Runoff is water that flows across the ground. The water “runs” downhill.
As rain falls, some water moves down into the soil. This is called “infiltration.” But sometimes rainwater has trouble seeping into the ground. This can happen when the soil is filled up with water, or “saturated.” It can also happen when it rains faster than the soil can soak it up. Or it can happen when the soil is hard, making it more difficult for rainwater to move into the soil.
Whatever the reason, when rainwater has trouble seeping into the ground, puddles form. If the land is sloping (going downhill), the water will move downhill. This is runoff water.
Runoff water flows downhill, carrying away millions of soil particles.
Photo: U of I College of ACES-ITCS