Estimation of Evapotranspiration in a Rice Irrigation Scheme in Peninsular Malaysia

The correct estimation of ET in the water balance equation allows for improved water management in rice cultivation. Eight évapotranspiration estimation methods (Penman, Penman-Monteith, Pan Evaporation, Kimberly-Penman, Priestley-Taylor, Hargreaves, Samani-Hargreaves and Blaney-Criddle) were tested with 30 years of daily data, at a study site in the west coast of Peninsular Malaysia. The estimation of évapotranspiration by all methods showed the same trend throughout the year. The Samani-Hargreaves method gave the highest estimation followed by the Priestley-Taylor and Hargreaves methods. The Penman-Monteith method gave the lowest estimations of évapotranspiration followed by the Blaney-Criddle method and then the Pan method. The Penman-Monteith, Blaney- criddle and Pan methods gave lower values of evapotranspiradon with no significant difference among them (P = 0.05). All the other estimation methods were significantly different from these three methods. The Penman method, though was different from the three methods in terms of development; however, it estimates évapotranspiration close to these three methods. The Penman-Monteith, Blaney-Criddle and Pan were found to be the better methods to estimate évapotranspiration in the study area. Results from this study showed that the Penman method can be used to get somewhat reasonable estimates though it tends to overestimate évapotranspiration. Comparisons of these selected methods against the Penman- Monteith method showed that they have good correlation. The Pan, Blaney- Criddle and Penman gave correlation coefficients of 0.87, 0.55 and 0.97 respectively. A simple correlation equation, developed using 30-year daily data, showed that direct measurement of net radiation can be used to estimate reference évapotranspiration with considerable accuracy (r^2 = 0.97).