SURFACE_WATER.surface_water_cd Hydrologic unit. Hydrologic unit. A hydrologic unit for which aggregate water-use data are stored. SURFACE_WATER.huc_cd SURFACE_WATER.surface_water_nm Record history. The points at which a database record was created and last modified, and the user names of the creators and modifiers. SURFACE_WATER.surface_water_cn/SURFACE_WATER.surface_water_cr/SURFACE_WATER.surface_water_mn/SURFACE_WATER.surface_water_md