Comparison of estimating ET0 with different methods in Hetao Irrigation District in Inner Mongolia

In order to obtain the proper calculation methods for reference crop evapotranspiration (ET0) in Hetao Irrigation District in Inner Mongolia and based on the micrometeorological data, four calculation methods (Priestly-Taylor. FAO Penman, Hargreaves-Samani, Irmark-Allen)　for ET0 were compared with FAO56 Penman-Monteith, respectively, and their applicabilities in Hetao Irrigation District were evaluated. The results show that calculation results of FAO Penman method is closest with FAO56 Penman-Monteith, The average absolute error and average relative error are 0.43 mm/d and 12.50%, respectively. Other methods have different positive and negative deviations in different seasons. In the total calculation periods, deviations between the results by Irmark-Allen, Hargreaves-Samani and Penman-Monteith method are larger, so they can not be used in the district for its weather conditions. the results by FAO Penman method are similar with FAO56 Penman-Monteith method and they can be applied to the most kinds of climate condition. Hargreaves-Samani method can be used in the district in which the difference in temperature is smaller. Irmark-Allen and Priestley-Taylor methods can be used in the area in which the relative humidity is larger.