Which of the following best defines a watershed?

A watershed is defined as a region or area in which all surface water converges and drains into a common outlet, such as a river, lake, or ocean. This definition encompasses the full interplay between the various water systems—streams, rivers, and lakes—within that geographical area. The significance of this concept lies in understanding how water flows, which can affect everything from ecological health to human usage, including agriculture and urban planning.

The other options do not accurately describe a watershed. The first choice, about surface water being diverted, refers more to water management practices rather than the natural characteristics of a watershed. The second choice discusses geothermal activity, which is unrelated to the concept of drainage and water flow associated with a watershed. Lastly, the choice referring to only underground water flow does not encompass the entire hydrological cycle that includes surface water, making it an incomplete and inaccurate description of a watershed.