What term describes run-off that flows under ground?

The term that describes run-off that flows underground is indeed groundwater flow. Groundwater flow refers to the movement of water through the soil and rock layers below the earth’s surface. This underground flow occurs when water, after infiltrating into the ground, moves through the pore spaces in soil and geological formations, often traveling significant distances. This is crucial for recharging aquifers, which can be sources of drinking water and support various ecosystems.

To distinguish this from other options: surface flow refers to water running off on the surface, which would not include underground movement. Stream flow encompasses the surface water in rivers and streams, while the water table is the upper surface of the saturated zone in the soil, indicating the level below which the ground is saturated with water. Therefore, groundwater flow is specifically focused on the movement of water beneath the surface.