Comparison of Some Empirical Estimating Methods of Reference Evapotranspiration in Tabriz Plain Using Lysimeter and Proposing a Model for its Determination from Climatic Data

Determining the evapotranspiration of plants is possible mainly by two methods, namely direct measuring and indirect estimation using climatic parameters. Evapotranspiration determination using direct method is difficult, costly and time consuming. For this reason, some indirect methods have been proposed for determination of crop water requirement with acceptable accuracy. A lysimetric study was conducted to determine the reference evapotranspiration (ETo) in Tabriz plain in a loamy soil. Evapotranspiration was measured in ten days and monthly intervals by water balance equation method and then compared with ETo obtained using some empirical methods. Results indicated that the average seasonal ETo and pan evaporation (Ep) for 9 months were 1226.9 and 1947.3 mm respectively. Comparison between the measured and estimated ETo by various methods showed that the pan evaporation (class A) and Hargreaves methods had the highest and lowest correlation with lysimeteric data, respectively. Based on the statistical analysis, Penman-Monteith method had the closest estimates to lysimeteric measurement. In general, Penman-Monteith was introduced as a suitable method for estimating reference evapotranspiration in Tabriz plain. Also, the best regression equation for estimating ETo using meteorological parameter was introduced.