Assessing Hydrological Dynamics in A Tropical Watershed: A Swat-Based Approach Using the Priestley-Taylor Method

Potential evapotranspiration (PET) and evapotranspiration (ET) are vital components of the hydrological cycle, particularly in tropical watersheds where water management relies on accurate predictions of water availability. This study employed the Priestley-Taylor method within the Soil and Water Assessment Tool (SWAT) to model long-term PET and hydrological responses in the Oyun Watershed, Kwara State, Nigeria, over a 30-year period (1991–2021). The model was parameterized using a 90-meter Digital Elevation Model (DEM), land use and soil maps, and daily climate data from the Nigerian Meteorological Agency (NiMet). The Priestley-Taylor method was selected for its suitability in data-scarce and humid regions. Model simulations revealed a total PET of 11,807.51 mm over the simulation period, with spatial variation across sub-basins ranging from 596.51 mm to 721.96 mm. Groundwater recharge was estimated at 8,050.46 mm, while surface runoff was 3,469.06 mm. Land cover, slope, and soil permeability significantly influenced the spatial variability of these outputs. Calibration and validation results demonstrated satisfactory model performance, with Nash-Sutcliffe Efficiency (NSE) and the coefficient of determination (R²) exceeding 0.7. These findings confirm the effectiveness of the SWAT model for predicting hydrological responses in data-scarce, humid regions, providing critical insights for water management in the Oyun Watershed.