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.
Highlights
Water-deficit stress, usually shortened to water or drought stress, describes the plant physiological responses induced by a lack of available water due to either soil water deficit or high evaporative demand of the atmosphere
120 and biochemical changes depending on the severity and duration of plant water deficit
Plant responses to water stress can be recognized as a sequence of physiological and biochemical changes depending on the severity and duration of plant water deficit
Summary
Water-deficit stress, usually shortened to water or drought stress, describes the plant physiological responses induced by a lack of available water due to either soil water deficit or high evaporative demand of the atmosphere. One of the most challenging tasks of our generation is to meet the second Sustainable Development Goal as defined in the 2030 agenda for sustainable development by the United Nations: “End hunger, achieve food security and improved nutrition and promote sustainable agriculture” [8] In this respect, agricultural processes need to be optimized and innovative farming methods must be developed to guarantee global food supply, as arable land and environmental resources have almost reached the limits of sustainability. It is desired to reduce the amount of water used per unit yield by reducing yield loss and by reducing the amount of water used for irrigation To achieve this goal, early detection and monitoring of plant responses to water stress in agricultural crops are mandatory. Precision agriculture promises great potential to close the yield gap by optimizing food production using the right management practice at the right place and the right time while keeping the consumption of resources at an environmentally sustainable level [13]
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