Fromsub- +‎basin.

subbasin (pluralsubbasins)

1. (geography) Abasin within a larger basin.
   • 2009 May 14, Felicity Barringer, “Rising Calls to Regulate California Groundwater”, inNew York Times‎^[1]:

     Since 2006 the surface of the aquifer, in the Kaweahsubbasin of the San Joaquin basin, has dropped 50 feet as farmers pumped deeper, Mr. Watte says.