A Comparative Study of Potential Evapotranspiration Estimation by Three Methods with FAO Penman–Monteith Method across Sri Lanka

Among numerous methods that have been developed to estimate potential evapotranspiration (PET), the Food and Agricultural Organization Penman–Monteith model (FAO P–M) is often recognized as a standard method PET. This study was conducted evaluate applicability of three other PET estimation methods, i.e., Shuttleworth–Wallace (S–W) model, Thornthwaite (TW) pan across Sri Lanka with respect FAO P–M model. The meteorological data, temperature, relative humidity, wind speed, net solar radiation, evaporation, recorded at 14 meteorologic stations, representing all climate topographic zones Lanka, were obtained from 2009 2019. models’ performances assessed based on statistical indicators: root mean squared error (RMSE), bias, Pearson correlation coefficient (R). In comparison estimates, seasonal annual estimates models show great differences. results suggested S–W perform better in dry zone country. Both underestimated over entire county seasons. TW does not consistent country, thus being found least reliable alternative. Although highly correlated application data-scarce region more constrained, it requires parameters than Thus, suggests employing alternative country instead one single