Abstract: Farmland evapotranspiration means the sum of the soil evaporation and the crop transpiration under the field conditions, it is a link of the water circulation in the soi-l plant-air system and is affected by many factors, such as the meteorological, crop and soil conditions. In this paper, a calculation model of the evapotranspiration from the cotton fields in the Aral Irrigated Area is developed based on the experiment of the current rotary combination design of square regression of 3 factors at 5 levels, which was carried out at Aksu Station for Water Balance Observation in Xinjiang in 2000, and a comprehensive consideration about the meteorological, crop and soil conditions as well as referred to the evaporation from water surface, biological functions of cotton and the availability of soil moisture. The results are as follows： (1) The relationship between the biological functions and the leaf area index is a power function; (2) The relationship between the soil moisture availability function and the content of the available soil moisture in root area is linear. The model can be used to calculate the farmland evapotranspiration by using the data of the routine meteorological and soil humidity observation only, so it is convenient to use. The comparison between the calculated and the measured results of the field evapotranspiration shows that the calculation with the model has a high precision.

Key words: cotton field, field evapotranspiration, calculation model