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The estimate methods of evapotranspiration was studied in several methods including field measurement, instrument measurement and Penman - Monteith formula. There are slight differences between the results calculated by the 3 methods. As a traditional method, field measurement has high credibility. By the analysis of the results between field measurement and instrument measurement, the maximum stage error is more than 10%, the minimum stage error is 2.7% and the average error is 3.11%. This conclusion indicates that the instrument measurement called ENVIdata which is automatic measurement system have an acceptable accuracy to estimate evapotranspiration. The stage error is about 5% between the evapotranspiration results of Penman Monteith formula calculation and field measurement, the average error in whole growth period is 4.24%. The result of Penman Monteith formula calculation is higher than the result of field measurement, analysis the causes of this phenomenon, the reason is that there is not any drought stress between the whole period as the background of formula calculation, but in fact drought stress may be occured during the irrigation intervals of surge irrigation.