Penman and Thornthwaite Equations for Estimating Reference Evapotranspiration Under Semi-Arid Environment

Abstract

The estimation of reference evapotranspiration (ETo) is required for effective development and management of agriculture water systems. In order to define the most accurate method to estimate ETo in semi-arid climatic environment of Faisalabad, Lahore and Peshawar. Penman ETo method and Thornthwaite ETo method are compared with standard Penman-Monteith (PM) ETo method. The statistical results show that the Penman ETo method overestimate ETo as compared to the PM ETo method in all the semi-arid climatic regions of Faisalabad, Lahore and Peshawar by 34.91%, 39.51% and 30.75%, respectively. The coefficient of determination (R²) were 0.98, 0.98 and 0.99 at Faisalabad, Lahore and Peshawar weather stations, respectively. The root mean square error (RMSE) are 2.47 mm/day, 2.64 mm/day and 2.19 mm/day at Faisalabad, Lahore and Peshawar weather station, respectively. The mean bias error (MBE) of-2.41 mm/day,-2.58 mm/day and-2.13 mm/day are noted at Faisalabad, Lahore and Peshawar weather stations, respectively. The statistical results of Thornthwaite (Th) ETo method with PM ETo method indicate underestimation of ETo in winter season and overestimation of ETo in summer season by 13.81%, 22.43% and 14.54% at Faisalabad, Lahore and Peshawar stations, respectively. The coefficient of determination (R²) of Thornthwait ETo method when compared with PM ETo method 0.92, 0.89 and 0.95 are noted at Faisalabad, Lahore and Peshawar weather stations, respectively. The root mean square error (RMSE) are 2.14 mm/day, 2.36 mm/day and 1.16 mm/day at Faisalabad, Lahore and Peshawar weather stations, respectively. The mean bias error (MBE) are-0.68 mm/day,-1.12 mm/day and 0.61 mm/day at Faisalabad, Lahore and Peshawar weather stations, respectively. Overall, Thornthwaite method gave better estimation of ETo than Penman ETo method at all the Weather stations.