statistically significant declining trends in annual minimum 7-day flows at two of three stations, with change points around 2005 coinciding with accelerated urban expansion in the upper catchment. Low-flow frequency estimates show a 23 percent reduction in Q95 between the pre-2005 and post-2005 periods. The 7Q10 low flow has declined from 0.82 to 0.61 m3/s at the main mainstem station. The study concludes that combined climate variability and urban expansion are reducing dry-season flows in the Upper Osun Basin and recommends minimum environmental flow requirements be established for water abstraction licensing in the basin.

Keywords: streamflow variability, low-flow frequency, Upper Osun Basin, Osun State, hydrological analysis