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Surface runoff often occurs because impervious areas (such as roofs and pavement) do not allow water to soak into the ground. Furthermore, runoff can occur either through natural or human-made processes. [5] Surface runoff is a major component of the water cycle. It is the primary agent of soil erosion by water.
The diagram also shows how human water use impacts where water is stored and how it moves. [ 1 ] The water cycle (or hydrologic cycle or hydrological cycle ) is a biogeochemical cycle that involves the continuous movement of water on, above and below the surface of the Earth .
Surface runoff often occurs because impervious areas (such as roofs and pavement) do not allow water to soak into the ground. Furthermore, runoff can occur either through natural or human-made processes. [1] Surface runoff is a major component of the water cycle. It is the primary agent of soil erosion by water.
Most water in Earth's atmosphere and crust comes from saline seawater, while fresh water accounts for nearly 1% of the total. The vast bulk of the water on Earth is saline or salt water, with an average salinity of 35‰ (or 3.5%, roughly equivalent to 34 grams of salts in 1 kg of seawater), though this varies slightly according to the amount of runoff received from surrounding land.
The diagram on the right shows some human impacts on the marine nitrogen cycle. Bioavailable nitrogen (Nb) is introduced into marine ecosystems by runoff or atmospheric deposition, causing eutrophication, the formation of dead zones and the expansion of the oxygen minimum zones (OMZs).
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The amount of runoff reaching surface and groundwater can vary significantly, depending on rainfall, soil moisture, permeability, groundwater storage, evaporation, upstream use, and whether or not the ground is frozen. The movement of subsurface water is determined largely by the water gradient, type of substrate, and any barriers to flow.
In addition to the pollutants carried in stormwater runoff, urban runoff is being recognized as a cause of pollution in its own right. In natural catchments ( watersheds ) surface runoff entering waterways is a relatively rare event, occurring only a few times each year and generally after larger storms.