Calibration and Evaluation of Alternative Methods for Reliable Estimation of Reference Evapotranspiration in South Korea

Reference evapotranspiration (ET) is crucial for effective agricultural and water resource management, with the FAO-56 Penman-Monteith (FAO PM) method being the standard for its estimation. However, in regions with incomplete or poor-quality meteorological data, alternative ET estimation methods may be necessary. This study evaluates the calibration and performance of seven alternative ET methods (Hamon, Hansen, Hargreaves–Samani, Jensen–Haise, Makkink, Priestley–Taylor, and Thornthwaite) across 117 sub-basins in South Korea. Monthly calibration coefficients were derived from FAO PM estimates to adjust these methods and enhance their accuracy. Results showed that, prior to calibration, deviations between alternative methods and FAO PM could exceed 70 mm per month. Post-calibration, these deviations were reduced to within 5 mm, indicating substantial improvements. Methods incorporating multiple climatic variables, particularly Priestley–Taylor, demonstrated higher accuracy compared to temperature-only methods. Although temperature-based methods like Hamon and Thornthwaite were effective in data-sparse regions, they required precise calibration. Applying sub-basin calibration coefficients to individual sites was a viable alternative to site-specific calibration, though it did not always match the precision of the latter. This study highlights the effectiveness of calibration in refining ET estimates and suggests combining site-specific and area-based approaches for robust ET estimation. Future research should continue to refine calibration techniques and explore advanced methods to improve ET estimation across diverse climatic conditions.