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Run-off, composed of a mixture of water and soil along with any other organic or inorganic substances that may exist in the land, is the product precipitation or other water coming in contact with the earth and carrying matter elsewhere.
A common context of run-off deals with agriculture. When farmland is tilled and bare soil is revealed, rainwater carries billions of tons of topsoil into waterways each year, causing loss of valuable topsoil and clogging of valuable waterways.
Another issue is loss of groundwater due to run-off cause by substantial land area being covered by pavement. The precipitation, which would have been absorbed into the soil and recharged groundwater, instead hits concrete, and flows, gathering momentum and finding its path.