Ten hydrogeologic provinces of California are represented by a region- class feature called "provinces" within this digital data set. These provinces were identified and defined in the USGS Water Resource Investigation Report (WRIR) 03-4166 (Belitz and others, 2003) titled, "Framework for a Ground-water Quality Monitoring and Assessment Plan for California". Hydrogeologic provinces are generalized areas where hydrology and/or geology have similar characteristics. The Southern Cal Islands group was not previously defined as a province in WRIR 03-4166, and is therefore identified as a group in this dataset.

These provinces were created by grouping polygons from two formally created digital data sets into respective provinces. New polygons were created where necessary. The two digital data set sources consisted of watershed polygons (calwater) and groundwater basin polygons (gw_basins) compiled to create an original publication of hydrogeologic province boundaries in the form of a region-class within the data set. By symbolizing on the region- class, the user can view the ten hydrogeologic provinces and one grouped province in their simplest representation, without watershed and ground- water basin boundaries obscuring the province areas.

Original polygons for both the watershed data set and the ground-water basin data set were retained as much as possible.