Study On The Time Lag Effect Of Canopy Temperature And Atmospheric Temperature And Their Effects On Soil Water Content

Quantitative characterization of the time lag effect between canopy temperature and atmospheric temperature and its controlling factors is helpful to improve the accuracy of soil water content inversion using canopy air temperature information.The reasons for the changes in the accuracy of canopy temperature monitoring soil moisture content at different times of the day were analyzed,and the soil moisture content monitoring model based on crop water stress index was finally established,which has important practical significance for the effective monitoring of soil moisture.Therefore,this paper took winter wheat as the research object,continuously monitored canopy temperature(Tc)under different irrigation levels(W1、W2、W3 and W4),and collected data of solar radiation(Rs),atmospheric temperature(Ta),relative humidity(RH),soil moisture content(SWC)and other environmental factors.The time delay between canopy temperature and atmospheric temperature was analyzed by synchronous day time series,and the lag time(LT)was calculated by hysteresis correlation analysis method.Multiple linear regression model is used to construct regression analysis time delay model based on solar radiation,atmospheric temperature,relative humidity and soil moisture content.Meanwhile,path analysis is used to study the interaction between various environmental factors on time delay effect.In addition,the correlation trend of canopy temperature with crop water stress index and soil water content over time was analyzed in one day,and the correlation trend of canopy temperature with crop water stress index and soil water content was mapped respectively.The correlation trend of canopy temperature with crop water stress index and soil water content before and after mapping was compared,and the mapping effect was verified according to the theoretical formula of soil water content.The main conclusions are as follows:(1)The variation of winter wheat canopy temperature and atmospheric temperature is not synchronous,and the daily cycle of canopy temperature and atmospheric temperature is clockwise.Under different weather conditions,the peak time of canopy temperature under W1,W2 and W3 irrigation levels is earlier than that under W4 irrigation level,resulting in significantly shorter lag time of W4 irrigation level than the other three irrigation levels.The difference between the mean peak time of canopy temperature and the mean peak time of atmospheric temperature was 47,37,36 and 33 minutes respectively under the irrigation level of W1,W2,W3 and W4 in rainy weather.Under the condition of cloudy weather,the mean peak time of canopy temperature was 81,76,80 and 71 minutes different from the mean peak time of atmospheric temperature under the irrigation level of W1,W2,W3 and W4.Under sunny weather conditions,the difference between the mean peak time of canopy temperature and the mean peak time of atmospheric temperature was 60,76,44 and 35 minutes respectively under W1,W2,W3 and W4 irrigation levels.(2)Under different weather conditions,the key driving factors of the time lag effect between canopy temperature and atmospheric temperature are different:under rainy weather,solar radiation is the main driving factor of the time lag effect;The relative humidity is the main driving factor of the time lag effect under cloudy and sunny weather.At the same time,the lag time of W4 irrigation level is significantly shorter than that of the other three irrigation levels,and there is a certain degree of correlation between soil moisture content and lag time only at W4 irrigation level(R~2=0.26～0.36).Multiple regression models show that solar radiation,atmospheric temperature,relative humidity,and soil moisture content together explain 68,48,and 64%of the time lag effects under rainy,cloudy,and sunny weather conditions.Path analysis shows that the main driving factors of delay effect(Rs and RH)can be enhanced by other indirect factors(Ta and SWC)in rainy weather.In cloudy weather,the main driving factors(RH and SWC)of the delay effect can be inhibited by other indirect factors(Ta).This mutual inhibition is more significant under sunny weather condition.(3)The accuracy of retrieving soil water content from canopy temperature and crop water stress index firstly increased and then decreased with time,which was caused by the relationship between crop water stress and soil water deficit with time.The best time of canopy temperature monitoring soil water content is 10:00～16:00(R~2>0.72),the best time of crop water stress index monitoring soil water content was 9:00～18:00(R~2>0.69).The normalized canopy temperature expression was used to characterize the relationship between crop water stress and soil water deficit over time.Based on this expression,the crop water stress index was mapped and then soil water content inversion was carried out,which could effectively avoid the phenomenon of low accuracy of inversion effect in the morning and evening.In the time period from 0:00 to 8:00,R~2 increased from 0.10～0.40 to 0.45～0.75,and RMSE decreased from 1.75～3.5 to 1.25～2.5.At the same time,R~2 increased from 0.30～0.50 to0.60～0.70 and RMSE decreased from 1.50～3.0 to 1.25～2.5 during 19:00 to 24:00.The theoretical expression of soil water content derived from crop water stress index also proved that considering the relationship between crop water stress and soil water deficit,the relative error in the morning and evening could be effectively reduced from 30%to 5%.This study is helpful to understand the reason why crop water stress index and soil water content change greatly during the day,and solve the problem of time limit of thermal infrared remote sensing monitoring of crop water stress.