Abstract: Evapotranspiration including evaporation and transpiration is a main expenditure of water budget at farmland, and it plays an important role in agricultural water management, especially in arid region. In Xinjiang, as a cotton growing area in arid region, water-saving irrigation of cotton is significant for agricultural production. Previous studies on evapotranspiration of cotton reported the total monthly or growth-stage values, however, the daily observations were unavailable. As a method with high time resolution, the eddy covariance has been widely used in measuring evapotranspiration. In this paper, the values of evapotranspiration in the whole cotton growing season were calculated and analyzed based on the observations by eddy covariance at a cotton field under drip irrigation with plastic film in North Xinjiang during the period from June 2009 to June 2010. Under the clear and cloudless terms, daily variation of evapotranspiration was highly consistent with net radiation at each cotton growth stage. The evapotranspiration was affected by net radiation during the period from sowing to emergence and at seedling and boll opening stages, and also by leaf area index at squaring, flowering and boll formation stages. Under the present irrigation regime, soil moisture content could meet the water demand for evapotranspiration in the whole cotton growing season except at boll opening stage. Daily mean evapotranspiration was 1.8, 1.7, 3.7, 3.8 and 1.2 mm/d during the period from sowing to emergence and at seedling, squaring, flowering, boll formation and boll opening stages, respectively. The total evapotranspiration in the whole cotton growing season amounted to 514.3 mm.

Key words: plastic film, drip irrigation, cotton, growth stages, evapotranspiration, eddy covariance, net radiation