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Question:

Why are there negative values observed over water in some Landsat surface reflectance products?

Answer:

Landsat atmospheric correction and surface reflectance retrieval algorithms are not ideal for water bodies due to the inherently low level of water leaving radiance, and the consequential very low signal to noise ratio. Similarly, surface reflectance values greater than 1.0 can be encountered over bright targets such as snow and playas. These are known computational artifacts in the Landsat surface reflectance products.  

Visit the Landsat Surface Reflectance web page for more information.