Authoritative source for 一 changing to 4th tone before 1st, 2nd or 3rd In another users answer https://chinese.stackexchange.com/a/2046/38 they said:


  一 is pronounced in the first tone when it stands alone. It is
  pronounced in the fourth tone when it precedes a first, second, or
  third tone. However, it is pronounced in the second tone when it
  precedes a fourth tone.


I was surprised to read this and didn't believe it. However, when searching the net I found several places where this is mentioned.

I checked Google translate and it gave me Yīzǎo for 一早 and Yī nián yǐhòu for 一年以后 which is what I always thought it was.

A search on Google for "yīnián" provides me with 106,000 results, but a search for "yìnián" only provides 6,330 results.

Obviously, the examples above only provide partial evidence of correctness. But, is there a reliable, authoritative resource which can confirm the claim that 一 should change to 4th tone?
These tone changes, known as tone sandhi, are not indicated according to Hanyu Pinyin rules:


  11.1 Only the original tones are indicated; tone sandhi is not indicated.


This is why your Google search for "yìnián" would not necessarily yield more results, because it's still supposed to be written as "yīnián" even when it's pronounced "yìnián".

EDIT: Here's more information on tone sandhi from The Chinese Lexicon: A Comprehensive Survey by Yip Po-Ching, p. 27:


  ...tone sandhi may also happen by convention in a specific set of words in the lexicon when they are followed by words of different tones. This set includes numericals: yī 'one', qī 'seven', bā 'eight', and the negator bù 'not'. For example:
  
  yī on its own or not followed by other tones
  
  yī → yí when followed by a fourth tone
  
  yī → yì when followed by other tones
  
  qī/bā on their own or followed by tones other than a fourth
  
  qī/bā → qí/bá when followed by a fourth tone
  
  bù on its own or followed by tones other than a fourth
  
  bù → bú when followed by a fourth tone
  
  ...
  
  All these tone sandhi applications are actually more or less rule-governed and will not generally be reflected in the pinyin or phonetic notations of the disyllabic words or expressions in question.