Crop coefficient for cotton under plastic mulch and drip irrigation based on eddy covariance observation in an arid area of northwestern China

Crop coefficient (Kc) is an important parameter in irrigation management. Kc 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 Kc 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 526mm, and the locally developed Kc values during the initial, mid, and late seasons are 0.23, 0.88, and 0.44, respectively. The relationships between Kc and leaf area index (LAI), Kc and growing degree days (GDD), and Kc and growing days after sowing (GD) were further analyzed. The relationship between weekly Kc and GDD, as well as weekly Kc and GD, during the growth stage after sowing is fitted to a third-order polynomial model (R2=0.96 and 0.95, respectively). Meanwhile, the relationship between weekly Kc and LAI is suitably described using a logarithmic model (R2=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 (R2=0.88, 0.88, 0.72 for GD, GDD, and LAI models, respectively) but accurate on a daily basis (R2=0.81, 0.81, 0.89 for GD, GDD, and LAI models, respectively). GD is a reliable indicator to estimate Kc 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.