droop vs run low

droop

verb
  • To proceed downward, or toward a close; to decline. 

  • To lose all energy, enthusiasm or happiness; to flag. 

  • To slowly become limp; to bend gradually. 

  • To allow to droop or sink. 

  • To hang downward; to sag. 

noun
  • A condition or posture of drooping. 

  • A hinged portion of the leading edge of an aeroplane's wing, which swivels downward to increase lift during takeoff and landing. 

  • Something which is limp or sagging. 

run low

verb
  • To near the end of a supply of something; to be nearly running out. 

How often have the words droop and run low occurred in a corpus of books? (source: Google Ngram Viewer )