In order to select a reliable hyperspectral index for winter wheat plant water diagnosis and determine the threshold of the selected hyperspectral index for an appropriate water state of plant, in this study, four winter wheat water treatments(irrigation amount of 0, 60, 120, and 180 mm) were set up. The data of canopy hyperspectral reflectance, plant water content, soil water content, and yield at key growth stages(regreening stage, jointing stage and filling stage) were obtained. The hyperspectral index was screened according to the correlation between the hyperspectral index and the water content of winter wheat plants. The selected hyperspectral index was used as the input variable to construct the water content estimation model of winter wheat plants by simple regression, partial least squares regression, random forest regression, and support vector regression. Considering the influence of soil moisture content on winter wheat plant moisture content, the relationship between plant moisture content and soil moisture content at different stages was further quantified. The hyperspectral index thresholds of winter wheat plantmoisture and soil moisture were determined according to the yield comparison method. The results showed that at regreening stage, jointing stage and filling stage, the accuracy of the univariate regression model(r2=0.673,RMSE=3.144%,RE=5.489%) was better. It could determine the threshold of hyperspectral index and realize the moisture diagnosis of winter wheat accurately and quickly. Among the machine learning algorithms, the accuracy of random forest regression model(r2=0.904, RMSE=1.701%, RE=3.606%) was the highest, but there were too many model parameters to give the hyperspectral index threshold. There was a strong positive correlation between the water content of plant on the day and the water content of the 0-50 cm soil layer the day before(r2=0.708, RMSE=2.436%, RE=7.755%). MCARI/OSAVI, PRI3, and VEG were the best hyperspectral indices for estimating the water content of winter wheat plants at the regreening stage, jointing stage and filling stage, respectively. When the plant was in a suitable water state, the corresponding hyperspectral index thresholds were 0.765 1-1.130 1,0.155 2-0.225 7, and 1.633 9-1.668 5, respectively. Therefore, the water status of winter wheat during the critical growth period can be determined based on the relationship between plant moisture content and soil moisture content, and corresponding countermeasures can be taken. |