Runoff is a compound word. Let us break it apart. Run - To move, Off - A direction. Away from, not on. So runoff means - to move away from, or to move off in a direction. So surface runoff means something that (moves away from the surface) or in this case, runoff. When combined with surface, it is usually referring to water. Rain is water. Some of it goes into the ground, the rest does not, and creates runoff :)

Probably you mean " Escorrentía". The flow of water usually rain's water flowing over the earth's surface until it gets rivers and water bodies. Let me know if it's the one you were looking for. Byee..

Eso refiere a agua que no va bajo tierra y se desborda hacia ríos en vez. Usualmente es agua mas sucio porque mueve encima de el superficie y no bajo tierra. No sé la palabra en español, es un término científico.