Comparison of the reference crop evapotranspiration estimated by the Penman-Monteith and Penman methods in Tarim River Basin

Based on the daily meteorological data in Aksu water balance experimental station in Tarim River Basin from 1989 to 1996, the reference evapotranspirations (ET₀) calculated by the Penman-Monteith and Penman methods were compared. The results showed that the annual ET₀ calculated by Penman method was a little higher than that obtained by Penman-Monteith method. The absolute deviation of yearly ET₀ between the two methods was 42~128 mm, the yearly relative deviation was 3.3%~9.8%. The difference of monthly ET₀ was smaller. The absolute deviation of monthly ET₀ between two methods was above zero in January, February and December, and was below zero from March to November. The relative deviation of monthly ET₀ was bigger in January and December, was smaller in February and November, the variation of the relative deviation of monthly ET₀ was small in other months. The main cause of the deviation of ET₀ was that different equations and parameters were used to calculate the radiation item and aerodynamic term in the equations. The correlation between ET₀ calculated by two methods had linear relationship.