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

Crop coefficient (Ksub(c)) is a very useful and widely used variable in evapotranspiration estimation in cropland. Traditional methods in calculating Ksub(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 Ksub(c) under new agronomy practice such as plastic mulching and drip irrigation in arid environments. This study calculated and analyzed Ksub(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 Ksub(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 Ksub(c) of mulched cotton for the entire growth season decreased by 16 to 39%. The largest reductions in Ksub(c) due to plastic mulch were found in the initial and developmental growth stage. Ksub(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 Ksub(c), particularly their critical values of 3.0 in LAI and 0.5 for ESW. Moreover, the results showed that irrigation significantly increased Ksub(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.