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Estimation of field crop evapotranspiration (

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) and its partitioning into evaporation and transpiration, are of great importance in hydrological modeling and agricultural water management. In this study, we used a dual crop coefficient model SIMDualKc to estimate the actual crop evapotranspiration (

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) and the basal crop coefficients over a cotton field in Northwestern China. A two-year field experiment was implemented in the cotton field under mulched drip irrigation. The simulated

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is consistent with observed

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as derived based on the eddy covariance system in the field. Basal crop coefficients of cotton for the initial, mid-season, and end-season are 0.20, 0.90, and 0.50, respectively. The transpiration components of

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are 96% (77%) and 94% (74%) in 2012 and 2013 with (without) plastic mulch, respectively. The impact of plastic mulch cover on soil evaporation is significant during drip irrigation ranging from crop development stage to mid-season stage. The extent of the impact depends on the variation of soil moisture, available energy of the soil surface, and the growth of the cotton leaves. Our results show that the SIMDualKc is capable of providing accurate estimation of

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for cotton field under mulched drip irrigation, and could be used as a valuable tool to establish irrigation schedule for cotton fields in arid regions as Northwestern China.