decadal vs decennial

decadal

adj
  • Of or related to the number ten. 

  • Of or related to a decade, in its various senses. 

decennial

adj
  • Of or related to a ten-year period, as a term of office. 

  • Occurring every ten years. 

noun
  • The tenth anniversary of an event or happening. 

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