rsync between mac and network drive

[Bob Lisbonne]

I would like to use rsync to backup my Mac to a network drive, and am  
hoping someone on the list can help straighten me out.

Since the network drive can't run any programs, I am first mounting  
the drive onto the Mac via SMB as /Volumes/backup. When I run:

rsync -av --delete --no-whole-file --progress /Users/bob/Documents / 
Volumes/backupbob

everything seems to copy successfully. However, if I immediately run  
the command again, it copies everything again, instead of doing an  
incremental backup. How do I get rsync to do incremental backups?  
Ideas anyone?

TIA,
/Bob