Spectral reflectance data from Earth-resource satellites have provided hydrogeologists with a new technical tool for monitoring natural environments. Because the satellite instruments record reflected solar radiation in digital format, the numerical data may be analyzed mathematically to identify and document changes in areal extent of surface water and areal extent and density/vigor of vegetation. Graphs can be constructed to show changes in measured environmental conditions with respect to time, or can be analyzed for changes over time in relations between environmental conditions and other parameters, such as groundwater pumped, annual precipitation, or land use. Principal advantages of the satellite image method for environmental monitoring include: a relatively long historic period beginning in 1985 for which information is available; the reliable numerical basis for the results; and the ability to assemble a historic environmental baseline and provide on-going monitoring for large or remote areas where standard monitoring methods may not be practical. The method requires processing and analysis of reflected solar radiation data obtained by Landsat 5 Thematic Mapper, and Landsat 7 Enhanced Thematic Mapper Plus multispectral scanners. Materials on the Earth's surface absorb or reflect different parts of the spectrum depending on their physical and chemical properties and, therefore, typically have characteristic spectral "signatures". Using these signatures, environmental parameters such as surface water and vegetation may be identified and analyzed. Because Landsat satellites 5 and 7 are in sun-synchronous, near-polar orbits that pass the satellites over the same point on Earth every 16 days, information is obtained at regular intervals allowing comparison of spectral data for the same area at different dates. Therefore, relatively continuous historic environmental parameters can be established for a location, and change in those parameters can be measured.
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