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Crop coefficient (K-c) is an important parameter in irrigation management. K-c values published by Food and Agriculture Organization (FAO) are subject to uncertainties and adjustment when applied to different areas under different climates and/or irrigation methods. This study investigated K-c of cotton in an experimental site under plastic mulch and drip irrigation in the Tarim River Basin in northwestern China through a 3-year crop evapotranspiration (ETc) observation by using the eddy covariance system. The average ETc during the growing seasons in 3 years is 526 mm, and the locally developed IQ values during the initial, mid, and late seasons are 0.23, 0.88, and 0.44, respectively. The relationships between K-c and leaf area index (LAI), K-c and growing degree days (GDD), and K-c and growing days after sowing (GD) were further analyzed. The relationship between weekly K-c and GDD, as well as weekly K-c and GD, during the growth stage after sowing is fitted to a third-order polynomial model (R-2 = 0.96 and 0.95, respectively). Meanwhile, the relationship between weekly K-c and LAI is suitably described using a logarithmic model (R-2 = 0.87). As LAI was not measured every day, daily and weekly models were reestablished based on LAI measurement days. The performance of the LAI model is low on a weekly basis (R-2 =0.88, 0.88, 0.72 for GD, GDD, and LAI models, respectively) but accurate on a daily basis (R-2 = 0.81, 0.81, 0.89 for GD, GDD, and LAI models, respectively). GD is a reliable indicator to estimate K-c of cotton under plastic mulch and drip irrigation and provides a basis for evapotranspiration estimation in cotton fields by using the FAO-56 method. This study provides valuable supplementary and reference information for efficient water management in cotton cropping systems in arid regions. (C) 2016 Elsevier B.V. All rights reserved.