Remote Sensing for Plant Water Content Monitoring: A Review

Abstract

This paper reviews the different remote sensing techniques found in the literature to monitor plant water status, allowing farmers to control the irrigation management and to avoid unnecessary periods of water shortage and a needless waste of valuable water. The scope of this paper covers a broad range of 77 references published between the years 1981 and 2021 and collected from different search web sites, especially Scopus. Among them, 74 references are research papers and the remaining three are review papers. The different collected approaches have been categorized according to the part of the plant subjected to measurement, that is, soil (12.2%), canopy (33.8%), leaves (35.1%) or trunk (18.9%). In addition to a brief summary of each study, the main monitoring technologies have been analyzed in this review. Concerning the presentation of the data, different results have been obtained. According to the year of publication, the number of published papers has increased exponentially over time, mainly due to the technological development over the last decades. The most common sensor is the radiometer, which is employed in 15 papers (20.3%), followed by continuous-wave (CW) spectroscopy (12.2%), camera (10.8%) and THz time-domain spectroscopy (TDS) (10.8%). Excluding two studies, the minimum coefficient of determination (R2) obtained in the references of this review is 0.64. This indicates the high degree of correlation between the estimated and measured data for the different technologies and monitoring methods. The five most frequent water indicators of this study are: normalized difference vegetation index (NDVI) (12.2%), backscattering coefficients (10.8%), spectral reflectance (8.1%), reflection coefficient (8.1%) and dielectric constant (8.1%).

Dynamic Characteristics of Canopy and Vegetation Water Content during an Entire Maize Growing Season in Relation to Spectral-Based Indices

“…In other words, the water taken in by the stems increased with the growth stage while that taken in by the leaves decreased throughout the growing seasons. This finding suggested that water storage in stems might play an important role in regulating the plant water status, as suggested by [29,58]. Moreover, all five SVIs showed an obvious dynamic trend of first increasing and then decreasing (Figure 4), which was also reported in another study [59].…”

“…However, soil moisture may not be the core issue when evaluating drought impacts on agriculture, as a water deficit does not necessarily result in biomass or yield loss [42,65,66]. In fact, in comparison to soil moisture, plant water status is more related to plant physiological functions (e.g., photosynthesis, transpiration, and respiration) [29,67] and is directly connected to biomass and yield production. In addition, leaf water content within a plant canopy plays an important role in light penetration and scattering [68].…”

“…Previous studies have suggested that CWC and VWC can be estimated by their regression relationship with various spectral vegetation indices (SVIs) derived from a combination of a reference band and/or water-absorption bands [13,[25][26][27][28]. The most frequent SVIs applied to estimate CWC, VWC, and other related physiological variables, e.g., EWT and leaf area index (LAI), are the normalized difference vegetation index (NDVI), normalized difference infrared index (NDII), normalized difference water index (NDWI), water index (WI), and optimal soil-adjusted vegetation index (OSAVI) [29,30]. For instance, VWC appeared to have a linear relationship with the NDVI and NDWI [31,32], while its relationship was exponential with OSAVI [30].…”

Technology and Data Fusion Methods to Enhance Site-Specific Crop Monitoring

“…However, the system only uses qualitative data, and withinfield color variations do not allow accurate quantitative estimation of crop parameters. Multispectral sensors are widely adopted for deriving crop vegetation indices (VIs) to predict crop yield [35,36]. However, the system is not able to provide rapid variations that occur in leaf chlorophyll [37].…”

Drought Stress: Responses and Mechanism in Plants

“…The functions of these as a medium for metabolic and physiological reactions in plants, in which metabolic and physiological activities can decrease when there is a lack of water and also plays a role as a medium for transporting essential nutrients and minerals from the soil so that a lack of water can reduce the rate of nutrient uptake from the soil by roots [5]. These is also one of the main factors determining plant production related to biomass production and transpiration rate [6]. Water will affect cell turgidity, thereby affecting the process of opening and closing stomata.…”