catchment area vs watershed

catchment area

noun
  • An area from which water drains into a particular lake, river, etc.; the catchment area of a large river includes its tributaries. 

  • The area served by a local institution, such as a hospital or school, or by extension, a railway station. 

watershed

noun
  • A region of land within which water flows down into a specified body, such as a river, lake, sea, or ocean; a drainage basin. 

  • The topographical boundary dividing two adjacent catchment basins, such as a ridge or a crest. 

  • The time after which material of more adult nature (violence, swear words, sex) may be broadcast on television or radio, either one laid down by law or one contrived from convention (e.g. when children are not watching) 

  • A critical point marking a change in course or development. 

adj
  • Serving to mark a significant development, change in direction, etc. 

How often have the words catchment area and watershed occurred in a corpus of books? (source: Google Ngram Viewer )