Exploring thermal spectroscopy for vegetation applications
The thermal infrared (TIR; 2.5 – 15 µm) domain affords two unique measurements of plant characteristics through emissivity and land surface temperature (LST) that are not possible using the visible-shortwave infrared spectrum (0.35 – 2.5 µm). These measurements are being used for vegetation research at multiple scales including the determination of leaf water content, plant evaporation rates , and large-scale drought monitoring. The increased availability of TIR sensors and technological advances have now made it increasingly possible to examine vegetation characteristics using emissivity and LST measurements. TIR emissivity signatures are unique from the visible-shortwave infrared because of how light interacts with the leaf. Due to the low energy of TIR wavelengths, leaves appear opaque because energy does not penetrate leaf surfaces. Measured spectral emissivity for a leaf is the product of light interactions with the outer layers of the leaf. In contrast, spectra in the visible-shortwave infrared domain are the product of light interactions with the internal leaf structure and biochemical components because the high energy wavelengths of this domain can penetrate leaf surfaces.
Examples of non-photosynthetic vegetation (NPV) reflectance spectra of several materials across the visible/shortwave infrared (0.35–2.5μm) and thermal infrared (2.0–15.4μm) wavelength ranges. Minor spikes or troughs centered at 4.3μm are due to residual carbon dioxide and when present, these wavelengths should be removed.
More thermal instruments are coming on line every year, including the ECOsystem Spaceborne Thermal Radiometer Experiment on Space Station (ECOSTRESS) was deployed on the International Space Station (ISS). ECOSTRESS provides data with a 38-m in-track by 69-m cross-track spatial resolution and five spectral bands in the 8 – 12.5 μm range with an additional band at 1.6 μm for geolocation. In order to understand the variability seen at these pixel sizes, it is often useful to examine leaf spectral variability. However, leaf spectra, especially in the TIR spectra, are not commonly available or only in small sample sizes. To assist researchers in addressing the ECOSTRESS science goals, laboratory spectra of vegetation and non-photosynthetic vegetation (NPV) in the visible/shortwave-infrared (VIS/SWIR; 0.35 – 2.5 µm) and thermal (TIR; 2.5 – 15 µm) spectral range are needed. Existing spectral libraries are limited in assisting this mission either by a lack of NPV spectra or TIR spectra of plant materials. To support ECOSTRESS activities, the Advanced Spaceborne Thermal Emission Reflection Radiometer (ASTER) spectral library was updated and renamed the ECOSTRESS spectral library version 1.0. The updated library represents a significant increase from the original four vegetation spectra and now contains 541 vegetation VIS/SWIR spectra, 472 vegetation TIR spectra, and 51 NPV VIS/SWIR and TIR spectra. The ECOSTRESS spectral library 1.0 has over 3000 spectra of lunar and terrestrial soils, man-made materials, meteorites, minerals, NPV, rocks, vegetation, and water/snow/ice. Available at https://speclib.jpl.nasa.gov/
HyTES imagery at 2 m resolution (1100 AGL flight altitude) from 2014 July 5 showing emissivity (left panel) and Land Surface Temperature (LST; middle panel). Emissivity is displayed with bands 10.1μm as red, 9.2μm as green, and 8.5μm as blue. Right panel shows high spatial resolution RGB imagery of the Huntington Gardens study area in San Marino, California, USA.
The thermal domain (TIR; 2.5 – 15 µm) delivers unique measurements of plant characteristics that are not possible in other parts of the electromagnetic spectrum. However, these TIR measurements have largely been restricted to laboratory leaf level or coarse spatial resolutions due to the lack of suitable data from airborne and spaceborne instruments. The airborne Hyperspectral Thermal Emission Spectrometer (HyTES) provides an opportunity to retrieve high spectral resolution emissivity and land surface temperature (LST) that can be exploited for canopy level vegetation research. This study has a high spatial resolution analysis of plant species’ emissivity and LST using HyTES imagery acquired in the Huntington Botanical Gardens. Leaf and canopy emissivity variation were identified among 24 plant species and used to determine leaf to canopy scaling capabilities. This research represents the analysis of vegetation characteristics using the NASA’s HyTES TIR sensor, opening the door for future remote sensing vegetation studies that include using the recently launched ECOsystem Spaceborne Thermal Radiometer Experiment on Space Station (ECOSTRESS) mission.
Meerdink, S.K., Roberts, D. A., Hulley, G., Gader, P.D., Pisek, J., Raabe, K., King, J., & Hook, S. J. (2019). Plant species’ spectral emissivity and temperature using the Hyperspectral Thermal Emission Spectrometer (HyTES) sensor. Remote Sensing of Environment, 224, 421–435.
Meerdink, S.K., Hook, S.J., Abbott, E.A., & Roberts, D.A. (2019). The ECOSTRESS Spectral Library 1.0. Remote Sensing of Environment. https://speclib.jpl.nasa.gov/