This map shows net migration from 2013 to 2014 for California counties based on tax return data (with the exception of Alpine County where the data is from 2012 to 2013). Number of exemptions claimed is a rough proxy for number of residents. The data is limited to people filing tax returns, so it doesn't reflect the entire population.
Sources: The IRS, governing.com, U.S. Census Bureau
County: County Name
Net Migration: Migration