Challenges and Future Perspectives of Multi-/Hyperspectral Thermal Infrared Remote Sensing for Crop Water-Stress Detection: A Review

Abstract

Thermal infrared (TIR) multi-/hyperspectral and sun-induced fluorescence (SIF) approaches together with classic solar-reflective (visible, near-, and shortwave infrared reflectance (VNIR)/SWIR) hyperspectral remote sensing form the latest state-of-the-art techniques for the detection of crop water stress. Each of these three domains requires dedicated sensor technology currently in place for ground and airborne applications and either have satellite concepts under development (e.g., HySPIRI/SBG (Surface Biology and Geology), Sentinel-8, HiTeSEM in the TIR) or are subject to satellite missions recently launched or scheduled within the next years (i.e., EnMAP and PRISMA (PRecursore IperSpettrale della Missione Applicativa, launched on March 2019) in the VNIR/SWIR, Fluorescence Explorer (FLEX) in the SIF). Identification of plant water stress or drought is of utmost importance to guarantee global water and food supply. Therefore, knowledge of crop water status over large farmland areas bears large potential for optimizing agricultural water use. As plant responses to water stress are numerous and complex, their physiological consequences affect the electromagnetic signal in different spectral domains. This review paper summarizes the importance of water stress-related applications and the plant responses to water stress, followed by a concise review of water-stress detection through remote sensing, focusing on TIR without neglecting the comparison to other spectral domains (i.e., VNIR/SWIR and SIF) and multi-sensor approaches. Current and planned sensors at ground, airborne, and satellite level for the TIR as well as a selection of commonly used indices and approaches for water-stress detection using the main multi-/hyperspectral remote sensing imaging techniques are reviewed. Several important challenges are discussed that occur when using spectral emissivity, temperature-based indices, and physically-based approaches for water-stress detection in the TIR spectral domain. Furthermore, challenges with data processing and the perspectives for future satellite missions in the TIR are critically examined. In conclusion, information from multi-/hyperspectral TIR together with those from VNIR/SWIR and SIF sensors within a multi-sensor approach can provide profound insights to actual plant (water) status and the rationale of physiological and biochemical changes. Synergistic sensor use will open new avenues for scientists to study plant functioning and the response to environmental stress in a wide range of ecosystems.

A Review of Current and Potential Applications of Remote Sensing to Study the Water Status of Horticultural Crops

“…The bands are generally designated for the visible and NIR region with extended capabilities in SWIR, TIR, as well as red edge region ( Table 1). The most widely used band combinations to study the water status of vegetation are the visible, NIR and TIR bands [23,25,53,54]. With the plethora of satellite systems currently available, user requirements on band combination may be achieved by using multiple satellites.…”

“…With the large volume of spatial/spectral data extracted from the hyperspectral data cube, machine learning will likely be adopted more widely in the horticultural environment to model water stress [141]. See Reference [54] for a comprehensive review of hyperspectral and thermal remote sensing to detect plant water status.…”

“…In an alternative (second) approach the range can be established based on meteorological data e.g., setting T dry to 5 • C above air temperature and T wet measured from an artificial surface. This method is also limited to local scale and presents a problem regarding the choice of material, which ideally needs to have similar to leaf emissivity, aerodynamic and optical properties [54,87]. The third method uses the actual temperature measurement range of the remote sensing image [33,97].…”

Review of Top-of-Canopy Sun-Induced Fluorescence (SIF) Studies from Ground, UAV, Airborne to Spaceborne Observations

“…However, to realize the potential of SIF and to explore its full spectrum using different RS observations, a complete document of existing SIF studies is needed [18]. In the past, several SIF-related review articles have been published, among which Meroni et al [1] and Mohammed at el.…”

Land Surface Temperature Retrieval for Agricultural Areas Using a Novel UAV Platform Equipped with a Thermal Infrared and Multispectral Sensor

“…Therefore, the LST is a key parameter for the physical description of the surface energy and water balance processes at the local to global scale [1,2]. The potential of this crucial parameter has been repeatedly demonstrated in various thermal infrared-based studies and applications, such as evapotranspiration [3,4], hydrological modelling [5], vegetation monitoring [6], 'urban heat island and urban development' [7][8][9], climate change and weather conditions [1,10], agriculture [3,11], and the monitoring of land use changes in wetlands [12]. In agricultural applications, precision farming has increasingly required thermal remote sensing techniques to detect water-stressed crops [3,13,14], plant diseases [13,15], and for irrigation management [13,14].…”