Spectral Reflectance of Crops and Soils
Spectral Reflectance of Soils
Soil spectral reflectance refers to the way soils reflect sunlight at different wavelengths. Spectral reflectance analysis is one of the most widely used techniques for soil property assessment. It involves measuring the reflectance of electromagnetic radiation across different wavelengths, typically in the visible and near-infrared regions. This allows researchers to analyze and map various soil properties like moisture content, organic matter, texture, and mineral composition with high spatial resolution. It involves measuring the reflectance of electromagnetic radiation across different wavelengths, typically in the visible and near-infrared regions. Different soil properties exhibit unique spectral signatures, allowing for their identification and quantification. Spectral sensing systems cover the visible (400–700nm), near-infrared (750–2,500nm), or mid-infrared (2,500–25,000nm) wavelengths or a combination of these wavelengths. Most studies examining quantitative relationships between remotely sensed data and soil properties have focused on the reflective region of the spectrum (400-2,500nm), with some relationships established from data in the thermal and microwave regions. By analyzing the spectral signature of soil, it's possible to infer physical and chemical properties using proximal or remote sensing tools (Table 10.2).
Soil Indices
Spectral soil indices are mathematical combinations (usually ratios, differences, or normalized differences) of reflectance values at specific wavelengths, designed to highlight certain soil properties. They are similar in concept to vegetation indices (like NDVI for plants), but tailored for soil features like moisture, organic matter, salinity, texture, and mineral content.
Normalized Soil Moisture Index (NSMI)
The Normalized Soil Moisture Index (NSMI) is a remote sensing index used to estimate soil moisture content, particularly in areas with bare or sparsely vegetated surfaces. It utilizes the short-wave infrared (SWIR) spectral bands to capture the moisture content of the soil.
Salinity Index (SI)
The Salinity Index (SI) is a measure of soil salinity that can be calculated using satellite data, specifically by analyzing the reflectance values in different spectral bands. It helps visualize the amount of salt present in soils and can be used to map areas with high salinity.
Soil Color Index (SCI)
The Soil Color Index (SCI) is a quantitative spectral index used to estimate soil color using reflectance values in the visible spectrum—especially important for interpreting soil organic matter, iron oxide content, and surface moisture. A common version of the SCI is: SCI = Red - Blue / Red + Blue
Clay Index (CI)
The Clay Index (CI), also known as the Consistency Index, is a soil property that indicates the soil's firmness or shear strength. It is calculated as the difference between the liquid limit and the natural moisture content, divided by the Plasticity Index.
Organic Carbon Index (OCI)
The Organic Carbon Index (OCI) is a spectral index designed to estimate soil organic carbon (SOC) content by leveraging differences in reflectance between wavelengths that are sensitive to organic matter absorption. A common version of the OCI is:
Bare Soil Index (BSI)
The Bare Soil Index (BSI) is a spectral index used in remote sensing to identify and differentiate bare soil from other land cover types. It combines blue, red, near-infrared (NIR), and shortwave infrared (SWIR) spectral bands to capture soil variations.
Soil Adjusted Vegetation Index (SAVI)
The Soil Adjusted Vegetation Index (SAVI) is a remote sensing vegetation index used to improve the accuracy of NDVI (Normalized Difference Vegetation Index) in areas with low vegetation cover. It minimizes the effects of soil brightness on vegetation readings by incorporating a soil-brightness correction factor, L, according to ClimateEngine.org.
Normalized Difference Tillage Index (NDTI)
The Normalized Difference Tillage Index (NDTI) is a spectral index used in remote sensing to assess the amount of crop residue cover, which in turn helps determine the intensity of tillage practices in agricultural fields.
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