Normally dry earth is not much help for grounding.
Grounding rods go down deep enough that you find moist dirt.
Often times copper lines are grounded or bonded in case a bare wire touches them.
A panel should have two grounding rods, and not rely on the plumbing.
Here, is we do a repipe on a home, we're required to have the panel grounded properly before we repipe.
Years ago they would use the copper pipe for grounding. We had to leave about ten feet out of the foundation. Too many copper thieves now for that to work.