What is the driving force behind excess runoff after a big precipitation event

Gravity is the driving force behind excess runoff after a big precipitation event.
Big precipitation event would be when clouds become too heavy and a big amount of rain, sleet, or snow falls. As a result water soaks into the soil and collects as groundwater, which is named as runoff . The driving force i this situation is force by which a planet or other body draws objects toward its center or in one term gravity. 

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