How can a field be eroded by heavy rain

Heavy rain on a fairly even slope creates "sheet runoff." The water flows down slope as an even sheet. Any dips in the slope will collect more runoff water, which will be able to erode more strongly. If the force of this water is not stopped it will produce a gully . The gully will collect more and more water and cause even more erosion. In time it will remove the soil down to solid bedrock. 

It’s possible if there is certain things in that rain water (like minerals) but I would have to know the question to sire

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