What is the HUC crosswalk?
CalWater maps fairly neatly to the federal Hydrologic Unit Codes (HUCS). Generally CalWater watersheds fit within HUCs, though there are some exceptions. The IDNUM to HUC cross-reference was developed with the following rules: For a given [state] CalWater watershed, overlapping [federal] Hydrologic Unit Codes (at the 8-digit Cataloging Unit [CU] level) are listed up to a maximum of 80% of the CalWater watershed OR three CUs, whichever comes first. For example: Where there is only one CU code listed for a given CalWater watershed code, that CU overlaps at least 80% of the CalWater watershed. Where there are two CU codes (CU and CU2) listed for a given CalWater code (only 18 watersheds fit this criterion), the largest CU alone overlaps less than 80% of the CalWater watershed, but combining CU2 then provides overlap of at least 80% of the CalWater watershed. Where there are three CU codes (CU, CU2, and CU3) listed for a given CalWater watershed (only 1 watershed fits this criterion), the
