a torrent (noun) is a sudden, strong wind a torrential (adjective) rainstorm is a storm with powerful winds There is imagery here as well as pure dictionary meaning. When the writer uses "a torrent of water" he/she is using the image of both these words to create the image in your mind...of strong, sudden, water rushing down the hill... It's a creative combination of two words not usually put together. The combination creates the right idea in the mind, though, so this is an example of skilful creative writing.