Political Science Professor Michael McDonald has a webpage that posts turnout data for each state. McDonald determines the number of ballots cast in each state, and divides that by the voting-eligible population. “Voting-eligible population” means not only the number of registered voters, but the number of people who could register if they chose to. The webpage shows turnout for all recent congressional and presidential elections. To see the 2014 data, see here. To see the 2010 data, see here.
When one compares 2014 to 2010, one sees that California easily had the sharpest drop in turnout of any state. The 2010 California turnout was 45.8%, and the 2014 California turnout was 30.0%. Comparing those percentages shows that the 2014 percentage was only 67.2% of the 2010 percentage. No other state had a ratio lower than .7. For the nation as a whole, the 2014 turnout was 88% of the 2010 turnout.
One likely reason that California’s turnout in 2014 slipped so badly relative to 2010 is that in November 2010, voters were given a choice of six parties for the statewide offices, whereas in November 2014 California was the only state in which it was impossible for anyone to vote in any statewide race unless that voter voted for a Republican or a Democrat. In November 2010, over 10% of the voters had voted for a minor party candidate for Lieutenant Governor or Insurance Commissioner.