Estimation of Evapotranspiration of a Jujube/Cotton Intercropping System in an Arid Area Based on the Dual Crop Coefficient Method

An accurate estimation of crop evapotranspiration (ET_c) in intercropping is critical for a theoretical basis for formulating an intercropping irrigation system of fruit trees and improving the soil moisture condition of orchards in arid regions of Southern Xinjiang. Herein, observational data such as soil moisture and plant physiological indicators were measured in jujube/cotton intercropping in the Aksu region from 2015 to 2017. The evapotranspiration of single-crop jujube and cotton was estimated using the modified dual crop coefficient method. Then, based on the proportion of intercropping crops, the soil water stress coefficient was introduced to estimate the evapotranspiration of the jujube/cotton intercropping. The results show that the model has good applicability to simulate single-crop jujube and cotton, and jujube/cotton intercropping. However, compared with single cropping, the accuracy of simulated daily evapotranspiration was decreased. In addition, adjusting the cotton irrigation amount caused the simulation accuracy to further decrease. From the perspective of interspecies complementarity and competition, cotton dominated the intercropping system and had better tolerance to external environmental changes than jujube trees. Intercropping had negative effects on jujube trees in general, and very obvious positive effects on cotton. Based on the 3-year crop yield, 5-year-old jujube trees are recommended for intercropping. During this time, the yield of cotton under the effect of interspecific complementation increased by 26.47%, and the yield of intercropping jujube was similar to that of single crop. As the jujube tree age increases, the effect of increasing cotton production gradually diminishes. The jujube trees also had a significant reduction in yield due to interspecific competition. Our research supports the dual crop coefficient method as appropriate to estimate crop ET_c in intercropping and may be further used to improve irrigation scheduling for jujube/cotton intercropping.