Abstract In order to improve estimation accuracy of evapotranspiration(ET)of crops in arid areas, the ET of spring maize in Shiyang River Basin in growth middle stage was estimated with FAO-56 in the paper. The effect of irrigation quota on estimation accuracy of ET was identified by comparing calculated values with measured ones. The estimation error was discussed by analyzing the spatial distribution of soil moisture and its change rule under different irrigation treatments. It was proposed that the average water moisture in the entire root zone rather than in the partial root zone should be used in the calculation of soil water stress coefficient. The results show that the FAO-56 can estimate ET accurately under low irrigation quota treatment, however, its estimation accuracy decreass with increasing irrigation quota. As a result, the estimation error in ET is as poor as -14.13% under high irrigation quota. The soil water stress coefficient is calculated with the average soil moisture in the soil moisture slowly changing layer and above. This method can improve estimation accuracy of ET of crops in arid areas under high irrigation quota treatment effectively.
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