Why are dry land areas-not just waterways and wetlands-included in a watershed?

Question

Can a possible answer be - with precipitation and elevation not all water flows out of the watershed and will often continue to flow where there is no direct runoff from recent precipitation.
Lab on water sheds and topographical maps.

Answer 1

yes, the water has to run off. Ever see a flash flood in the New Mexico desert? I was in one once, we had to escape for two hours to an overpass, as the I10 was flooded with swift water.