If you are just starting out with the Cumulus Software or wish to Calibrate the Year Rainfall click on Configuration from the top menu and select Station Settings.

Bottom middle – ‘Annual Rainfall’.
Put your total for the year so far in ‘YTD Amount’.
Add the current ‘Year’ underneath.

If you want your annual rainfall total to be counted from the start of a particular month (e.g. your rainfall season might be August to July rather than January to December),  you can specify the start month under – ‘Rainfall Season Start’ on the right.

For most it is January to December.
Click on Ok and check Cumulus’s Frontend to ensure correct.