A multi-decadal national scale assessment of reference evapotranspiration methods in continental and temperate climate zones of South Korea

Agricultural water resource management and hydrological models rely heavily on reference evapotranspiration (ET0), which is typically calculated using FAO-56 Penman-Monteith (FPM) method. However, FPM is restricted in its applicability by comprehensive meteorological data requirements, motivating identification of alternative approaches for estimating ET0 that require fewer meteorological inputs. This study compares 30 empirical models for estimating ET0 over 50 years (1972-2021; at daily scale) at 30 stations spanning continental (22 stations) and temperate (8 stations) climate zones across South Korea. Candidate empirical models were categorized based on their composition (temperature-based (n = 5), radiation-based (n = 15), temperature-radiationbased (n = 2), and mass-transfer-based (n = 8)) and complexity with models requiring between one and four meteorological variables. Model performance did not vary significantly across mapped climate zones (i.e., continental vs. temperate), and more complex models (i.e., those requiring 4 meteorological inputs) did not necessarily yield better prediction. Temperature-radiation-based models were consistently effective, whereas the models based on mass-transfer were always poor. Given the lack of long-term spatially extensive meteorological data in most developing countries in continental and temperate climate zones, this study establishes best practices for estimating ET0 using empirical models, which will aid water resources management decisionmaking in regions with limited meteorological data.