Analysis Of Crop Canopy Temperature Variation And Its Hysteresis Effect To Environmental Factors

Crop canopy temperature is an important index to characterize crop canopy energy balance and physiological and ecological status,which is of great significance to diagnosis of farmland water status,crop selection and breeding,etc.Studying the change law of canopy temperature and its response to environmental factors has certain practical significance for strengthening water resources management and effectively monitoring crop growth.Therefore,in this study,winter wheat and summer corn from 2019 to 2020 were taken as the research objects,and the canopy temperature of winter wheat and summer corn during the growth period was continuously monitored by the precision infrared temperature sensor(SI-411),and the data of environmental factors such as solar radiation,atmospheric temperature and relative humidity in the experimental site were obtained synchronously.By dividing different weather conditions(typical sunny days and typical rainy days)and setting different moisture treatments(I1:85%～100% of field capacity,I2:70%～85% of field capacity and I3:50%～65% of field capacity),the diurnal variation law of canopy temperature was analyzed.The Gaussian peak detection algorithm was used to analyze the peak time difference between canopy temperature and environmental factors under different conditions,and the time lag between canopy temperature and environmental factors was calculated by the time lag cross-correlation analysis method.Finally,the influence of time lag effect on the accuracy of canopy temperature simulation and soil moisture monitoring was analyzed.The main conclusions are as follows:(1)The variation law of winter wheat canopy temperature under different weather conditions and its response to environmental factors such as solar radiation,atmospheric temperature and relative humidity are different.On typical sunny days,the diurnal variation time series of canopy temperature and various environmental factors are single-peak curves,while on typical rainy days,they are multi-peak curves.On typical sunny days,the response curve of canopy temperature to various environmental factors is circular,that is,the rising curve does not coincide with the falling curve,in which the response loop of canopy temperature to solar radiation and relative humidity is counterclockwise,while that to atmospheric temperature is clockwise,but there is no obvious circular curve in rainy days.There is an obvious time lag effect between canopy temperature and various environmental factors.In sunny days,canopy temperature lags behind solar radiation for 50 min,and is ahead of atmospheric temperature,relative humidity for 40 min and 30 min respectively.There is no time lag effect between canopy temperature and soil heat flux.In rainy days,canopy temperature lags behind solar radiation for 30 min and is ahead of atmospheric temperature for 20 min,and there is no time lag effect between canopy temperature and relative humidity as well as soil heat flux.(2)There are some differences in canopy temperature changes of summer maize under different water treatments,and their lag response time to environmental factors is also different.There are some differences in the peak values of canopy temperature under different water treatments.The peak values of canopy temperature gradually increase with the decrease of soil water content,and the time to reach the peak values is delayed.Gaussian peak detection algorithm is used to determine the peak time difference between canopy temperature and various environmental factors,and the normalized delay loop area between them is calculated.It is found that the normalized delay loop area of different water treatments has significant difference,and the larger the normalized delay loop area,the larger the peak time difference.By using the time-lag cross-correlation analysis method,it is calculated that the time of the canopy temperature of I1,I2 and I3 moisture treatments is 50,40 and 80 min behind the solar radiation,and the time of the canopy temperature is 40,40 and 20 min ahead of the atmospheric temperature and relative humidity,respectively.The relationships between various environmental factors and canopy temperature are in good agreement with the quadratic equation of one variable,and the fitting accuracy of canopy temperature can be obviously improved by considering the time lag effect.(3)Considering the time lag effect of canopy temperature and atmospheric temperature can improve the accuracy of monitoring soil moisture content by canopy-air temperature difference.Taking summer maize at jointing-heading stage and filling stage as the research object,it was found that the canopy temperature and atmospheric temperature at jointing-heading stage reached the peak later than that at filling stage,and the time lag between canopy temperature and atmospheric temperature at jointing-heading stage was about 20 min longer than that at filling stage.Comparing the mean values of the canopy temperature difference from 11:00 to 13:00,before and after considering the time lag effect,it is found that considering the time lag effect will significantly reduce the canopy temperature difference,and the canopy temperature difference considering the peak time difference is the smallest,followed by considering the time lag,without considering time lag is the maximum.Monitoring soil moisture content at different depths in different growth stages by using canopy-air temperature difference shows that considering the time lag effect of canopy temperature can improve the monitoring accuracy of soil moisture content to a certain extent.At jointing-heading stage,the monitoring accuracy of soil moisture content at different depths by considering the peak time difference is the highest,followed by considering the time lag,and the accuracy without considering the time lag is the lowest.At the filling stage,the monitoring accuracy of soil moisture content in 0-10 cm and 0-20 cm soil layer is improved higher by considering the peak time difference,and in 0-40 cm and0-60 cm soil layer is improved higher by considering the time delay.This provides a new reference for monitoring farmland water status by using plant physiological characteristics.