In hydrology, run-on refers both to the process whereby surface runoff infiltrates the ground as it flows, and to the portion of runoff that infiltrates. Run-on is common in arid and semi-arid areas with patchy vegetation cover and short but intense thunderstorms. In these environments, surface runoff is usually generated by a failure of rainfall to infiltrate into the ground quickly enough (this runoff is termed infiltration excess overland flow). This is more likely to occur on bare soil, with low infiltration capacity. As runoff flows downslope, it may flow onto ground with higher infiltration capacity (such as beneath vegetation) and become run-on.
Run-on is an important process in the hydrological and ecohydrological behaviour of semi-arid ecosystems. Tiger bush is an example of a vegetation community that develops a patterned structure in response to, in part, the generation of runoff and run-on.
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