Change heavy and varied aquifers to light aquifers.
This script behaves differently depending on whether it’s called pre-embark or post-embark. Pre-embark, it changes all aquifers in the world to light ones, while post-embark it only modifies the active map tiles, leaving the rest of the world unchanged.
If you don’t ever want to have to deal with heavy aquifers, you can enable the
light-aquifers-only command in the “Autostart” tab of gui/control-panel
so it will be run automatically whenever you start a new fort.
When run pre-embark, this script changes the drainage of all world tiles that would generate heavy aquifers into a value that results in light aquifers instead, based on logic revealed by ToadyOne in a FotF answer: http://www.bay12forums.com/smf/index.php?topic=169696.msg8099138#msg8099138
Basically, the drainage is used as an “RNG” to cause an aquifer to be heavy about 5% of the time. The script shifts the matching numbers to a neighboring one, which does not result in any change of the biome.
When run post-embark, this script simply clears the flags that mark aquifer tiles as heavy, converting them to light.