RUNOFF

That portion of precipitation or irrigation on an area which does not infiltrate or evaporate, but instead is discharged from the area.

Dissolved chemicals, etc, included in such water.

"The runoff of nitrates is poisoning the lake."

A second or further round of an indecisive election, after other candidates (often all but the last two) have been eliminated.

"There will now be a runoff as neither front runner received more than 50% of the vote."