Modeling Of Soil Moisture Content In Maize Root Fields By UAV Thermal Infrared Remote Sensing

Quick and accurate acquisition of field soil moisture content information is essential for the development of water-saving and high-efficiency agriculture.UAV thermal infrared remote sensing has the characteristics of real-time,high-efficiency,and rapidity.It can quickly obtain regional surface temperature information and effectively diagnose soil moisture content,greatly improving the monitoring efficiency of field soil moisture content and reducing detection costs.Therefore,this study takes corn processed with different water gradients as the research object,and uses the drone remote sensing platform to obtain visible and thermal infrared images of the key growth periods（trilobal period,jointing period,tasseling and silking period,and milk maturity period）of corn.Otsu threshold method,RGRI index method,EXG threshold method and other methods are used to extract corn canopy temperature information and soil temperature information,and further construct crop water stress index（CWSI）,canopy soil temperature range ratio(the ratio of The canopy temperature range to the soil temperature range（CS）,comprehensive water deficit index（CWDI）and other indicators,analyze their correlation with the soil moisture content at different depths in each growth period of maize,and then evaluate the accuracy.The following results were obtained:（1）A new model of soil moisture content inversion is proposed,which eliminates the soil background method.The RGRI index method and the Otsu threshold method are used to eliminate the soil background in the thermal infrared image.The comparison shows that the RGRI index method can eliminate the soil background better,and the extracted canopy temperature has the highest correlation with the measured temperature.The CWSI established based on the three methods and the soil moisture content shows a negative correlation response trend as a whole.Compared with CWSI_Otsuand CWSI _soil,CWSI_RGRIhas a higher linear correlation with soil moisture content,and the inversion effect of soil moisture content is better.It performed good with soil moisture content in the depth of0～10cm,0～20 cm,0～30 cm in the jointing stage,and its R²were on the rise which reached the best in the depth of 0～30 cm.（2）A new method for extracting canopy temperature based on the idea of??coverage is proposed—EXG threshold method,and a new index for inversion of soil moisture content—canopy-soil temperature range ratio（CS）is constructed.The canopy temperature and soil temperature are extracted by the EXG threshold method,and then the crop water stress index CWSI and the canopy soil temperature range ratio CS are established to invert the soil moisture content.The results show that CWSI_EXGin the three-leaf stage and jointing stage has a better effect than CWSI_Otsuin retrieving soil moisture content under semi-covered conditions.R²both increased first and then decreased.It reached the maximum at the depth of 0～20 cm in the three-leaf stage and 0～30cm at the jointing stage respectively.The R²of CS with the soil moisture content in the three-leaf stage,jointing stage,tasseling and silking stage at each depth were all above 0.55,and showed a trend of firstly increasing and then decreasing with the increase of soil depth.This indicates that under the condition of semi-coverage,the CWSI_EXGand canopy soil temperature range ratio CS established based on the EXG threshold method can be used as an index for the inversion of soil moisture content.（3）A comprehensive temperature index CT combining canopy temperature and soil temperature based on the coverage ratio is proposed,and a comprehensive water deficit index CWDI is constructed to invert the soil moisture content.Based on the EXG index method,the canopy temperature,surface soil temperature and coverage information were extracted,and the comprehensive temperature CT and comprehensive water deficit index CWDI were further constructed.Comparing the traditional canopy temperature and the crop water stress index CWSI,in the three-leaf stage and the jointing stage,R²of the comprehensive temperature CT with the measured temperature reached 0.829 and 0.916,respectively,which performed well;in the tasseling and spinning stage,the comprehensive temperature CT and the crown The R²of the layer temperature was very close;CWDI performed better in the jointing and tasseling stages under the half-covered condition,and the R²of the soil moisture content at each depth was higher than CWSI;in the three-leaf stage with lower coverage,CWDI performed worse than CWSI.