Disability or industrial disability retirement is available for members unable to continue working due to illness or injury.
If you receive a disability pension - it is your taxable income - and you should receive a form 1099-R from the payer reporting that income - http://www.irs.ustreas.gov/pub/irs-pdf/f1099r.pdf
It is taxable on the federal and on the state levels - but if you actually owe any taxes - depends on your total income, filing status, deductions, etc.
If you determine that you will owe taxes - you might want the payer to withhold federal and/or state income tax with each paycheck - here is the form to use - https://www.calpers.ca.gov/mss-publication/pdf/xqwsO2GSlf6ZA_taxwithhold.pdf
If you need any help to estimate your tax liability - let me know.