Spectral water indices
Spectral water indices enable scientists and researchers to detect changes in water bodies, assess soil moisture, and monitor vegetation health with unprecedented precision and scale.
Remote sensing and spectral indices provide a powerful set of tools for water management. Spectral water indices are numerical indicators derived from remote sensing data that help in the detection, monitoring, and analysis of water bodies and moisture content in soils and vegetation. These indices utilize the spectral signatures captured by satellite or aerial sensors across various wavelengths of the electromagnetic spectrum.
Water bodies typically absorb most of the visible light (400-700 nm) and reflect more in the near-infrared (NIR) and short-wave infrared (SWIR) regions. This distinctive characteristic is key to identifying and analyzing water bodies and water content in vegetation from satellite imagery.
Examples of spectral water indices
Note: This list is not exhaustive. Please refer to the TranzAI platform documentation for a complete list of indices available in the platform.
Normalized Difference Water Index (NDWI)
This index is designed to enhance the presence of water bodies in remotely sensed imagery. It is calculated using the formula:
NDWI=(Green+NIR)/(Green−NIR)
where Green represents the reflectance in the green light band, and NIR is the near-infrared band. Water bodies absorb more near-infrared light and reflect more green light, resulting in positive values for water and negative values for land.
Modified Normalized Difference Water Index (MNDWI)
The MNDWI modifies the NDWI by using the Green and Short-Wave Infrared (SWIR) bands in its calculation:
MNDWI=(Green+SWIR)/(Green−SWIR)
This modification helps in reducing the effects of built-up land, thus improving the detection of water bodies.
Normalized Difference Moisture Index (NDMI)
The NDMI is used to assess moisture content in vegetation and soil. It is calculated as
NDMI=(NIR+SWIR)/(NIR−SWIR)
where NIR is the near-infrared band, and SWIR is the short-wave infrared band. Higher values indicate more moisture content.
Water Ratio Index (WRI)
The WRI utilizes multiple bands to enhance the detection of water bodies, calculated as
WRI=(NIR+SWIR)/(Green+Red)
where Red is the reflectance in the red light band. This index helps in distinguishing water from vegetation and soil.
Working with spectral water indices in the TranzAI platform
Definition of indices in the ontology
Spectral indices are predefined entities of the environmental ontology available in the TranzAI platform.
As part of the ontology, spectral indices can be used to instantiate master data fields (semantic layer) and feature tables.
Their metadata is closely aligned with the metadata of the Earth Observation data sources referenced in the TranzAI platform. This coherence between the metadata of the ontology and the metadata of the data sources makes it possible to automatically extract spectral indices from rasters (stored as GeoTIFF files in the TranzAI spatial data store) with a pure no-code interface. Feature extraction pipelines dedicated to the calculation of spectral indices can be easily defined by the data source (e.g. Sentinel-2) and the spectral index to be calculated, as specified at the ontology level.
The synergy between advanced remote sensing techniques and environmental science is significantly enhancing our ability to address some of the most pressing challenges related to water scarcity, ecosystem health, and climate change. For instance, by accurately mapping and monitoring wetlands, water bodies, and irrigation patterns, policymakers and conservationists can make informed decisions that promote the efficient use of water resources, protect biodiversity, and mitigate the impacts of climate variability.
