Installation of water lines must be done according to code.
The proper depth required to dig and install a water line is decided by your location. In colder climates a water line should be deeper. You can get information on the proper water line depth for your area from local building codes.
To prevent freezing a water line must be installed below the frost line. The depth of the frost line can be variable between different counties in a state.
In the State of Maryland a water line must be installed at least 36 inches beneath the ground according to the building code. In Indiana the water line should be 36 inches to 60 inches deep beneath the ground.
In Placer County, California a water line must be installed at least 12 inches beneath the ground according to the building code. Measurement should be done from the top of the line.