It’s an undeniable fact that the world is going through a water crisis today. The matter is not the fact that there is a shortage in the availability of water, but it is the fact that the available water is not being sustainably managed. Modern techniques such as Remote Sensing can be used to develop an effective and integrated approach to managing these water resources. Water quality is the process of determining the chemical, physical, and biological characteristics of various water bodies. It also includes the identification of possible contamination sources that degrade the water quality.
Advancing Water Quality Monitoring: Overcoming Challenges with Remote Sensing Technology
There could be various reasons for water quality getting degraded such as the influence of waste discharges, nutrients, heavy metals, sediments, and microorganisms. However, the real challenge is getting to know the minimum or maximum limit that these external factors are allowed to be present in water bodies without degrading them. That is why there are different water quality standards that have been developed by various monitoring agencies for water.
However, in addition to being time-consuming and expensive, conventional methods of studying water quality frequently fail to adequately represent heterogeneous and patchy areas . The traditional in situ methods of obtaining water quality parameters by obtaining samples are laborious, costly, and time-consuming, bringing many challenges. Remote Sensing technology can be effectively used to ascertain the spatial and temporal changes in water quality even in inaccessible locations. Owing to high-frequency data acquisition and large-scale coverage, various spaceborne sensors with visible, infrared, and microwave wavelengths can be used to monitor water quality.
Basis of remote sensing
Remote sensing is defined as the science and art of obtaining information about an object, area, or phenomenon through the analysis of data acquired by a device that is not in contact with the object, area, or phenomenon under investigation. Remote Sensing techniques measure the spectral signature backscattered from water. The amount of backscattering from the water surface changes with the particular substance present in it. Remote sensing techniques depend on measuring these changes in the spectral signature backscattered from water and relating these measured changes by empirical or analytical models to a water quality parameter. The optimal wavelength used to measure a water quality parameter depends on the substance, its concentration, and the sensor characteristics.
Factors Affecting Water Quality
Some of the major factors that play a major role in affecting the quality of water are the presence of algae (i.e., carotenoids, chlorophyll), suspended sediments , dissolved organic matters, some chemicals (such as nutrients, pesticides, and metals), pathogens, aquatic plants, and oils. These factors change the energy spectra of the reflected and emitted radiations coming from the water surfaces of ponds, lakes, and rivers. These radiations are then measured using various satellite sensors. Thus, these remote sensing techniques can easily help make spatial and temporal analyses of surface water quality parameters, which may not be possible to obtain directly by field measurements
Spectral reflectance curve for water bodies
The optical properties of pure water are required for remote sensing of surface water. Knowledge of the spectral properties of components in the surface water is required for accurate interpretation of measured reflection and attenuation spectra in terms of their concentrations. One of the most relevant properties of water that helps in remote sensing is its optical properties. The optical properties of water can be categorized as: Inherent Optical Properties (IOPs) and Apparent Optical Properties (AOPs).
Optical Properties of Water
Inherent optical properties (IOPs) include the absorption coefficient, scattering coefficient, volume scattering function, index of refraction, beam attenuation coefficient, and single-scattering albedo. In contrast, apparent optical properties (AOPs) include diffuse attenuation coefficients and irradiance reflectance. IOPs depend on the medium through which light travels, while AOPs depend on the medium and its spatial distribution.
Light Reflectance in Water and Snow
Water has very low spectral reflectance in the visible part of the electromagnetic spectrum, whereas snow or ice has very high spectral reflectance in the visible and near-infrared regions. Pure water absorbs nearly all incident energy in both near-infrared and middle-infrared wavelengths. The low reflectance of water in the visible and near-infrared bands is advantageous in remote sensing, as water becomes clearly distinguishable from vegetation or soil cover.
Remote Sensing in Water Quality Assessment
Satellite imagery has been successfully used to determine various water quality parameters such as total suspended solids, turbidity, chlorophyll content, color, and temperature by utilizing the visible, reflected infrared, and occasionally thermal infrared bands of the electromagnetic spectrum. This is accomplished by measuring spectral reflectance.
Spectral Reflectance
The ratio of the energy reflected by the Earth’s surface to the incident energy, measured as a function of wavelength, is known as the spectral reflectance of the surface, also referred to as the albedo. It can vary from 0% to 100%. The energy reflected by features on the Earth’s surface across various wavelengths provides their spectral responses in remote sensing systems.
