Learn English – split town — meaning

How much water a boat draws is a measure of the depth of water required to float it: a rowboat ‘draws’ only a few inches, a fully loaded flat-bottomed barge may ‘draw’ eight or ten feet, an ocean-going vessel with a very deep keel may ‘draw’fifty feet.

Figuratively speaking, how much ‘water’ a man ‘draws’ is a measure of his presence and influence. Jackie Treehorn is a very important man in his community; Lebowski is a nobody.

• ADDED:
  Galactic Cowboy suggests that the idiom here is one of drawing water from a well or other source, and that in water-poor Southern California a man entitled to "draw a lot of water" is marked as particularly influential. He may well be right: one of the earliest instances I have found of the idiom is from a 1938 crime novel set in LA and written by a former LA policeman in 1938 - about ten years after the California Water Wars. One seamier sidelight of that episode was the profits supposed to have been realized by insiders who bought up land in the San Fernando Valley, knowing that LA would have to annex the district to secure rights to the water. However, I also find the idiom in a report of an air show in the Pacific Northwest in the same year.

Only so often is an idiom.

It means sometimes or occasionally with a slight emphasis on it being a rare occasion.

It is similar to every so often, which also means sometimes or occasionally but with a general feeling of being more common.

If I get my hair cut every three months, I might say:

Every so often I get my hair cut.

If my husband only shaves once every two years, I might say:

It's only so often I get to see my husband without a beard.