The fraction of incoming electromagnetic radiation reflected by a surface at specific wavelengths. Spectral reflectance signatures of water, soil, and vegetation are the foundation of satellite-based water resource monitoring.
Spectral reflectance is the ratio of reflected to incident electromagnetic radiation at a given wavelength, expressed as a value between 0 and 1 (or 0-100%). Every material on Earth's surface has a characteristic spectral reflectance curve or signature that depends on its physical and chemical properties. Water has very low reflectance in near-infrared and shortwave infrared wavelengths due to strong absorption, which is exploited by water indices like NDWI to map surface water bodies. Turbid or sediment-laden water shows elevated reflectance in visible wavelengths, enabling estimation of suspended sediment concentrations from satellite data. Vegetation has a distinctive signature with low reflectance in red (chlorophyll absorption), high reflectance in near-infrared (cell structure scattering), and low reflectance in shortwave infrared (water absorption). Soil reflectance varies with moisture content, organic matter, mineralogy, and texture, with wet soils showing consistently lower reflectance than dry soils. Understanding spectral reflectance is essential for interpreting satellite imagery, developing retrieval algorithms for water quality parameters, and calibrating radiometric measurements. Atmospheric correction of satellite data to convert at-sensor radiance to surface reflectance is a critical preprocessing step for quantitative hydrological remote sensing.