Components of Captured Energy
The energy radiance captured by a sensor comprises roughly two components: scattered energy from the atmosphere and radiant energy reflected from the water body. The energy reflected from the atmosphere is essentially considered noise and is thus removed for accurate remote sensing quantification of water quality parameters.
Spectral Signature
Each type of feature/object has a unique spectral response/reflectance characteristic, known as a spectral signature, which can be used to identify the respective surface features and study their properties. The graphical representation of the spectral response of an object over different wavelengths of the electromagnetic spectrum is termed the spectral reflectance curve.
Types of Sensors
Observing sensors are categorized into two primary types, depending on their platforms. Space-based sensors are deployed via spacecraft or satellites to positions beyond Earth’s atmosphere. Meanwhile, airborne sensors are installed on platforms in Earth’s airspace, such as balloons, aircraft, helicopters, boats, or Unmanned Aerial Vehicles (UAVs). These sensors play a crucial role in capturing images and related data while digitally recording the interaction of electromagnetic waves with matter.
Image Resolution in Remote Sensing
Image resolution is a vital characteristic of remote sensing, determining the ability to discern fine details within the image. Remote sensing has the following resolution types: spectral, spatial, radiometric, and temporal resolution. Spectral resolution is the wavelength interval captured by a detector, defined as the wavelength interval recorded at 50% of the peak response of the sensor. Spatial resolution pertains to the capability to differentiate between closely spaced objects in an image. Radiometric resolution represents the number of discrete levels, or bits, that an imaging system uses to record variations in a given range of values. Temporal resolution denotes the time interval between successive images captured of the same area.
Space-Borne Sensors
Space-borne sensors applied in water quality monitoring typically operate passively.
These sensors offer extensive and periodic scanning of the Earth’s surface, ranging from daily to monthly revisits due to the consistent orbital patterns of satellites. This frequent coverage is particularly beneficial for studies requiring regular monitoring of water quality trends in specific areas. Further, space-borne sensors have been classified into two types depending on the count of spectral bands they use. These types are multispectral sensors and hyperspectral sensors.
Multispectral sensors gather data across a defined number of distinct spectral bands, typically spanning a few spectral bands, generally ranging from 3 to 10 bands. The images generated by these sensors are called multispectral images and represent the main type of imagery captured by remote sensing (RS) radiometers. Compared to panchromatic (black and white) images, multispectral images contain additional data; each pixel captures the total intensity of visible radiation from its corresponding section of the scene. Typically, satellites include the red–green–blue (RGB) region, which consists of three or more wavelength bands within the 0.4–0.7 μm range. Infrared wavelengths, spanning from 0.7 to 10 μm or beyond, are further categorized into near-infrared (NIR), middle-infrared (MIR), and far-infrared (FIR or thermal) groups. While multispectral sensors provide adequate spectral coverage for numerous applications, their limited number of bands may constrain the level of detail that can be captured compared to hyperspectral sensors .
Conclusion
Water quality is undeniably an important indicator of the health of an environmental system. Its correct estimation is not only essential for human consumption but also necessary for the well-being of the entire ecosystem. Remote Sensing, coupled with Geographic Information System, has proven to be a powerful tool for monitoring water quality and water pollution. This technology has been tremendously successful in its application for management and planning despite its limitation of parameters under consideration. Remote Sensing reads and maps the spectral reflectance that it receives from a water body, and by using various wavelength bands such as visible, NIR, SWIR, and TIR, it identifies the relevant water quality characteristic. Several types of research and literature are available where the data obtained from these airborne or spaceborne remote sensors are used for planning, monitoring, management, and prediction of water characteristics across the globe. Sometimes in situ measurement is coupled with Remote Sensing to ensure the accuracy of the quantifications being made. The technology has also been used extensively to monitor groundwater quality aspects. Remotely sensed data come with the added advantage of being readily available even at the remotest of locations. While assessing water quality, remote sensing parameters such as suspended sediments, algae, turbidity, and chlorophyll concentration can easily be monitored. They can enable consultants and natural resource managers to develop management plans for a variety of natural resource management applications.
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Reference
1- Applications of remote sensing in water quality assessment
https://www.sciencedirect.com/
2-Sensors Used in Remote Sensing Water Quality Monitoring
