Diagnosis Research Of Cotton Water Status Based On Infrared Thermal Imaging

The diagnosis method of crop water status that based on canopy temperature, as it can rapidly measure crop population in a large range and has the advantage of no destruction to plants, is one of the effective methods to diagnose water deficit of field crops, and is also the basis of monitoring regional crop water status using satellite or space remote sensing data. In recent years, the infrared thermal imaging technology has been developed rapidly, and it can accurately reflect the temperature distribution of the observation area, and provide a reliable method for obtaining accurate crop canopy temperature. When the infrared thermal imager is used in the field, the shooting direction, angle, distance and other factors have different effects on the observation results, but the related research at home and abroad is not considered systematically. In this paper, the influential factors(observation direction, angle and distance) of canopy temperature observation were studied by analyzing the correlation relationships between the crop water stress index(CWSI) and soil water content, leaf water potential, stomatal conductance, a suitable method for monitoring or diagnosing of cotton water status based on the infrared thermal imaging was put forward. On this basis, by analyzing and comparing the relationship between the temperature at the dry/wet reference surface and the canopy temperature, a suitable and simplified calculation model of cotton CWSI in the North China Plain was suggested, to promote and strengthen the research work in this aspect in China.After research and analysis on experimental data, the main conclusions are as follows:(1) The influential factors(emissivity, background temperature, angle and distance) on the canopy temperature measurement using the infrared thermal imager were studied, as the emissivity reduced at 0.1 unit, the canopy temperature showed a downward trend, and the difference was more and more big; the commonly set the emission rate of 0.96, 0.97 and 0.98 as the research object, the temperature difference between the three emission rate was small, so 0.96~0.98 were suggested as the emission rate of infrared imager in cotton experiment. When the background temperature was set up near the target object temperature, canopy temperature acquired was the largest and most accurate, While the background temperature was lower or higher than the target temperature, canopy temperature observations were low, especially when the background temperature was higher than the target temperature, canopy temperature observations reduced significantly. It has been suggested that background temperature is set up close to or slightly lower than the target temperature. The shooting angle of thermal imaging had an effect on the canopy temperature, shooting vertically to object could obtain the maximum temperature, the canopy temperature decreased with the reduction of the angle, so the vertical shooting angle to the object was recommended. The canopy temperature gradually decreased with the increase in shooting distance, there was a good power function relationship between the shooting distance and canopy temperature, the canopy temperature obtained at the far distance shot could be calibrated by referring to this formula.(2) The determination method of dry and wet reference surface in CWSI experience calculation model was proposed. It is concluded that the leaf temperature after spraying water 1-1.5 minute is selected as the lower boundary temperature, and the leaf temperature that smeared vaseline 1-1.5 minutes as the upper boundary temperature, which can respectively represent the leaf temperature that in a state of potential transpiration and the one in stomata closed completely without any transpiration.(3) Cotton canopy temperature and CWSI were acquired by the same and opposite direction of the sun with the observation angle of 30°, 45° and 90°, and the correlation relationships between the CWSI, observed using different methods, and soil water content(SWC), leaf water potential(LWP) and stomatal conductance(gs) of cotton in different water treatments were analyzed. Results indicated that the CWSI which being measured in the opposite direction of the sun with the observation angle of 45°, had a good correlation with LWP, gs and SWC, and it could be shown that this was a suitable method of observing canopy temperature.(4) By analyzing and comparing the relationship between the temperature at dry/wet reference surface and the canopy temperature, a suitable and simplified model of cotton CWSI based on Jones experience model was developed which taking cotton canopy temperature as the only parameter. The CWSI value calculated by the formula can better evaluate the crop water status, which has a certain practical value in North China Plain.