Characteristics and influencing factors of crop coefficient for drip-irrigated cotton under plastic-mulched condition in arid environment

Crop coefficient (K-c) is a very useful and widely used variable in evapotranspiration estimation in cropland. Traditional methods in calculating K-c are based on field water balance, which is limited by long measurement interval and small study area. In addition, there is the need for K-c under new agronomy practice such as plastic mulching and drip irrigation in arid environments. This study calculated and analyzed K-c of a drip-irrigated and plastic-mulched cotton field in Aksu Oasis of the arid Tarim River Basin, China, and its relationships with several crop-, soil-and management variables such as relative growth days (RGD), leaf area index (LAI), extractable soil water (ESW), and irrigation, based on two years' observations. The results showed that daily K-c varied within the range of 0.08-1.28, with an average of 0.54 for the entire cotton growth season, in 2013 and 2014. Compared to non-mulched condition already published, the K-c of mulched cotton for the entire growth season decreased by 16 to 39. The largest reductions in Kc due to plastic mulch were found in the initial and developmental growth stage. K-c could be calculated by a third-degree polynomial model in relation to RGD, which was satisfactorily validated and can be used in other studies under the same or similar pedo-climatic and management conditions. Cotton LAI and ESW were found to be important factors influencing K-c, particularly their critical values of 3.0 in LAI and 0.5 for ESW. Moreover, the results showed that irrigation significantly increased K-c, i.e., 29 on average, partly due to arid advection. This study provided up-to-date and detailed information on cotton crop coefficient under plastic mulching and drip irrigation conditions in arid environment, and it is useful for improved management of agricultural water resources.