The Exergy Destruction Principle as a Theory to Detect Crop Stress Using Thermal Remote Sensing

Abstract:

Thermal remote sensing is widely used in many applications; as an assessment tool for urban heat island, as an ecological indicator of ecosystem development, and as a water stress detection tool. In this study, the exergy destruction principle is applied as a theory to explain the expected inverse relationship between surface temperature and crop stress. Two hypotheses are developed as predicted by the exergy destruction principle. It is hypothesized that agricultural crops experiencing greater growth and providing greater yield will have lower surface temperatures. The second hypothesis is that crops grown under less stress growing conditions will have lower surface temperatures compared to crops grown under highly stress conditions, which include nitrogen stress as well as other crop stressors. The two proposed hypotheses are tested under variable greenhouse and field conditions at three different scales (i.e., leaf, canopy and over a plot area of 60 m²). It is found that surface temperature decreases during the day as the rate of nitrogen increases yielding a shallow, but statistically significant (P < 0.05) negative slope. Surface temperature measurements, however, are highly variable. This variability is the result of many external and weather dependent variables that affect crop surface temperature. The exergy destruction principle (EDP) provides a theoretical background from which thermal remote sensing can be applied to detect physiological stress in crop plants at early growth stages, before any visual indicators appear on a plant surface. Furthermore, spectral reflectance measurements were conducted on corn leaves in the thermal infrared waveband, and the spectral emissivity was calculated. An average of 0.96±0.006 emissivity for corn leaves in the 7.5-14 µm waveband is determined from multiple laboratory experiments using the Surface Optics Corporation (SOC 400T) infrared reflectometer and the Bruker Fourier transform infrared spectrometer. This study is important to enhance the precision agriculture in the application of nutrients, herbicides, and pesticides to crop plants at an optimal time, rate and location, which subsequently increase the production, reduce the cost of excessive input application, and reduce harmful impacts on the environment.