Abstract
The FAO-56 Penman-Monteith model is recognized as the standard method for estimating reference evapotranspiration (ETo) which requires daily meteorological data as inputs. Among all input data, vapour pressure deficit (VPD) is one of the critical parameter that drives evapotranspiration (ETo), and is of fundamental importance in crop models. In this study effort has been made to compare six vapour pressure models during four seasons. Three vapour pressure models (Models 1-3) selected as mentioned in Irrigation and Drainage Paper-56 of the Food and Agriculture Organization (FAO-56) and Models 4-6 has been selected from literature survey. Model 1, which uses daily maximum and minimum temperature, relative humidity (RH), is the preferred method to estimate actual air pressure (AE) hence it is used as standard for comparing other models. The effectiveness of vapour pressure models were measured by statistical tools and ranked according to Global Performance Indicator (GPI) where higher value of GPI represent best model. The ranking order using GPI shows that Model 5 resulted in best estimation capability with a GPI of 2.77. Moreover, the effect of variation in wind speed on the performance of the vapour pressure models in ETo estimation is also assessed.